File 22: Mineral and Organic Constituents

Mineral and organic constituents belong to the basic components observed in soil thin sections. They can appear, for instance, as large rock fragments, or single minerals as sand grains; they can constitute large areas of micromass formed by clay minerals or display parts of plant roots or leaf fragments, i.e. organic material. These constituents comprise the body of the soil itself, and in soil micromorphology, they belong to the groundmass , as well as the material constituting the pedofeatures (see “File 9”). Two types of basic components are recognized by Stoops (2003, 2021), those recognizable at the magnifications of the optical microscope and those which are not. Stoops (2003, 2021) pointed out the problem of the optical microscope resolution and the thickness of conventional thin sections. Indeed, it is preferable not to have a standard size limit between coarse and fine materials. Consequently, there are three main types of basic components: the coarse mineral constituents, the fine mineral phase, and the organic matter-related constituents.

After a description of the sorting and shape of coarse grains, five sections present the main rocks encountered in soils. During their weathering phase, rocks can free mineral grains in the soil environment, and this is illustrated in five other sections. Three sections of rocks show the large diversity of the micromass and the other constituents of the groundmass. Some minerals do not originate from rocks but from living organisms, such as plants, bacteria, fungi, and animals: they are known as “biominerals” and three sections report the most common of them. Organic matter plays a fundamental role in soils and it leaves many traces of its impact at the microscale: three sections describe its various characteristics. Finally, soils were the foundations of all civilizations: they often contain the traces of humankind, which are called anthropogenic features. These are illustrated by two sections.

The diversity of constituents, from minerals to organic or anthropogenic features. All microphotographs are in PPL.

File 23: Particle Size and Sorting

The proportions of coarse and fine materials, according to their size , their degree of sorting , and their shape , constitute the fundamental parameters related to the soil texture in thin sections. All these terms are currently used in sedimentary petrography to describe terrigenous clastic sedimentary rocks; they are also used in soil science, as far as physical soil properties are concerned. Moreover, regarding particle sizes, it must be stressed that it is difficult to measure the sizes of constituents in thin sections, as they depend on the orientation of the object and its cross-cut plane in respect to the thin section surface. SeeSee Grain size

Captions start with the ternary plot at the top. Round images below the ternary plot are listed from the upper left corner to the lower right corner.

1.:

Ternary plot of soil particle sizes and texture classes according to the World Reference Base for Soil Resources (IUSS and Working-Group-WRB 2014). Examples of texture microphotographs (in PPL) are shown for each type of texture, i.e. clayic SeeSee Grain size , siltic SeeSee Grain size , arenic, and loamic SeeSee Grain size . These examples are illustrated by actual soil samples for which analyses of particle-size distributions have been performed. Clayic texture: soil from the Ligurian coast (northern Italy); siltic texture: soil from the Loess Plateau (China); loamic texture: soil from the northern Apennines (Italy); arenic texture: soil from the Central Sahara (Libya).

2.–6.:

After a grain-size distribution analysis of a primary material, the classes obtained have been selectively mixed together in order to reconstruct visual examples with suitable distributions of grain sizes. All views in XPL.

2.:

Only one size fraction of quartz sand is present, making the sorting perfect.

3.:

Only up to 10% of grain-size fractions other than the dominant quartz sand size fraction are present, resulting in a well sorted soil texture.

4.:

Fractions other than the dominant quartz sand size fraction represent 10–30% of the mineral grains, resulting in a moderately sorted soil texture in the thin section. At least two different fractions are obvious: fine and very coarse.

5.:

The well sorted quartz sand fraction is no longer the dominant one, resulting in poor sorting. No single size is clearly dominant.

6.:

A variety of quartz sand sizes is present in similar proportions. The sorting is very poor and the sediment is qualified as unsorted.

File 24: Shape of Grains: Equidimensionality

Equidimensionality refers to the way particle sizes are organized regarding the three perpendicular dimensions of space and how equal they are. However, particle shapes can only be described according to two dimensions in thin sections; therefore, the real three-dimensional morphology of a particle must be deduced or inferred very carefully, because it depends on the orientation of the object and its cross-cut plane in respect to the thin section surface.

Captions from upper left corner to lower right corner.

  1. 1.

    Equant quartz sand grains surrounded by thin layers of micrite and cemented by microsparite (Bizerte coast, Tunisia; XPL). The equant morphology is characterized by the same amplitude along the three axes.

  2. 2.

    Three-dimensional scanning electron microscope image of a single equant quartz sand grain of aeolian origin (Chobe Enclave, Botswana).

  3. 3.

    Planar mica grains dispersed in a yellowish grey micromass (northern Po Plain, Italy; PPL). In the planar morphology, one axis is much shorter than the two others, giving a flat shape to the object.

  4. 4.

    Three-dimensional scanning electron microscope image of a planar mica crystal of the same size as in 3. (Jura Mountains, Switzerland).

  5. 5.

    Prolate rootlet fragment inside a channel in a loamy soil (northern Apennines, Italy; XPL). Such morphologies are usually oblong, with one axis longer than the others.

  6. 6.

    Three-dimensional scanning electron microscope image of a prolate rootlet fragment in a coarse sandy soil (central Po Plain, Italy).

  7. 7.

    Acicular crystals of needle-fibre calcite SeeSeeCalcite , coating a channel in a silty soil (Loess Plateau, China; XPL). Like the calcite crystals shown in the photograph, the acicular shape refers to needle-like morphologies.

  8. 8.

    Three-dimensional scanning electron microscope image of acicular crystals of needle-fibre calcite (Jura Mountains, Switzerland).

File 25: Shape of Grains: Roundness and Sphericity

The roundness of a particle is determined by the sharpness of its edges and corners, independently of the shape of the particle itself. The sphericity of a particle is determined by its overall form, independently of the sharpness of its edges and corners. Both properties are commonly evaluated by means of visual estimation charts, even if today, image processing software can automatically generate parameters describing the roundness and sphericity of individual particles (see “File 77”).

Captions start with a visual estimation chart at the top. Then, they are listed from upper left corner to lower right corner.

  1. 1.

    Visual estimation chart of both roundness and sphericity, redrawn from Powers (1953). Six classes of roundness and two classes of sphericity are provided. In the original work of Powers (1953), the particles were modelled from clay to make possible the addition of details like shape, sphericity, and roundness of particles. Many other charts have been drawn since, but whatever the type, they all are based on the same approach as Powers (1953); they usually add some intermediate steps in the variability of parameters. The roundness classes of Powers (1953) are based on a specific ratio \(\rho = \frac {r}{R}\), where r is the radius of curvature of the largest inscribed circle and R is the radius of the smallest circumscribing circle. The ratio’s values range from the “very angular” to the “well rounded” classes, as follows: 0.12 to 0.17, 0.17 to 0.25, 0.25 to 0.35, 0.35 to 0.49, 0.49 to 0.70, and 0.70 to 1. Based on Wadell’s work (Wadell 1932), Krumbein (1941) proposed to estimate the sphericity by calculating \(\varPsi = \left (\frac {bc}{a^2}\right )^{1/3}\), where a, b, and c are the long, intermediate, and short axis dimensions (respectively) of the particle. Today, quantitative image processing automatically allows particles to be selected and shape parameters to be calculated (Heilbronner and Barrett 2014).

