Zincobotryogen, ZnFe3+(SO4)2(OH)⋅7H2O: validation as a mineral species and new data

Zincobotryogen occurs in the oxidation zone of the Xitieshan lead-zinc deposit, Qinghai, China. The mineral is associated with jarosite, copiapite, zincocopiapite, and quartz. The mineral forms prismatic crystals, 0.05 to 2 mm in size. It is optically positive (2Vcalc = 54.1°), with Z ‖ b and X ∧ c = 10°. The elongation is negative. The refractive indices are nα = 1.542(5), nβ = 1.551(5), nγ = 1.587(5). The pleochroism scheme is X = colorless, Y = light yellow, Z = yellow. Microprobe analysis gave (in wt%): SO3 = 38.04, Al2O3 = 0.04, Fe2O3 = 18.46, ZnO = 13.75, MgO = 1.52, MnO = 1.23, H2O = 31.06 (by calculation), Total = 104.10. The simplified formula is (Zn,Mg)Fe3+(SO4)2(OH)⋅7H2O. The mineral is monoclinic, P121/n1, a = 10.504(2), b = 17.801(4), c = 7.1263(14) Å, and β = 100.08(3)°, V = 1311.9(5) Å3, Z = 4. The strongest lines in the powder X-ray diffraction pattern d(I)(hkl) are: 8.92 (100)(110), 6.32 (77)(−101), 5.56 (23)(021), 4.08 (22)(−221),3.21 (31)(231), 3.03 (34)(032), 2.77 (22)(042). The crystal structure was refined using 2816 unique reflections to R1(F) = 0.0355 and wR2(F2) = 0.0651. The refined formula is (Zn0.84Mg0.16)Fe3+(SO4)2(OH)⋅7H2O. The atomic arrangement is characterized by chains with composition [Fe3+(SO4)2(OH)(H2O)]2− and ~ 7 Å repeat distance running parallel to the c-axis. The chain links to a [MO(H2O)5] octahedron (M = Zn, Mg) and an unshared H2O molecule, and forms a larger chain building module with composition [M2+Fe3+(SO4)2(OH)(H2O)6(H2O)]. The inter-chain module linkage involves only hydrogen bonding.

In the present study, using the original sample of zincobotryogen from the Xitieshan lead-zinc deposit, chemical analyses were performed by an electron microprobe analyzer and the crystal structure was reinvestigated. All atoms including hydrogen atoms have been located and the hydrogen bounding system is discussed. The (Zn, Mg) site population has been refined, confirming that zincobotryogen is deserved as a mineral species. The new mineral and its name have been approved by the CNMNC, IMA (IMA No. 2015-107). The mineral is named after its chemical composition and relationship to botryogen. The type specimen of zincobotryogen has been deposited in the mineralogical collection of the Museum, Institute of Geology and Geophysics, Chinese Academy of Sciences with registration number, KDX076.

