Crystallographic Controls on the Alteration of Microcline Perthites from the Spruce Pine District, North Carolina

Abstract

Abstract--Altered pelthites 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 Ixm scale. Plagioclase ranges from large lamellar vein and film albite (visible in the light microscope) to cryptoperthite whose size ranges from wm to perhaps 100 ~. The smallest-scale albite appears to be a Iate-stage phase of exsotution 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 haUoysite suggest that once fluids penetrate the crystal, alteration may proceed preferentially in more strongly twinned regions.

Characterization of mineral surfaces using FIB and TEM: A case study of naturally weathered alkali feldspars

“…Previous TEM-based studies of feldspar weathering have described abundant intragranular clay mineral alteration products (e.g., Eggleton and Buseck 1980;Tazaki 1986;BanÞ eld and Eggleton 1990;Sheets and Tettenhorst 1997). We found no evidence for accumulations of clay minerals >2.5 nm thick on the outer surfaces of Shap feldspars, but it may be expected that solutions within the intragranular nanopore networks would readily reach saturation with respect to weathering products and become clogged.…”

Diffusion-controlled and replacement microtextures in alkali feldspars from two pegmatites: Perth, Ontario and Keystone, South Dakota

“…Aberdam (1965) carried out pioneering work on the relationship between dislocations and exsolution lamellae. Fitz Gerald and McLaren (1982) investigated Albite and Pericline twinning in microcline from several pegmatites and Sheets and Tettenhorst (1997) studied replacement of pegmatitic feldspar by clay minerals. Sánchez-Muñoz et al (1998) studied six adularia crystals from Alpine veins using high-resolution TEM (HRTEM) and nuclear magnetic resonance spectroscopy, confirming that they have the coherent tweed domain microtexture Albite twinned albite lamellae slightly oblique to b and fine-scale 'tartan', combined Albite and Pericline twinning, in microcline.…”

Halloysite occurrence at the karstified contact of Oligocene sands and Cretaceous calcarenites in Hinnisdael quarries, Vechmaal (NE of Belgium)

“…Its geological occurrence has been primarily linked to soil and weathering environments, by the weathering and alteration of volcanic rocks (Vaughan et al, 2002;Velde & Meunier, 2008), the alteration of clay minerals like montmorillonite or biotite (Hill, 2000;Papoulis et al, 2009) or weathering of feldspars (Sheets & Tettenhorst, 1997;Adamo et al, 2001). Halloysite is also a common mineral constituent in karst and paleokarst environments as a result of acid weathering (Polyak & Güven, 2000;Joussein et al, 2005).…”