  2. 2.

    High sphericity, very angular : amphibole grain from an Andosol, tropical eastern Africa; PPL.

  3. 3.

    Low sphericity, angular: weathered feldspar grain in a dense dark micromass from an Andosol, tropical eastern Africa; XPL.

  4. 4.

    Low sphericity, subangular: micritic to microsparitic nodule from a Vertisol, northern Cameroon; XPL.

  5. 5.

    Low sphericity, sub-rounded : quartz grain in a carbonate micritic micromass of a Calcisol, Morocco; PPL.

  6. 6.

    Low sphericity, rounded: quartz grain associated with calcium carbonate pellets in a Calcisol, northern Tunisia; XPL.

  7. 7.

    High sphericity, well rounded: aeolian quartz grain observed in a Kastanozem, Botswana; XPL.

File 26: Basalt, Granite, and Gabbro

It is very common to observe large pieces of fragmented rocks (lithoclasts ) in soil thin sections. The recognition of their texture and nature is therefore important as they emphasize the role of soil parent material or can be the result of reworking of an observed soil horizon, indicating an allochtonous origin of the soil material. This section describes three common igneous rocks , i.e. basalt , granite, and gabbro .

Captions from upper left corner to lower right corner.

  1. 1.

    Basalts are dark volcanic igneous rocks with a microlitic texture, mainly composed of tiny crystals of plagioclases , forming the fine groundmass in the picture. Bright blue and orange crystals in XPL are phenocrysts of olivine minerals, and the whitish ones are calcic clinopyroxenes; Massif Central, France.

  2. 2.

    Vesicular basalt has the same texture and minerals as in 1., but with large voids due to the presence of gas. Such a microstructure makes this type of basalt more prone to weathering; Massif Central, France.

  3. 3.

    Basaltic pumice stone with large voids trapping air, separated by thin layers of basaltic groundmass with bubbles; Massif Central, France.

  4. 4.

    Volcanic ash deposited close to the eruption source. Large crystals are surrounded by weathered glass, an amorphous product constituting the micromass (extinct in XPL). The two large crystals are feldspar (in white in PPL) and amphibole (green in PPL and with birefringence colours typically up to middle second order in XPL); Massif Central, France.

  5. 5.

    Granite is a plutonic igneous rock with a granular texture. In this example from Forez, France, quartz is the dominant mineral (clear whitish crystals in PPL) associated with feldspars , such as microcline SeeSee Feldspar (microcline exhibits minute multiple twinning, which forms a cross-hatched pattern in XPL). Some micas are identified by their light-reddish orange or green colour in XPL.

  6. 6.

    Granite with large quartz grains (PPL and XPL); the crystals appearing clear and dusty in PPL, and grey in XPL, are feldspar . Brown (in PPL) and dark brown to reddish orange minerals are micas (biotite) ; Quebec, Canada.

  7. 7.

    Granite composed of quartz , orthoclase SeeSee Feldspar (a potassium feldspar), and plagioclases. Orthoclase appears as dusty crystals in PPL and grey in XPL. The bright yellow crystal is a mica. This granite underwent a degree of weathering evidenced by the fact that rare calcite crystals (dusty light-yellowish white in XPL) secondarily precipitated in some pores; Swiss Alps.

  8. 8.

    Gabbro is a common mafic igneous rock, generally greenish and dark-coloured. This sample from Ontario, Canada, contains many plagioclases (whitish in PPL and from white to grey and extinct in XPL) and minor amounts of olivine (light green in PPL and bright yellow in XPL).

File 27: Schist, Gneiss, and Amphibolite

Some large pieces of lithoclasts in soil thin sections originate from metamorphic rocks. The recognition of their texture and nature is therefore important as they emphasize the role of soil parent material or can be the result of reworking of an observed soil horizon, indicating an allochtonous origin of the soil material. This section describes four common metamorphic silicate rocks , i.e. schist , gneiss , amphibolite , and greenschist .

Captions from upper left corner to lower right corner.

  1. 1.

    Typical schist texture showing sheet-like grains with a preferred orientation, i.e. NW–SE. The clear areas in PPL are mainly composed by quartz grains, whereas the fine layers, dark in PPL and pink to yellow in XPL, are oriented micas, i.e. biotite and muscovite SeeSee Mica SeeSee Mica ; Italian Alps.

  2. 2.

    A mica-schist facies similar to 1. but with abundant thin layers of micas intertwined with thin quartz bands. Quartz is white, grey, and extinct in XPL, whereas mica appears in second order colours; Italian Alps.

  3. 3.

    A garnet-mica schist showing a garnet crystal in its centre (isotropic in XPL, i.e. black). The high-order coloured bands in XPL are comprised of muscovite mica with some brown crystals of biotite mica. The grey and white crystals in XPL are quartz with some feldspars; Italian Alps.

  4. 4.

    Example of a schist SeeSee Mica containing significant amounts of sericite , a fine-grained type of mica (high-order coloured small crystals in XPL). Remains of a plated structure of schistosity can be seen in the middle; Ontario, Canada.

  5. 5.

    An orthogneiss resembles a granite, as it results from the metamorphism of igneous rocks, with low amounts of mica (biotite in brown in XPL and muscovite with high-order coloured bands in XPL) and large crystals of quartz and feldspar (white, grey, and extinct in XPL); Swiss Alps.

  6. 6.

    A paragneiss designates a gneiss derived from a sedimentary rock. In this sample, micas are very abundant with bands of granoblastic interlocking crystals of quartz (white, grey, and extinct in XPL); Swiss Alps.

  7. 7.

    Metamorphic amphibolite formed by a dominant amphibole mineral, hornblende, associated with rare plagioclases . Hornblende is green in PPL and displays bright interference colours of second order in XPL. Such rocks result from the metamorphism of igneous rocks (basalts or gabbros ); Ontario, Canada.

  8. 8.

    Greenschist is a metamorphic rock usually produced by regional metamorphism. Its colour comes from the presence of large amounts of green minerals such as chlorite (green in PPL and second order interference colours) or epidote (small light green equant crystals in PPL). The schistosity is emphasized by the plate structure formed by the green minerals in PPL. Other minerals include feldspar and amphibole, as well as rare quartz; Ontario, Canada.

File 28: Quartzite and Marble

Some large lithoclasts in soil thin sections originate from metamorphic rocks. The recognition of their texture and nature is therefore important as they emphasize the role of soil parent material or can be the result of reworking of an observed soil horizon, indicating an allochtonous origin of the soil material. This section describes two common rocks that result from the metamorphism of former sedimentary rocks, i.e. quartzite and marble .

Captions from upper left corner to lower right corner.

  1. 1.

    When a sandstone undergoes metamorphism, it can be transformed into a quartzite rock (example from Ontario, Canada). This picture taken in PPL does not show any particular pleochroism , all of the thin section being composed by colourless quartz.

