SynopsisPermian sandstones from the Cock of Arran area contain small amounts of sphene cement. The sphene occurs as a minor phase (up to 2%) in small patches cementing several grains, and as individual crystals with well developed crystal faces grown into pore space from quartz, feldspar or lithic grain surfaces. The sphene postdates the growth of red grain pellicles and chlorite rosettes and probably formed at a time of calcite dissolution and creation of secondary porosity. Sphene is apparently absent from Permian sandstones at Corrie and Brodick. It is postulated that the sphene was deposited from convective fluids associated with the intrusion of the Arran Granite in Tertiary times. Ti was probably derived from the alteration of Ti-bearing oxides and volcanic rock fragments in the sandstones. porous and permeable sandstones from Corrie and Brodick (Fig. 1).Scott.
Synopsis Roscoelite, a vanadium mica, occurs in the cores of reduction spots within the Lower Old Red Sandstone that crops out in Gamrie Bay (Moray Firth). The finely crystallized mineral is seen both as a cement in the sandstone and replacing plagioclase feldspar in a lithic clast of granitic composition. It is suggested that roscoelite was formed by a complex reaction involving a change in the oxidation–reduction potential of the groundwater and the presence in the sandstone of sensitive associations of minerals containing vanadium-rich iron and titanium oxides. The reduction spots formed by diffusion as part of the same process.
On computer-drawn projections of the structure of diopside (CaMgSi?O?) the PBCs are sought. These are <001>, <100>, <010>, <110>, <10[-2]>, <102>, <11[-2]>, <112>, <012> and <011>. Using a broken bond model the attachment energies can be calculated and so the F faces (containing two or more PBCs) can be classified in the order of decreasing importance. In the case of free atoms in the crystallizing phase one obtains : {110}, {010}, {100}, {111}, {[-2]21}, {111}, {021}, {001}, {221} and {011}. The results are identical for the other monoclinic pyroxenes : hedenbergite, augite, jadeite and acmite. Drawings of the theoretical growth form of the crystals are given assuming the attachment energy to be proportional to the growth rate of a face. When the eventual presence of Si?O? groups in the crystallizing phase is considered as an external factor, the growth form, as far as it depends on the crystal structure is the one calculated on the basis of free atoms. Diopside, hedenbergite and augite are bound by {110}, {010}, {[-1]11}, {111} and jadeite and acmite by {110} and {[-1]11}. It is questionable whether the various forms appearing in nature are always due to external factors or whether the hypothesis that EaH is directly proportional to the growth rate is justified.
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