Macro- and micro-perthitic microclines from pegmatites from Perth, Ontario (Wards catalogue 46 E 0510) and Keystone, South Dakota (Wards 46 E 5125) have been studied using light and electron microscopy. A sample of the type perthite from Perth, Ontario (Hunterian Museum, Glasgow, M2361)was compared using light microscopy. It differs in bulk composition and microtexture from the Wards sample. The Perth sample from Wards is a mesoperthite, with sub-periodic ∼mm-thick albite veins near (100), with irregular surfaces. The microcline has regular tartan twins and formed fromorthoclase by a continuous process. The Keystone sample is a microperthite, with non-periodic albite veins mainly in {110}. Irregular tartan twins, volumes of irregular microcline and subgrains suggest that the microcline formed by dissolution–reprecipitation. Microcline in both samplescontains semicoherent cryptoperthitic albite films that formed after the development of tartan twins. The bulk compositions of these intergrowths imply exsolution below ∼400°C. Diffusion parameters imply sustained heating for between 0.11 My at 400°C, 1.5 GPa and 8.4 My at 300°C,1 GPa. Unrealistic times are required at 200°C. Subsequently, the crystals reacted with a fluid leading to replacive growth of the vein perthites. Unusually, Albite twin composition planes in replacive subgrains have sub-periodic dislocations, formed by coalescence of advancing growthtwins. Processes that might lead to periodic, replacive intergrowths are discussed. The Perth and Keystone feldspars have been used for experimental work on dissolution during weathering and on anomalous thermoluminescence fading. Their microtextures make them unsuitable for obtaining propertiesthat can be extrapolated to feldspars in general.

The Miocene dolomite-chert microbialites studied here offer a complete record of the geochemical cycles of silicate weathering and the subsequent formation of secondary products. The microbialites were formed in lacustrine systems during the Miocene of the Duero Basin, central Spain. Mineralogical, chemical and petrographic results provide evidence of the mediation of microbes in early weathering and by-product formation processes. Irrespective of the composition, the surfaces of the grains were subject to microbial attachment and concomitant weathering. Palaeo-weathering textures range from surface etching and pitting to extensive physical disaggregation of the minerals. Extreme silicate weathering led to the complete destruction of the silicate grains, whose prior existence is inferred from pseudomorphs exhibiting colonial textures like those recognized in the embedding matrix. Detailed petrographic and microanalytical examinations of the weathering effects in K-feldspars show that various secondary products with diverse crystallinity and chemical composition can coexist in the interior of a mineral. The coexistence of by-products is indicative of different microenvironmental conditions, likely created by microbial reactions. Thus, the presence of varied secondary products can be used as a criterion of biogenicity. Intensive alteration of P-bearing feldspars suggests that mineral weathering may have been driven by the nutrient requirements of the microbial consortium involved in the precipitation of dolomite. The rock record provides useful information on mineral weathering mediated by microbes.