  2. 2.

    Same view as in 1. in XPL. All the quartz crystals display either colours from the first order or are extinct. The minerals form a granoblastic structure typical of quartzite, i.e. the crystals have very irregular boundaries, and they are interlocking and appear as a mosaic. This quartzite is composed of quartz phenocrysts .

  3. 3.

    Same type of rock as in 1. in PPL. This quartzite from Ontario, Canada, is also composed solely of quartz grains.

  4. 4.

    Same view as 3. in XPL. The granoblastic structure is still present, but the size of the average quartz grain is much smaller than in 2., although some large crystals can be observed at the top of the photograph.

  5. 5.

    Marble is a metamorphic rock composed of recrystallized carbonate rocks made up of calcite or dolomite . In PPL, it is possible to see only the border of the phenocryst of calcite, the mineral being colourless in PPL; Quebec, Canada.

  6. 6.

    Same view as in 5. in XPL. The phenocrysts of sparite clearly appear as interlocking crystals of calcite with bright twin bands visible in extinct minerals. Calcite usually displays high-order colours of birefringence; Quebec, Canada.

  7. 7.

    Fine-grained marble, a metamorphic rock composed of recrystallized calcite from a primary sedimentary mud. There are only a few crystals of sparite, the rock fabric being mainly composed of fine microsparite. PPL, Quebec, Canada.

  8. 8.

    Same view as in 7. in XPL. The multiple and various high-order colours of birefringence denote the presence of numerous small crystals oriented in various directions; these directions can be mapped using electron backscatter diffraction (EBSD; see “File 2”) SeeSee Electron backscatter diffraction . Some twinning zones can be observed in the larger crystals at the centre of the microphotograph; Quebec, Canada.

File 29: Calcium-Bearing Sedimentary Rocks

Calcium is the fifth most abundant element on Earth. It is a major compound of numerous sedimentary rocks. Deciphering its origin remains a fundamental issue in soil science, as calcium is a crucial element in many pedogenic processes and biogeochemical pathways (Rowley et al. 2018). Although calcium is incorporated in many structural formulae of silicates, its supply from calcium-bearing sedimentary rocks largely prevails in soil development.

Captions from upper left corner to lower right corner.

  1. 1.

    Fine-grained limestone composed of micrite . Such limestones formed as mud and became hardened during diagenesis. When extremely fine, calcite has a greyish brown colour in PPL and high-order colours of birefringence in XPL; Jura Mountains, Switzerland.

  2. 2.

    Oolitic limestone with two generations of cements: an isotropic and early diagenetic rim of small and pointy crystals and large sparitic crystals infilling voids due to late diagenetic precipitation. Such limestones are observed in agitated marine environments where oolites (spherical grains composed of concentric layers) form. PPL; Jura Mountains, Switzerland.

  3. 3.

    Silicified limestone from the Aquitaine Basin, France. The micritic limestone underwent a late diagenetic process during which two siliceous cements precipitated: small crystals of chalcedony coat the pores, and large chalcedony spherulites infill the voids. The second order colours of the siliceous crystals in XPL are due to the use of a gypsum plate , which shifts the birefringence of minerals by one wavelength.

  4. 4.

    Senonian chalk sample composed of micrite and round foraminifera microfossils (Globigerina sp.). PPL; Galilee, Israel.

  5. 5.

    Meteogene travertine , according to Pentecost (2010)’s definition, formed by stacked irregular shrubs precipitated by Schizothrix fasciculata (Freytet and Verrecchia 1998) with some internal dark laminations, which can be seen in PPL and XPL; Aquitaine Basin, France.

  6. 6.

    Marl is a sedimentary rock in which a calcium carbonate phase is mixed with some clays and quartz grains. In XPL, some quartz grains appear in white, grey, or are extinct (Negev Desert, Israel).

  7. 7.

    Gypsum is a common deposit of evaporitic environments from the past. Gypsum can form large beds or invade cracks in rocks. Swiss Alps.

  8. 8.

    Lamina of calcium phosphate interstratified in an organic-rich marly mudstone from California, USA. The mudstone is dark brown in PPL, whereas phosphates appear as light yellow to brownish micro-nodules coated by clays. The calcium phosphate phase, with low birefringence, appears extinct in XPL, and clay coatings are emphasized by their light rims in XPL.

File 30: Sand and Sandstone

Sands are usually composed by loose detrital particles of rock fragments between 0.05 and 2 mm. Commonly, quartz grains predominantly comprise sands at the surface of continents. During diagenesis, sands undergo hardening by cementation, forming sandstones . Cements can have various mineralogical compositions, from silica to calcium carbonate, from iron oxyhydroxides to sulphates.

Captions from upper left corner to lower right corner.

  1. 1.

    Loose grains of medium to coarse sand, mainly quartz in origin, from an aeolian deposit. Grains are separated by packing voids, resulting from the simple arrangement of the loose particles. Italian Alps.

  2. 2.

    Very fine fluvial sand from an alluvium of the Rhone River, Switzerland. Layers of different grain sizes, emphasized by the sizes of the packing voids , separate the succession of short sedimentary events of various intensities.

  3. 3.

    Pure fine sandstone composed solely of very well-sorted quartz grains. A thin siliceous cement keeps the quartz grains attached; Fontainebleau Forest, France.

  4. 4.

    Pure medium sandstone made of well sorted quartz grains with rare micas. As in 3., the sandstone is hardened by a siliceous cement. Swiss Alps.

  5. 5.

    A calcareous sandstone made of quartz grains cemented by very large monocrystals of calcite . The large crystalline nature of the cement is emphasized in the XPL view in which three single sparitic crystals can be identified by their polarising colours: pinkish, blueish, and extinct. Such cements have generally two possible cementation origins: (a) in a phreatic environment or (b) during late diagenesis after burial. Sample from a cone-in-cone structure (Aassoumi et al. 1992), Morocco.

  6. 6.

    A calcareous sandstone made of well rounded and sorted quartz grains, coated with micrite and cemented by microsparite . Bizerte coast, Tunisia.

  7. 7.

    Ferruginous sandstone with heterometric quartz grains cemented by iron oxyhydroxide , which are dark brown in PPL and seem extinct in XPL. Aquitaine, France.

  8. 8.

    Well sorted quartz grains cemented by malachite forming a green sandstone. Malachite is a copper carbonate easily identified by its green colour in PPL. Negev Desert, Israel.

File 31: Mineral Grains in the Soil I: Quartz and Chalcedony

Siliceous grains include quartz , its various polymorphs, and amorphous to cryptocrystalline siliceous minerals, for example chalcedony in chert. In volcanic material, siliceous grains can appear as cristobalite or tridymite.

Captions from upper left corner to lower right corner.

  1. 1.

    PPL view of individual and monocrystalline quartz grains in an arenic grain-size soil characterized by a chito-gefuric c/f relative distribution with packing voids . Quartz grains are white to light beige, sub-rounded with a low sphericity (see “File 25”). The fine material is made of clays associated with iron oxyhydroxides. Paleosol, Libyan Sahara.

