Abstract
Altered perthites from a weathered pegmatite in the Spruce Pine District, North Carolina, were characterized by electron microprobe as a K-rich microcline host with lesser Na-rich plagioclase having a lamellar morphology. Light-optical and transmission electron microscopy (TEM) show microtextural elements such as phase boundaries, holes and microfractures that could serve as potential nucleation sites for alteration to clay minerals.
The host microcline contains albite and pericline twinning textures that vary in character; the amount of each twinning type and/or the size of twin individuals changes on a u.m scale. Plagioclase ranges from large lamellar vein and film albite (visible in the light microscope) to cryptoperthite whose size ranges from u.m to perhaps 100 Å. The smallest-scale albite appears to be a late-stage phase of exsolution in which lamellae have nucleated heterogeneously on albite-twin composition planes in the microcline.
Alteration is concentrated in vein and film albite, especially along grain boundaries with microcline. Powder X-ray diffraction (XRD) patterns of intensely altered pegmatite show halloysite. Holes, microfractures, vein albite/host microcline boundaries and microcline/halloysite boundaries trend parallel to the traces of (010) and {110}, suggesting that these directions are pathways along which fluids migrate. Cleavage and microfractures occur along, and holes are bounded by, these directions. Holes are associated with dislocations and the latter are observed at feldspar/clay boundaries. Twin domains and cryptoperthitic albite are less susceptible to alteration than coarse lamellar albite and regions containing negative crystals and microfractures. However, microtextures in some areas containing halloysite suggest that once fluids penetrate the crystal, alteration may proceed preferentially in more strongly twinned regions.
Similar content being viewed by others
References
Andersen O. 1928. The genesis of some types of feldspar from granite pegmatites. Nor Geol Tidsskr 10:10–207.
Akizuki M. 1972. Electron-microscopic investigation of microcline twinning. Am Mineral 57:57–808.
Banfield JF, Eggleton RA. 1990. Analytical transmission electron microscope studies of plagioclase, muscovite, and K-feldspar weathering. Clays Clay Miner 38:77–89.
Berner RA. 1981. Kinetics of weathering and diagenesis, Chapter 3. In: Ribbe PH, editor. Kinetics of geochemical processes. Rev Mineral vol. 8. Blacksburg, VA: Mineral Soc Am. p 111–133.
Berner RA, Holdren GR, Jr. 1977. Mechanism of feldspar weathering: I. Some observational evidence. Geology 5:5–372.
Brindley GW. 1951. The kaolin minerals. In: Brindley GW, editor. X-ray identification and crystal structures of clay minerals. London: Mineral Soc. p 32–75.
Brobst DA. 1962. Geology of the Spruce Pine district, Avery, Mitchell, and Yancey counties, North Carolina. US Geol Surv Bull 1122A. 26 p.
Brown WL, Macaudiere J. 1984. Microfracturing in relation to atomic structure of plagioclase from a deformed metaanorthosite. J Struct Geol 6:6–586.
Brown WL, Parsons, I. 1983. Nucleation on perthite-perthite boundaries and exsolution mechanisms in alkali feldspars. Phys Chem Mineral 10:10–61.
Brown WL, Parsons I., 1984b. Exsolution and coarsening mechanisms and kinetics in an ordered cryptoperthite series. Contrib Mineral Petrol 86:86–18.
Burgess R, Kelley SP, Parsons I, Walker FDL, Worden RH. 1992. 40Ar-39Ar analysis of perthite microtextures and fluid inclusions in alkali feldspars from the Klokken syenite, South Greenland. Earth Planet Sci Lett 109:147–167.
Butler JR. 1973. Paleozoic deformation and metamorphism in the Blue Ridge Thrust Sheet near Spruce Pine, North Carolina [abstract]. Geol Soc Am Abstr with Prog 5:382.
Casey WH, Banfield JF, Westrich HR, McLaughlin L. 1993. What do dissolution experiments tell us about natural weathering? Chem Geol 105:1–15.
David F, Walker L. 1990. Ion microprobe study of intragrain micropermeability in alkali feldspars. Contrib Mineral Petrol 106:106–128.
