MICROTEXTURAL CONTROLS OF WEATHERING OF PERTHITIC ALKALI FELDSPARS

The relationship between the microtexture and dissolution behaviour of fresh, HF acid-etched and naturally weathered alkali feldspar phenocr ysts from the Lower Devonian Shap granite has been investigated by SEM and TEM. A novel resin impregnation technique has revealed the three dimensional shape and interconnectivity of etch pits beneath the weath ered crystal surface. Further electron microscope work suggests that S hap phenocrysts are representative of the alkali feldspar in the proto lith of many soils. Fresh and unweathered Shap feldspars have a comple x microtexture, comprising areas of pristine cryptoperthite and lamell ar microperthite cross-cut by volumes of microporous altered feldspar or ''patch perthites.'' Cryptoperthites are made up of <75 nm wide alb ite exsolution lamellae (platelets) in tweed orthoclase, whereas lamel lar microperthites contain >75 nm wide albite films. The platelets are coherent, but albite films have numerous edge dislocations along thei r interface with orthoclase; (001) and (010) cleavage surfaces interse ct similar to 2-3 edge dislocations/mu m(2). In three dimensions, thes e edge dislocations form an orthogonal net in the ''Murchison plane'' of easy fracture, close to ((6) over bar 01). Patch perthites are irre gular, semicoherent to incoherent intergrowths of albite and irregular microcline subgrains, with similar to 0.65-0.70 sub-mu m to mu m-size d pores/m(2). Microporous patch perthites form by dissolution-reprecip itation reactions with magmatic or hydrothermal fluids and pores are p resent before the alkali feldspars enter the weathering regime. Dissol ution of Shap feldspars during natural weathering and laboratory acid etching is controlled by their microtexture, especially by dislocation s and exsolution lamellae. The core and strain energy associated with dislocation outcrops on (001) and (010) cleavage surfaces promotes rap id dissolution at those sites and formation of nm-sized etch pits afte r <30 s of laboratory etching with HF acid vapour. With progressive HF etching, crystallographically controlled differences in the reactivit y of etch pit walls cause them to expand more rapidly into orthoclase than albite. Naturally weathered feldspars were collected from the gla cial erratic boulders, fine gravels surrounding exposed granite surfac es, and from peat soil overlying the granite. During natural weatherin g, etch pits on microperthites enlarge almost exclusively by dissoluti on of albite and resin casts demonstrate that they can penetrate great er than or equal to 15 mu m below the cleavage surface, forming an int erconnecting, ladder-like grid of submicrometer wide channels in the M urchison plane. Coherent albite platelets and volumes of albite betwee n dislocations in films dissolve uniformly, but faster than orthoclase . This is probably because the albite lamellae have significant elasti c coherency strain, but this is much less, per unit volume of albite, than the core and strain energy associated with edge dislocations. Pat ch perthites etch in HF vapour and weather rapidly in nature to produc e a honeycomb-like texture of interconnecting nanometer- to micrometer -sized pits, which nucleate at preexisting micropores or incoherent su bgrain boundaries. The size and density of etch pits on microperthite surfaces, which is determined by rates of growth and coalescence, may be a useful progress variable for natural and experimental dissolution . All alkali feldspars are highly heterogeneous materials whose chemic al composition and microtexture can vary on a submicrometer scale. The se microtextures are critical variables with regard to the origin of s urface roughness of fresh and weathered grains, the controls on absolu te dissolution rates and why they commonly change over time, the nonli near variation of dissolution rate with grain size, the ratio of alkal i ions released into solution, and disparities between laboratory diss olution rates and those observed in the field.