  2. 2.

    XPL view of the same field as in 1. Quartz grains appear in white, grey, or extinct (black), with weak birefringence to first order greys (see “File 8”). Grains are rimmed by a brownish to orange coating made of clays associated with iron oxyhydroxides.

  3. 3.

    Rock fragment composed of multiple quartz grains from a metamorphic rock (see “File 7”). The quartz grains appear colourless and non-pleochroic , i.e. they do not show any variation in colour when rotating the circular sample stage. Crystals have various shapes, without any clear cleavage, and display a very low relief. The fragment floats in a silty–clayey groundmass. Cambisol, Jura Mountains, Switzerland.

  4. 4.

    Same view as in 3. in XPL. Quartz in the fragment displays various first order greys, from white to black (extinct). However, the extinction of some individual quartz crystals are not uniform (shadowy), a common feature of quartz in deformed rocks. A closer view reveals that some quartz grains have lobate and interfingering boundaries, and others show dark and light veinlets penetrating into the crystal, emphasizing twin lamellae.

  5. 5.

    Aggregate of clear quartz grains with small and dark inclusions (PPL view). Jura Mountains, Switzerland.

  6. 6.

    Same view as in 5. in XPL. The centre of the quartz grains displays typical features such as large open cracks, embayments, and what were probably melt inclusions that were later infilled with weathering products, such as clays and amorphous silica (likely chalcedony). Such quartz is probably of volcanic origin.

  7. 7.

    View in PPL of a colourless flint flake in a hockey stick shape embedded in a loamy groundmass. Jura Mountains, Switzerland.

  8. 8.

    Same view as in 7. in XPL. Chalcedony is characterized by very low grey to whitish colours. The shaft of the “hockey stick” is made of length-slow chalcedony, whereas the blade shows spherulites of length-fast and zebraic chalcedony.

File 32: Mineral Grains in the Soil II: Feldspar and Mica

Feldspars are tectosilicates commonly found in igneous and metamorphic rocks (see “File 26”). They are characterized by two mineral families: the K-feldspars and the plagioclases SeeSee Feldspar , which form a continuous solid solution with various amounts of sodium and calcium, from albite (a sodium feldspar) to anorthite (a calcium feldspar). During weathering, calcium, sodium, and potassium ions are freed in the soil solution, and feldspar partially transforms into clays and∕or oxyhydroxides . Micas are phyllosilicates frequently associated with granite and granodiorite but also with metamorphic rocks, such as schist and gneiss (see “File 26” and “File 27”). Muscovite is frequently observed as grains in soils, whereas biotite is rarer, as it is easily weathered and transformed into clay. This difference between the two micas is related to their structural chemical formula, the aluminium in muscovite being much more refractory to weathering than the magnesium and iron in biotite.

Captions from upper left corner to lower right corner.

  1. 1.

    Translucent grains in a fine brownish groundmass. It is difficult to clearly identify the mineralogical nature of the grain, although some faint parallel lines can be made out on the right hand side of the central grain. PPL view, Vertisol, Cameroon.

  2. 2.

    Same view as in 1. in XPL. The central grain is a plagioclase characterized by its polysynthetic twins of alternating dark and light lamellae. The two other grains are quartz. The groundmass is formed of clay minerals, fine quartz grains, and a small flake of light blue muscovite.

  3. 3.

    Large translucent grain with thin parallel and alternating white and extremely pale pink lamellae. PPL view, Fluvisol, Cameroon.

  4. 4.

    Same view as in 3. in XPL. Crystal of microcline showing its characteristic twins closely interwoven forming a typical cross-hatched (“tartan”) pattern.

  5. 5.

    It is extremely difficult to find a fresh crystal of biotite in soils, due to their fast weathering. This brownish platelet preserves some characteristics of biotite (see “File 26”). PPL view, Fluvisol, Cameroon.

  6. 6.

    Same view as in 5. in XPL. The crystal of biotite is surrounded by quartz (from white to grey, or extinct), clays, and oxyhydroxides and still displays a greenish moiré colour due to exfoliation combined with formation of secondary minerals (probably vermiculite).

  7. 7.

    Translucent crystal of muscovite formed by visible thin sheets with a relatively high relief, showing cleavage . PPL view, Cambisol, Switzerland.

  8. 8.

    Same view as in 7. in XPL. The crystal appears in blue and pink bright birefringence colours typically up to middle third order. The alternating bright blue and pink colours underline the sheet structure of such crystals.

File 33: Mineral Grains in the Soil III: Inosilicates and Nesosilicates

Inosilicates are formed of interlocking chains of silicate tetrahedra and include two main groups of minerals: pyroxenes , as single chain silicates, and amphiboles , as double chain silicates. They are commonly igneous rock-forming minerals but can also be associated with high-temperature metamorphic rocks. In nesosilicates , the silicate tetrahedra are isolated and bound to each other by ionic bounds, making their structure particularly dense, providing them some resistance to weathering. Olivines (minerals found in high-temperature igneous rocks) and garnets (minerals prevalent in metamorphic rocks) form two common members of nesosilicates. Both of these rock-forming silicate groups, inosilicates and nesosilicates, can be observed in soils as residual mineral grains in the coarse fraction.

Captions from upper left corner to lower right corner.

  1. 1.

    Isolated grain of pyroxene, pale coloured with a subtle greenish pleochroism. The crystal is characterized by a high relief with two sets of cleavages at 90. The pyroxene grain is surrounded by a brown, cloudy, and undifferentiated micromass. PPL view, Andosol, Central Africa.

  2. 2.

    Same view as in 1. in XPL. The colour of the crystal changes to light brown with the cleavage remaining perfectly visible. The micromass is totally extinct and corresponds to cryptocrystalline to amorphous phases .

  3. 3.

    Fragments of amphibole crystals showing brownish to pale yellowish green pleochroic colours in PPL. The shape of the pointy and half-diamond crystal (bottom part of the micrograph) is due to the 60 and 120 cleavage angles of amphiboles, emphasizing potential weakness of the crystal lattice during its weathering in soils. Andosol, Central Africa.

  4. 4.

    Same view as in 3. in XPL. Amphibole crystals show interference colours in the upper first or lower second order. The single orientation of cleavage in the upper crystal is due to its orientation. Other crystals in dark and light grey are feldspars.

  5. 5.

    Amphiboles sometimes display second order interference colours, here bright yellow to pink with shades of blue. Andosol, Central Africa.

  6. 6.

    Olivine with weathering products in cracks (light brown in PPL) formed by iddingsite . Olivine, usually colourless in PPL, displays typical second order interference colours in XPL (here bright blue). Andosol, Massif Central, France.

  7. 7.

    Garnets are colourless minerals with a high to extreme relief, forming equant crystals, without any observable cleavage. PPL view, Cryosol, Lombardy (Italy).

  8. 8.

    Same view as in 7. in XPL. As an optically isotropic mineral, garnet remains extinct in XPL. The micromass is formed of small quartz and muscovite crystals due to the weathering of metamorphic pebbles.