Mineral occurrence
The Xietieshan lead-zinc deposit is located at the northern margin of the Qaidam Basin, Qinghai Province, China, and distributed in the Upper Ordovician Tanjianshan Group volcanic-sedimentary rocks. The Tanjianshan Group is separated by a fault from the Meso-to Neo-Proterozoic Dakendaben Group mica-quartz schists to the northeast and unconformable overlain by Devonian-Carboniferous purple conglomerates. The Tanjianshan Group is divided into three formation units from bottom upward: (1) volcanicsedimentary rocks, (2) purple sandstone, and (3) intermediate-basic volcanic rocks. The Pb-Zn ore bodies are hosted in the marble and greenschists of the lower formation unit. The minerals in the deposit are mainly sphalerite, galena, pyrite and calcite; the minor minerals are quartz, dolomite,  (Wang et al. 2008).
In the area of the Xietieshan lead-zinc deposit the climate is very arid. The average annual precipitation is below 100 mm and the evaporation capacity is usually up to 2000 mm. The oxidation zone of the lead-zinc deposit is well developed, and the thickness varies from 4 to 20 m. The oxidation zone can be divided into three vertical subzones in the profile from top downward: (1) Limonite-hematite subzone with the thickness of 2-5 cm, (2) Jarosite subzone with the thickness of 3-20 m, consisting of jarosite, quartz, gypsum, sulfur, anglesite, copiapite, zincocopiapite, sideronatrite and fibroferrite, and (3) Gypsum-sulfur subzone with the thickness of 1-4 m, consisting of gypsum, sulfur, anglesite, quartz, melanterite, roemerite and halotrichite. Zincobotryogen occurs in the Jarosite subzone, associated with jarosite, copiapite, zincocopiapite, and quartz (Tu and Li 1963;Tu et al. 1964a, b).
The spatial distribution of sulphate minerals shows a rather definite pattern in the oxidation zone of the Pb-Zn deposit: ferric sulphates are observed in the Jarosite subzone containing copiapite, sideronatrite and fibroferrite, while ferrous sulphates have their prominent development in the Gypsum-sulfur subzone containing melanterite, roemerite and halotrichite. Zn-bearing sulphate minerals, such as zincocopiapite and zincobotryogen, mainly occur in the Jarosite subzone, Mg-bearing sulphate minerals, such as pickeringite, in the Gypsum-sulfur subzone.

Physical and optical properties
The mineral forms prismatic crystals elongated in [001] from 0.5 to 2 mm in length and 0.05 to 0.2 mm in diameter, and commonly occurs in radial or globular aggregates (Fig. 1). The crystals are transparent; their colors are light to dark Parameters of the most intense powder diffraction lines are quoted bold 3.55 %, 6.51 % No. of refined parameters 232 GoF on F 2 1.197 RigakuFour-circle diffractometer equipped with a Saturn 724+ CCD detector, Mo tube, graphite monochrometor, φ-scans for distinct ω-angles, Δφ = 0.5°/frame, frame size: binned mode, 34 μm/2048 × 2048 pixels, detector-to sample distance: 45 mm. Unit-cell parameters were obtained by least-squares refinements of 2θ values orange-red. The streak of zincobotryogen is light yellow, and its luster is vitreous. The density of crystal aggregates measured by using a micro-torsion balance is 2.20(1) g/cm 3 and the calculated crystal density (for Z = 4) is 2.266 g/cm 3 . The mineral is soluble in hot water similar to botryogen. Zincobotryogen is prismatic, with observed forms: {010}, {1 -01}, {120}, and {110}. The a:b:c ratio calculated from the single-crystal unit cell parameters is 0.5901:1:0.4003.

