Minerals Are Not Just Chemical Compounds

Abstract

Mineral grains are not just chemical compounds; they possess microtexture. The properties of crystallographically nearperfect "gem-quality" alkali feldspar crystals are contrasted here with those of ordinary, "garden-variety" specimens, exemplifi ed by perthitic phenocrysts from the granite at Shap, northern England. These are representative of common alkali feldspars in the crust. Two well-studied gem-quality materials (Madagascar orthoclase and Eifel sanidine) vary at the source in microtextural and other properties. Some crystals of Eifel sanidine exhibit much more rapid Si-Al ordering than ordinary feldspars, probably because of the presence of a hydrogen species. The complex microtextures of two feldspars from pegmatites that have been used in studies of dissolution rate, Perth perthite and Keystone microcline, are described and compared with textures in the Shap feldspars. Microtextures, in particular misfi t dislocations on regular exsolution-lamellae of albite, are crucially important in chemical dissolution and mechanical degradation during natural weathering in acid peat soils at Shap. In contrast, during laboratory dissolution-rate experiments carried out far from equilibrium, natural defects are less important than edges and steps produced during the grinding of the samples. Diagenetic replacement-type reactions in an overlying conglomerate at Shap are also strongly sensitive to microtexture. In arkosic sandstones in the Jurassic Fulmar Formation of the central North Sea, the detrital alkali feldspar inventory has been subject to microtextural winnowing, with relatively defect-free varieties of alkali feldspar being relatively more abundant than would be expected from the distribution of rock types known in the North Sea catchment.Keywords: mineral microtexture, alkali feldspar, exsolution, dislocations, dissolution-reprecipitation, dissolution, replacement, gem-quality minerals, weathering, diagenesis. SOMMAIRELes grains de minéraux ne sont pas simplement des composés chimiques; ils possèdent une microtexture. Les propriétés de cristaux gemmes de feldspath alcalin sont ici comparées avec celles de cristaux perthitiques tout-à-fait ordinaires provenant du granite de Shap, au nord de l'Angleterre. Ces derniers seraient représentatifs des feldspaths communs de la croûte. Deux matériaux gemmes bien étudiés, l'orthose du Madagascar et la sanidine d'Eifel, sont pris de populations variables à la source dans leur microtexture et leurs propriétés. Certains cristaux provenant de l'Eifel font preuve d'une mise en ordre Si-Al beaucoup plus rapide que ces feldspaths ordinaires, probablement à cause d'une espèce hydrogénée. Les microtextures complexes de deux feldspaths provenant de pegmatites et utilisés dans les études du taux de dissolution, la perthite de Perth et le microcline de Keystone, sont décrites et comparées aux textures des feldspaths provenant de Shap. Ces microtextures, en particulier les dislocations dues au décalage le long des lamelles d'exsolution d'albite, sont très importantes dans ...

The role of phase separation and related topography in the exceptional ice-nucleating ability of alkali feldspars

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Sodium-potassium interdiffusion in potassium-rich alkali feldspar II: Composition- and temperature-dependence obtained from cation exchange experiments

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Chemical weathering of a marine terrace chronosequence, Santa Cruz, California. Part II: Solute profiles, gradients and the comparisons of contemporary and long-term weathering rates

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