File 34: Mineral Grains in the Soil IV: Carbonates

Carbonate minerals are common features of soils. They can be either inherited from the carbonate parent material of the soil (see “File 29” for some examples) or precipitated as secondary pedogenic features (see e.g. “File 72”). In this section, carbonate grains found in soils are inherited from the bedrock. These elements in the soil coarse fraction constitute lithoclasts and not pedofeatures . Carbonate lithoclasts in soils are usually inherited from three main types of rocks: limestones , marls , and dolomites .

Captions from upper left corner to lower right corner.

  1. 1.

    Two large limestone lithoclasts integrated inside a soil rich in detrital carbonate grains. These lithoclasts are formed by inherited fragments of micritic (brown fragment) and microsparitic limestones. The brownish micromass is a mixture of micrite, clays, and very small quartz grains. PPL view, Calcaric Cambisol, Jura Mountains, Switzerland.

  2. 2.

    Same view as in 1. in XPL. The microsparitic nature of the bottommost fragment is emphasized in XPL by the high-order interference colours of calcite. Grey-coloured grains are quartz. Calcaric Cambisol, Jura Mountains, Switzerland.

  3. 3.

    Inherited sparitic detrital grain in the same type of soil as in microphotograph 1. Sparitic crystals are colourless with a low relief. PPL view, Calcaric Cambisol, Jura Mountains, Switzerland.

  4. 4.

    Same view as in 3. in XPL. The sparitic nature of this fragment is emphasized in XPL by the high-order interference colours of calcite. This large lithoclast is surrounded by small calcite and quartz grains in a brownish micromass. Calcaric Cambisol, Jura Mountains, Switzerland.

  5. 5.

    Fragments of an inherited and fossiliferous limestone with detrital grains of Bryozoans (e.g. top) and shells (bottom; probably an oyster shell) in a calcareous micromass. PPL view, Calcaric Cambisol, Jura Mountains, Switzerland.

  6. 6.

    Same view as in 5. in XPL. The calcium carbonate and partially fibrous nature of the bioclasts (organism fragments) are indicated in XPL by the high-order interference colours, as well as the interwoven and rolling extinctions. Calcaric Cambisol, Jura Mountains, Switzerland.

  7. 7.

    Inherited dolomitic crystals in a red clayey and iron oxyhydroxide-rich micromass. PPL view, Paleosol, Liguria, Italy.

  8. 8.

    Same view as in 5. in XPL. Dolomite crystals display their greyish interference colours although these colours are partially altered by the presence of red clayey and iron oxyhydroxide-rich thin coatings. Paleosol, Liguria, Italy.

File 35: Mineral Grains in the Soil V: Chlorides and Sulphates

Although many inherited chlorides and sulphates could have been easily dissolved during weathering, some soils can preserve their clasts or imprints. They must not be interpreted in terms of pedogenic processes when they are inherited from the bedrock. It is why their recognition as lithoclasts is paramount. This plate shows examples of a chloride (halite) and three different types of sulphate: barite , anhydrite , and gypsum . In this plate, all these minerals are inherited from the bedrock.

Captions from upper left corner to lower right corner.

  1. 1.

    Crystals of halite in a carbonate and clayey micromass. They are recognizable by their square or rectangular shape due to their isometric crystallographic system (cubic form). As such, they appear white in PPL with a very low relief. Dead Sea, Israel.

  2. 2.

    Same view as in 1. in XPL. Halite is isotropic, i.e. the crystal does not display any interference colour and stays extinct in XPL. Dead Sea, Israel.

  3. 3.

    Colourless crystals of barite (a barium sulphate), showing a high relief, and associated with calcium carbonate (micritic) lithoclasts in a large pore of a palustrine limestone; PPL view, Chobe Enclave, Botswana.

  4. 4.

    Same view as in 3. in XPL. Crystals of barite are fibrous to columnar, up to first order pale yellow. Palustrine limestone in potential hydrothermal settings. Chobe Enclave, Botswana.

  5. 5.

    Lithoclasts of calcium sulphate anhydrite in a loessic groundmass. These mineral fragments are directly inherited from the bedrock and have been incorporated in the upper soil layer by bioturbation . These granular clusters of anhydrite remain colourless in PPL. Soils developed on Triassic gypsic sedimentary rocks. Saint-Léonard, Valais, Switzerland.

  6. 6.

    Same view as in 5. in XPL. The two large clusters of anhydrite display bright second and third order interference colours, mostly purple, red, and yellow, but green is also common. In the lower lithoclast, some first order grey and white interference colours indicate the presence of gypsum crystals (see below).

  7. 7.

    Lithoclast of colourless gypsum crystals in a clayey and carbonate micromass. The low-relief crystals form various habits, from elongated to granular shapes. PPL view, soils developed on Triassic gypsic sedimentary rocks. PPL view, Saint-Léonard, Valais, Switzerland.

  8. 8.

    Same view as in 7. in XPL. Gypsum is characterized by typical first order grey and white interference colours. The lenticular to acicular shapes of crystals are emphasized in the upper part of the cluster, whereas the lower part displays a saccharoid (granular) habit.

File 36: Biominerals I

Stoops (2003, 2021) defines biominerals as inorganic residues of biological origin. They include fragments of internal or external skeletons of animals, as well as direct or indirect mineral products of organism metabolism—for more details, see Mann (2001) and Skinner and Ehrlich (2017). Phytoliths are produced in specific plant cells, in intercellular spaces, or associated with cell walls Stoops (2003). Calcium oxalate SeeSee Oxalate druses are aggregations of crystals with a roughly overall spherical shape (Baran and Monje 2008), and they also originate from plants. Other forms of oxalate crystals exist, such as equant or needle-shaped morphologies, but both can be associated with plants and fungal filaments (Verrecchia et al. 1993). These minerals indicate the presence of organisms, even if the organic matter has been totally decayed.

Captions from upper left corner to lower right corner.

  1. 1.

    Clusters of opal phytoliths (transparent and elongated bodies) associated with plant residues (in brown). These phytoliths have been disarticulated and reworked during pedoturbation. PPL view, Libyan Sahara, archaeological deposit.

  2. 2.

    Assemblage of elongated opal phytoliths, transparent in PPL and with a low birefringence to extinct in XPL. The arrangement of the crystals precipitated inside the plant tissues is well preserved. Libyan Sahara, archaeological deposit.

  3. 3.

    Calcite spherulites associated with some oxalate druses in a dung deposit. In XPL, spherulites are characterized by a black cross, emphasizing their fibro-radial nature. Druses have a more radial structure with irregularly shaped borders. In PPL, these biominerals are difficult to observe.

  4. 4.

    Assemblage of essentially oxalate druse crystals with a radial structure and irregularly shaped borders in XPL. Same source as in 3.

  5. 5.

    Plant fragment in a dung deposit from the Sahara Desert. Part of the plant organic matter is decomposing, i.e. it appears as a dark colour. In the central part of the microphotograph, some transparent crystals are clustered, but difficult to identify. PPL view.

  6. 6.

    Same view as in 5. in XPL. Note the presence of equant crystals of calcium oxalate inside the remains of plant tissues.