Chemical composition
The quantitative chemical composition of zincobotryogen was measured with a JXA-8100 electron microprobe analyzer in wavelength-dispersive spectroscopic (WDS) mode. Accelerating voltage and specimen current were kept at 15 kVand 10 nA. The beam diameter was 5 μm. The chemical composition was determined from ten electron microprobe analyses shown in Table 1. The H 2 O content was calculated from charge balance and theoretical content of water molecules. The high analytical total is attributed to partial dehydration under vacuum either during carbon coating or in the microprobe chamber. This H 2 O loss results in higher concentrations for the remaining constituents than are to be expected for the fully hydrated phase.
The Our TG and DTA data indicate that zincobotryogen loses most of (H 2 O) at 149°C i.e., 28.2 wt% (Yang and Fu 1988a). The loss of (OH) is initiated at 475°C and is complete at 578°C associated with a weight loss of 2.0 %. The sharp absorption peak of the IR spectrum for zincobotryogen at 3550 cm −1 could be attributed to the OH-stretching mode, the wide peak at 3420 cm −1 and the sharp peak at 1635 cm −1 to H 2 O shown in Fig. 2 (Yang and Fu 1988a).
A Mössbauer spectrum had been measured by Yang and Fu (1988a) to determine the Fe valence state, revealing that there are two quadrupole doublets. The refined hyperfine parameters are IS (isomer shift) = 0.390 mm/s, QS (quadrupole splitting) = 1.131 mm/s for inner peaks; IS = 0.374 mm/s, QS = 1.629 mm/s for outer peaks. It indicates that iron atoms in this mineral belong to Fe 3+ on two independent atomic sites. The ratio of areas of inner peaks to outer peaks is 36.3 : 63.7. X-ray crystallography and crystal-structure refinement Generalities The X-ray powder diffraction data on zincobotryogen were obtained using a Bruker Smart APEX instrument with a CCD detector, monochromatized MoKα (0.7107 Å) radiation at 45 kVand 35 mA, and using the GADDS program (Häming 2000). Observed d spacings are given in Table 2 A single anhedral crystal was examined under a polarizing microscope with no indication of twinning. Single-crystal Xray data for zincobotryogen were collected using monochromatic MoKα-radiation on a Rigaku RA-Micro7HF diffractometer with a Saturn 724+ CCD detector. A total of 1000 frames were recorded by a combination of several ω and φ rotation sets with a 0.5°scan width.
As the Mn and Fe 2+ contents are low, only the Zn:Mg ratio at the octahedral M site was refined, assuming full site occupancy; the respective formula is (Zn 0 . 8 4 Mg 0 . 1 6 ) Fe 3+ (SO 4 ) 2 (OH)⋅7(H 2 O). The approximate positions of the H atoms of the H 2 O and (OH) groups could be localized in difference-Fourier maps and were included in the final refinement, with isotropic atomic displacement parameters. Anisotropic displacement parameters were used for all other atoms. The crystal data, data-collection information and refinement details for zincobotryogen are listed in Table 3. The atomic coordinates, displacement parameters and site occupancy factors are shown in Tables 4 and 5. The relevant bond lengths and angles are shown in Table 6.

Structure description
The crystal structure of zincobotryogen corresponds to the model reported by Süsse (1967Süsse ( , 1968, Yang and Fu (1988a) and Majzlan et al. (2016). The structure of zincobotryogen is shown in a projection along the b-axis in Fig. 3 (1) is coordinated to four oxygen atoms of sulfate groups and to two of hydroxyl groups, Fe (2) ( (Palmer et al. 1972;Hawthorne et al. 2000). In the structure of botryogen, the average S-O distances are 1.471 and 1.470 Å (Majzlan et al. 2016).

System of hydrogen bonds
All hydrogen atoms have been approximately located in good agreement with the model proposed by Majzlan et al. (2016). Bond-valence theory allows estimating the reliability of a structure model. The bond-valence sums including contributions of hydrogen bonds, calculated from the O···O distances as suggested by Ferraris and Ivaldi (1988) (Tables 7 and 8), show satisfactory agreement with the valence-sum rule (Brown 2002). The valence sums of the bond strengths reaching each oxygen are quite satisfactory. The range of variation in S-O bond-valences is 1. 421-1.588 v.u. (valence units), which is in accord with the bond-valence curve for S-O bond-valences (1. 13-1.92 v.u.) given by Brown (1981) and Hawthorne et al. (2000).

Relationships to other minerals, and concluding remarks
Zincobotryogen belongs to the botryogen group (Strunz and Nickel: 07.DC.25; Dana: 31.09.06), which consists of two isotypic members: zincobotryogen is the Zn-end member, while botryogen, MgFe 3+ (SO 4 ) 2 (OH)⋅7(H 2 O), is the Mg-end member. In the structure of zincobotryogen, zinc, magnesium, manganese, and ferrous iron were assigned to the M site, ferric iron fully occupies the Fe sites. The calculated site scattering value (26.62 epfu) is in good agreement with the refined one (27.12 epfu), confirming the reliability of the model. BBotryogen^from the Rammelsberg mine, Germany, should be renamed as zincobotryogen, since zinc is the predominant element in the M site (Zemann 1961;Süsse 1967Süsse , 1968). The samples from Al-caparrosa locality, Antofagasta Province, Chile, and from Nuevo Cuyo, Argentina are botryogen, as magnesium is the predominant element in the M site (Frost et al. 2011;Majzlan et al. 2016).