Debat P, Soula J-C, Kubin L, Vidal J-L. 1978. Optical studies of natural deformation microstructures in feldspars (gneiss and pegmatites from Occitania, southern France). Lithos 11:11–145.
Eggleton RA. 1986. The relation between crystal structure and silicate weathering rates. In: S. M. Coleman and D., P. Dethier, editors. Rates of chemical weathering of rocks and minerals. London: Academic Pr. p 21–39.
Eggleton RA, Buseck PR. 1980. High-resolution electron microscopy of feldspar weathering. Clays Clay Miner 28:28–178.
Fitz Gerald JD. 1993. Slowly-cooled, orthoclase-rich alkali feldspars: Microstructures and implications for Ar-Diffusion [abstract]. NATO ASI Series Meet; Edinburgh, Scotland.
Fitz Gerald JD, Harrison TM. 1993. Argon diffusion domains in K-feldspar I: Microstructures in MH-10. Contrib Mineral Petrol 113:113–380.
Fitz Gerald JD, McLaren AC. 1982. The microstructures of microcline from some granitic rocks and pegmatites. Contrib Mineral Petrol 80:80–229.
Gandais M, Williame C. 1984. Mechanical properties of feldspars. In: Brown WL, editor. Feldspars and feldspathoids, NATO ASI Series C 137. Dordrecht: D. Reidel. p 207–246.
Gard JA. 1972. The electron-optical investigation of clays. Mineral Soc Monograph 3 (Clay Miner Group). Oxford: Alden Pr. 383 p.
Goldberg SA, Trupe CH, Adams MG. 1992. Pressure-temperature-time constraints for a segment of the Spruce Pine thrust sheet, North Carolina Blue Ridge [abstract]. Geol Soc Am Abstr with Prog 24:17.
Kohyama N, Fukushima K, Fukami A. 1978. Observation of the hydrated form of tubular halloysite by an electron microscope equipped with an environmental cell. Clays Clay Miner 26:26–40.
McLaren AC. 1978. Defects and microstructures in feldspars. Chem Phys Solids Surf Chem Soc 7:7–30.
Parham WE. 1969. Formation of halloysite from feldspar: Low temperature artificial weathering versus natural weathering. Clays Clay Miner 17:17–22.
Parker JM III. 1949. Geology and structure of part of the Spruce Pine District, North Carolina. NC Dept Conserv and Devel Bull 65. 26 p.
Parsons I, Brown WL. 1984. Feldspars and the thermal history of igneous rocks. NATO ASI Series C137:317–71.
Parsons I. 1993. Fast routes for isotopic exchange in alkali feldspars [abstract]. NATO ASI Series Meet; Edinburgh, Scotland.
Rowe GL, Jr. Brantley SL. 1993. Estimation of the dissolution rates of andesitic glass, plagioclase and pyroxene in a flank aquifer of Poas Volcano, Costa Rica. Chem Geol 105:71–87.
Smith JV, Brown WL. 1988. Feldspar Minerals, vol. 1. New York: Springer-Verlag. 828 p.
Sverdrup HU. 1990. The kinetics of base cation release due to chemical weathering. Sweden: Lund University Pr. 246 p.
Swoboda-Colberg NG, Drever J. 1993. Mineral dissolution rates in plot-scale field and laboratory experiments. Chem Geol 105:105–69.
Tibballs JE, Olsen A. 1977. An electron microscopic study of some twinning and exsolution textures in microcline amazonites. Phys Chem Miner 1:1–324.
Worden RH, Walker FDL, Parsons I, Brown WL. 1990. Development of microporosity diffusion channels and deuteric coarsening in perthitic alkali feldspars. Contrib Mineral Petrol 104:104–515.
Zeitler PK, Fitz Gerald JD. 1986. Saddle-shaped 40Ar/39Ar age spectra from young, microstructurally complex potassium feldspars. Geochim Cosmochim Acta 50:1185–1199.
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Sheet, J.M., Tettenhorst, R.T. Crystallographic Controls on the Alteration of Microcline Perthites from the Spruce Pine District, North Carolina. Clays Clay Miner. 45, 404–417 (1997). https://doi.org/10.1346/CCMN.1997.0450310
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1346/CCMN.1997.0450310