  7. 7.

    Monoclinic oxalate crystals formed in specific cells of a tropical wood (iroko tree, Millica excelsa). Cellulose appears in light yellow and corresponds to the cambium. PPL view, Ivory Coast.

  8. 8.

    Slightly decayed iroko tree fragment, including oxalate crystals. In XPL, cellulose is characterized by a very high birefringence, whereas oxalate crystals display second order colours.

File 37: Biominerals II

Stoops (2003, 2021) defines biominerals as inorganic residues of biological origin. They include fragments of internal or external skeletons of animals, as well as direct or indirect mineral products of organism metabolism—for more details, see Mann (2001) and Skinner and Ehrlich (2017). This section mainly refers to three types of spherulites: (1) calcitic faecal spherulites result from animal digestive processes; they are more abundant in herbivore dung (Durand et al. 2018); (2) another type of calcitic and radial spherulites are found in desert laminar crusts SeeSee Laminar crust , likely as a by-product of photosynthetic activity by cyanobacteria (Verrecchia et al. 1995); and (3) in bird and reptile droppings, it is possible to find spherulites, composed of uric acid (Canti 1998) SeeSee Spherulite . These minerals indicate the presence of former biological activity, even if organic matter is not directly observed.

Captions from upper left corner to lower right corner.

1.–2.:

Faecal calcitic spherulites dispersed in a groundmass of dung in an archaelogical deposit, Libyan Sahara. In PPL (1), the crystals are almost invisible, whereas in XPL (2), they form an extinction cross due to their radial structure.

3.–4.:

Faecal calcitic spherulites, densely clustered, in archaeological dung, Libyan Sahara. Note the presence of a quartz grain at the bottom centre of the microphotograph.

5.–6.:

Uric acid spherulites dispersed in an organic groundmass found on a cave wall bed, Libya, Central Sahara. In PPL (left), they are not visible in the phosphatic groundmass. In XPL, they show a black extinction cross or a pseudo-isogyre resembling a biaxial interference figure (Canti 1998), visible in the centre of the microphotograph.

7.–8.:

Two examples of calcitic spherulites observed in laminar crust thin sections. Left: dense cluster of radial spherulites in a Pleistocene laminar crust from the Shephelah, Israel. Right: a thick layer of spherulites with a perfectly round fibro-radial crystal in the centre. Pleistocene laminar crust, southern Morocco. For their genesis, see details in Verrecchia et al. (1995).

File 38: Biominerals III

Stoops (2003) defines biominerals as inorganic residues of biological origin. They include fragments of internal or external skeletons of animals, as well as direct or indirect mineral products of organism metabolism—for more details, see Mann (2001) and Skinner and Ehrlich (2017). Mollusc shells are genetically induced biominerals (Mann 2001). The identification of the molluscs as originating from marine or continental environments is a clue to the nature of the soil parent material. Egg shells and bones also belong to genetically induced biominerals (Mann 2001): both of them are common features of archaeological sites (Durand et al. 2018). Finally, some clear and small crystalline aggregates are often found in temperate soils. In thin section, they appear as calcitic spheroids . They are usually formed by earthworms as metabolic by-products in their casts (Becze-Deak et al. 1997; Durand et al. 2018).

Captions from upper left corner to lower right corner.

1.:

Mollusc shell fragment showing a cross-section of the two layers composing the structure: the prismatic layer at the top and the lamellar layer at the bottom. Shell fragments can be made of either aragonite or calcite . Physical weathering of the prismatic layer can lead to the formation of calcite needle-fibres in soils (Villagran and Poch 2014). Archaeological site, Syria.

2.:

Cross-section of a shell with well-preserved chambers inside a micritic groundmass. This example is a freshwater snail (probably from the Planorbidae family). Holocene carbonate swamp deposits, Syria.

3.–4.:

Fragments of bird egg shells : the shell is formed by columnar calcite crystals (the “palisade layer”) , oriented perpendicular to the surface of the egg, often with a narrow fan-like fabric.

5.–6.:

Fragment of a bone showing low interference colours (first order grey). This colour can be due to the remains of collagen, and not only apatite , which forms the mineral part of bones. Haversian canals (as channels) are frequently preserved (in 6. for example) and surrounded by fibres of apatite. Bone fragments are generally coloured from pale yellow to yellow or yellowish brown in PPL.

7.–8.:

Two examples of earthworm biospheroids from Cambisols, Switzerland. Calcitic biospheroids have mostly ellipsoidal shapes with a sharp and smoothly undulating or rough boundary, depending on the earthworm species. Their internal fabric resembles geodes, composed of drusic centripetal calcite crystals, the core being occupied by smaller crystals. Their density in thin section can vary greatly from one or two, to sometimes more than twenty.

File 39: Anthropogenic Features I

Many types of features in soils can be directly related to anthropogenic activities. In archaeological sites, their presence is obvious, but they can also be encountered in “natural” soils, providing evidence of the incidental presence of humankind (Nicosia and Stoops 2017). Pottery fragments are commonly found in archaeological sites. Chunks of brick can be present in more recent anthropogenic soils . Moreover, not only can artefacts be found in soils, but also secondary chemical deposits, such as amorphous phosphate or vivianite crystals . These deposits are frequently associated to reducing environments enriched in organic matter as, for instance, cesspits and latrines.

Captions from upper left corner to lower right corner.

1.–2.:

Ceramic shards from a Bronze Age archaeological site, northern Italy. It is mainly composed of a fired-clay matrix with some inclusion of coarse material and voids. The nature and content of compounds have varied over time and space and conditions of firing, but common traits indicate a heating effect on the clays, which can change the colour, the amount, and shape of planar voids. In 1., these changes can be seen as two yellow external layers bordering a brown central part. In 2., this change in colour is reversed.

3.:

Fragment of a raw (i.e. not fired) mud brick from a Bronze Age site, central Syria. It is composed of a clay matrix with some inclusions of coarse mineral grains and plant residues.

4.:

Fragment of a fired mud brick from a Medieval archaeological site from northern Italy. It is composed of a clay matrix with inclusions of coarse mineral grains with planar voids.

5.:

Fibro-radial cluster of vivianite crystals in a loamy groundmass. Same site as in 4. Its blue colour in PPL is diagnostic of this mineral. In XPL, vivianite displays a birefringence in the first order colours with a well-expressed pleochroism.

6.:

Cluster of vivianite crystals in elongated shapes of various sizes from a Medieval archaeological site from northern Italy. The blue colour of the crystal in PPL is diagnostic of this mineral.

7.:

A mix of amorphous phosphate and vivianite (blue in PPL) in a phosphatized clay and silty groundmass, from a Medieval archaeological site from northern Italy. In XPL, the b-fabric is undifferentiated and related to the impregnation of phosphate into the groundmass.

8.:

Same as in 5, except for the absence of vivianite and the coarser groundmass.

