Abstract

The ore skarn zones in the dolomite invaded by the Beinn an Dub-haich granite, Skye, often contain residual chert nodules (“sponge forms”) immediately encased by wollastonite not infrequently associated with clinopyroxene. Study of the optical properties of the wollastonites of this encasing zone shows that their composition varies according to the nature of the associated clinopyroxene. Wollastonites associated with strongly coloured clinopyroxene show the optical characters of the artificial iron wollastonites, whereas the wollastonite associated with a colourless diopsidic pyroxene possesses the normal, chemical and optical properties of this mineral. The coloured clinopyroxenes found in the skarn zone are hedenbergite rich solid solutions with green to yellow green pleochroism (γ = yellow green, α = green) more or less strongly marked. The wollastonite (white) is intergrown with these or may form a separate band between two seams of the pyroxene. The remaining mineral found in close association particularly with the pyroxene is a coloured grossular-andradite. From such a skarn the wollastonite and pyroxene have been separated for analysis in order to reveal more precise data of the paragenesis. The separated wollastonite fraction yielded a product of varying specific gravity, the bulk of the material having a value of 3.09. This material provided homogeneous crystals, colourless in transmitted light, with the following optical properties: α = 1.640, γ = 1.653, 2Vα60°±3°, α'∧c = 44° in section perpendicular to the zone of cleavages. Sections perpendicular to the acute bisectrix gave extinction angles of 5°-6° from the zone of cleavages. Simple twins on {100} (old monoclinic orientation). Analysis of such material gave the results set out in the accompanying table. In the separation a small amount of a heavier fraction (Sp. Gr. >3.10) showed a maximum value of γ=1.660, pointing to still higher contents of iron in solid solution in some of the material. A partial analysis of the associated hedenbergitic pyroxene (Sp. Gr. 3.55) which contained a small quantity of impurity including some fine garnet, gave SiO2 46.62, A12O3 0.59, Fe2O3 1.99, FeO 16.81, MnO 1.04, CaO 24.46, MgO 5.63. If alumina and ferric oxide are removed as grossular-andradite, this pyroxene analysis recalculated gives SiO2 50.7, FeO 18.5, MnO 1.1, CaO 23.5, MgO 6.2, yielding an approximate composition Di33Hd67 (Diopside33 Hedenbergite67). This is a bulk composition of somewhat variable material. The maximum value of γ recorded is 1.742 equivalent to Hd76, the pyroxene as with the wollastonite showing a range of composition around the average of Hd67.

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