File 40: Anthropogenic Features II

Many types of features in soils can be directly related to anthropogenic activities (Nicosia and Stoops 2017). At archaeological sites, their presence is obvious, but they can also be encountered in “natural” soils. Indeed, a fire can be triggered by nature (e.g. summer forest fires or lightning strikes) or by humans. In this section, the features shown are related to anthropogenic activities induced by fire, i.e. charcoal , ashes , heated bones , and heated rock fragments .

Captions from upper left corner to lower right corner.

1.–2.:

Fragments of wood charcoals, opaque in PPL (as well as in XPL, not shown) with very fine (in 1.) and coarse (in 2.) porosity patterns, respectively. These patterns can be used by botanists to identify the type of wood. Moreover, the microstructure is fine granular and related to biological activity. Paleosol, northern Apennines, Italy.

3.–4.:

Ash deposits from an archaeological site in the central Sahara, Libya. Ashes, grey in PPL, are constituted by microcrystalline aggregates of calcium carbonate , clearly observed in XPL. The groundmasses of these examples also include numerous coarse grains of quartz and some elongated charred plant remains (in brown or black).

5.–6.:

Heated bone fragments found in a firepit with burned pebbles from a Neolithic site, northern Italy. In 5., the bone colour is dull orange-reddish brown in PPL and bright orange-reddish in XPL. This colour could correspond to a heating temperature of 400 C (Villagran et al. 2017). In 6., the heated zone is limited to the upper left side where the colour is brownish, while the bone displays a light to medium yellow colour in PPL, corresponding to a lower heating temperature (Villagran et al. 2017).

7.–8.:

Heated flint/chert SeeSee Chert shards found in a firepit with burned pebbles from a Neolithic site, northern Italy. They exhibit changing colours due to oxidation. In addition, these shards are surrounded by a clayey micromass .

File 41: Organic Matter I

Organic matter is a common feature of soils worldwide. Soil organic matter includes remains of all plant parts, as well as small animals (insects, arthropods, etc.), fungi , and bacteria. Organic horizons are characterized by their dark colour caused by the melanization (darkening) SeeSee Melanisation of the soil organic matter as well as the formation of specific organic molecules. Plants are mainly composed of lignin and cellulose, giving some organic material a birefringence belonging to the first order colours in XPL. In this section, fungal features are described as well as plant material from leaves to roots .

Captions from upper left corner to lower right corner.

1.:

Fungal filaments covered by oxalate crystals in a Calcisol, Nazareth, Israel. The groundmass is composed of micrite . Such filaments are covered by weddellite, a hydrated calcium oxalate (at high magnification, the shape, size, and the way the crystals are attached to the filament are good clues to identify such oxalates), which is frequently confused with calcium carbonate. Therefore, the terms “calcified filaments” should be avoided, as they refer to calcium carbonate and not oxalate (Bindschedler et al. 2016). XPL view.

2.:

Fungal sclerotium SeeSee Fungi in an A horizon from a podzol (L’Isle-Adam forest, France). Sclerotia are brown objects with sharp limits and composed of multiple and agglomerated small rounded cells. The border is generally thick and can be broken, opening the sclerotium. The groundmass is composed of sandy quartz grains arranged in a chitonic c/f related distribution . PPL view.

3.:

Cross-sections of leaves in a humus sample (O horizon from a Brunisol, Switzerland). These organ fragments are composed of various thin elongated and brown tissue types, i.e. the cuticle and the mesophyll. PPL view.

4.:

Cross-section of stems in an Ah horizon from a calcaric Leptosol (Jura Mountains, Switzerland). The dark brown border corresponds to the epidermis and the sclerenchyma of a dicot plant. The groundmass is made of large proportions of brown gel-like organic material. PPL view.

5.–6.:

Longitudinal sections of roots in a loamy Leptosol, northern Apennines, Italy. Only the external part (epidermis) is preserved in 5., whereas the epidermis and some cortex cells are visible in 6. Some decaying organic material is partly melanized (dark to black parts), and root disappearance after total decomposition can form channel voids . PPL view.

7.–8.:

Cross-sections of roots in a loamy Leptosol, northern Apennines, Italy. The internal structure of the roots is exceptionally well preserved, as it is possible to see the epidermis, the cortex cells, the pericycle, as well as the endodermis. PPL view.

File 42: Organic Matter II

Roots are the most common organic residues observed in soils. Roots not only appear differently depending on the angle at which they were cut (see “File 20”), but they can also be differentiated according to their preservation or decay. Three examples of similar root sections are given for three different ways of decaying and preservation. In addition, it is possible to find a biogenic product, starch , preserved in soils at certain locations, e.g. in arid zone soils.

Captions from upper left corner to lower right corner.

1.:

Longitudinal section of a root epidermis showing the network of its cells in a brown colour due to the presence of lignin. PPL view.

2.:

Same view as in 1. in XPL. The organic material has a low birefringence. The soil groundmass is scarcely enriched in calcium carbonate.

3.:

Longitudinal section of a root epidermis after total darkening. The organic matter appears in dark black (melanization) but preserves the structure of the root tissue. A loess-like deposit constitutes the groundmass. Loess Plateau, Baoji, north central China.

4.:

Same view as in 3. in XPL. The root mass appears completely extinct.

5.:

Example of a root in which cell vacuoles have been replaced by calcite SeeSee Calcite . These calcified root cells (Jaillard et al. 1991) are common features of soils and paleosols. Here, the preserved cell morphologies correspond to epidermal tissues (Jaillard et al. 1991). Calcified root cells form when the vacuole is infilled by a calcite crystal, which replaces the entire cell, fossilizing the root tissue. A loess-like deposit constitutes the groundmass. Loess Plateau, North Central China.

6.:

Same as in 5. in XPL. The polarizing colours correspond to calcite.

7.–8.:

Dung deposit from the central Sahara (Libya), in which organic compounds are extremely well preserved, including storage tissue cells. In these cells, starch is present as spheres that are perfectly visible in PPL and XPL. In XPL, the spheres show a black extinction cross but in an abnormal greyish birefringence colour.

File 43: Humus

Humus forms result from decomposed organic matter lying at the surface of the soil, or present in the uppermost 30 cm. It mainly consists of combinations of Oi, Oe, Oa, and A horizons (IUSS and Working-Group-WRB 2014). The state of preservation and decomposition of plant tissues allows the type of humus form to be recognized. Humus forms are also often associated with earthworm casts and faecal pellets of small animals. A progression from litter to A horizon is presented in this section made from thin sections from a folic Umbrisol and a calcaric Leptosol, Côte de Ballens, Jura Mountains, Switzerland.

Captions from upper left corner to lower right corner. All views are in PPL.

1.–2.:

Thin sections of an Oi horizon showing cross-sections of leaves, twigs, and tissue fragments of a branch or bark. The organic material is not, or only very slightly, decomposed and made of large plant parts at this scale.

3.:

Thin section of an Oe horizon, where organic materials are fragmented by soil mesofauna. Some larger tissues or organs are still recognizable, but most of the objects are cut into small pieces and mainly represent the organic layers most resistant to decay. The comparison with 1. and 2. at the same scale emphasizes the decreasing size of the organic material.

4.:

Thin section of an Oe horizon with the same general characteristics as in 3. In the centre, cross-section of a stem. Some small aggregates of unrecognizable organic matter can be seen on the left and right edges of the microphotograph.

5.:

Thin section of an Oa horizon in which most of the organic material can no longer be recognized. Humification processes are at work. Only a few resistant or newly incorporated pieces of plant tissues still appear as continuous shapes.

6.:

Thin section of an Oa horizon with the same general characteristics as in 5. The only difference is the presence of small black pieces of charcoal , and close to the right edge, a triangular preserved tissue.

7.:

Thin section of an A horizon showing organomineral aggregates, forming crumb peds. The differences in the darkness are related to the degree of incorporation of humified material. The largest crumb includes some moderately decayed organic matter.

8.:

Detail of a crumb aggregate made by an earthworm in an A horizon. The groundmass is composed by a micromass of humified organic matter (as granular micro-aggregates) with the presence of some moderately decomposed plant tissues. A fungal sclerotium can be seen at the bottom and centre-left part of the picture.

File 44: Micromass

According to Bullock et al. (1985), micromass is a general term used to denote the finest material of the groundmass . Stoops (2003, 2021) describes the micromass as being characterized by the presence of crystalline or amorphous clay minerals , associated with oxyhydroxides or not, amorphous organic matter , and possibly the presence of small crystals of calcite (i.e. micrite) or mica . Micromass can be described using its colour, its transparency (i.e. its limpidity), and the interference colours (i.e. its b-fabric ). Limpidity ranges from limpid to opaque, with intermediate states such as cloudy, speckled, and dotted. This section presents examples or micromass colours and limpidity, which have to be observed in PPL. In addition, the next two sections show examples of b-fabrics observed in XPL.

Captions from upper left corner to lower right corner.

  1. 1.

    Red micromass: the red colour is due to the presence of hematite mixed with clays . Chromic Luvisol, Liguria, Italy.

  2. 2.

    Yellow micromass: the bright yellow colour is due to the presence of goethite mixed with clay minerals. Lixisol, tropical Africa.

  3. 3.

    Very dark brown micromass: the brown colour is due to the presence of iron and organic matter forming the clay-humic complex. Luvisol, northern Apennines, Italy.

  4. 4.

    Brownish grey and cloudy micromass: the pale colour corresponds to a calcite-rich (micrite) micromass. Calcaric Leptosol, Jura Mountains, Switzerland.

  5. 5.

    Black and opaque micromass from a peat soil developed in central Syria. Melanized organic matter gives the black colour.

  6. 6.

    Greyish and limpid micromass: the reduction processes removed all oxidized iron, giving the micromass this light and limpid aspect. Gleysol formed on glacio-lacustrine clays, Jura Mountains, Switzerland.

  7. 7.

    Yellow and cloudy micromass: the bright yellow colour is due to the presence of goethite . Ferralsol from tropical Africa.

  8. 8.

    Brownish and speckled micromass: the micromass is not uniform, compared to the other examples. Cambisol, Apulia, Italy.

File 45: B-Fabric I

According to Bullock et al. (1985), micromass is a general term used to indicate the finest material of the groundmass . Stoops (2003, 2021) describes the micromass as being characterized by the presence of crystalline or amorphous clay minerals , associated with oxyhydroxides or not, amorphous organic matter , and possibly the presence of small crystals of calcite (i.e. micrite) or mica . Micromass can be described using its colour, its transparency (i.e. its limpidity), and the interference colours (i.e. its b-fabric ). The b-fabric describes “the origin and patterns of orientation and distribution of interference colours in the micromass” (Bullock et al. 1985). This section shows examples of thin sections observed only in XPL.

Captions from upper left corner to lower right corner.

  1. 1.

    Undifferentiated b-fabric induced by the dominance of short-range order minerals, e.g. allophanes , in a volcanic soil (Andosol). The two largest minerals are olivine and amphibole crystals .

  2. 2.

    Undifferentiated b-fabric induced by a mass of fine crystals of oxyhydroxide. Paleosol (paleo-Oxisol), central Sahara, Libya.

  3. 3.

    Crystallitic b-fabric made of small birefringent calcite (micrite ). Calcaric Leptosol, Jura Mountains, Switzerland.

  4. 4.

    Crystallitic b-fabric composed of large amounts of very fine mica crystals. Soil developed on loess, northern Italy.

  5. 5.

    Mosaic-speckled b-fabric with randomly arranged clusters of oriented clays resulting in a mosaic-like pattern. Paleosol (paleo-Luvisol), central Sahara, Libya.

  6. 6.

    Stipple-speckled b-fabric with randomly arranged but isolated clusters of oriented clays . Chromic Luvisol, tropical Africa.

  7. 7.

    Unistrial b-fabric formed by a parallel oriented micromass and displaying a typical imprint of a sedimentary pattern. Fluvisol, Rhône Valley, Switzerland.

  8. 8.

    Bistrial b-fabric formed by two preferred directions of orientation in the micromass. It is typical of a sedimentary pattern. Fluvisol, Rhône Valley, Switzerland.

File 46: B-Fabric II

According to Bullock et al. (1985), micromass is a general term used to indicate the finest material of the groundmass . Stoops (2003, 2021) describes the micromass as being characterized by the presence of crystalline or amorphous clay minerals , associated with oxyhydroxides or not, amorphous organic matter , and possibly the presence of small crystals of calcite (i.e. micrite) or mica . Micromass can be described using its colour, its transparency (i.e. its limpidity), and the interference colours (i.e. its b-fabric ). The b-fabric describes “the origin and patterns of orientation and distribution of interference colours in the micromass” (Bullock et al. 1985). This section shows examples of thin sections observed only in XPL, displaying the presence of elongated areas in which clays are approximately simultaneously extinct.

Captions from upper left corner to lower right corner.

  1. 1.

    Porostriated b-fabric in which a clayey assemblage is distributed parallel to the surface of pores. Chromic Paleo-Luvisol, Sardinia, Italy.

  2. 2.

    Granostriated b-fabric in which a clayey assemblage is distributed parallel to the surface of quartz grains. Chromic Paleo-Luvisol, Sardinia, Italy.

  3. 3.

    Parallel striated b-fabric showing a parallel arrangement of clayey assemblages. Chromic Paleo-Luvisol, Sardinia, Italy.

  4. 4.

    Monostriated b-fabric showing an isolated and independent alignment of clays in the micromass. Chromic Paleo-Luvisol, northern Italy.

  5. 5.

    Cross-striated b-fabric showing the intersections of multiple sets of clay alignments. Chromic Luvisol, Liguria, Italy.

  6. 6.

    Random striated b-fabric does not show any organization in the arrangement of clay alignments, giving an irregular pattern. Chromic Paleo-Luvisol, Sardinia, Italy.

  7. 7.

    Concentric striated b-fabric displaying multiple concentric rings of oriented clay material. Paleosol, Libya, central Sahara.

  8. 8.

    Circular striated b-fabric in which a clayey assemblage is distributed in a circular arrangement, for instance in a ring shape. Luvisol, Apulia, Italy.