Isotropic Minerals - Lakehead University

Isotropic Minerals - Lakehead University

Isotropic Minerals fluorite A C B D

Fluorite is a fairly common mineral in hydrothermal veins and occurs as an accessory mineral in some granitic rocks. The perfect {111} cleavage is visible in A in the euhedral to subhedral fluorite cubes in a matrix of barite. Fluorite is isotropic and viewed under crossed nicols it is black (extinct), as in C, in which it is intergrown with high-order white calcite. It has the lowest refractive index (n ~ 1.434) of all the common minerals and typically stands out with high negative relief, as seen relative to barite in A and calcite in B. Fluorite can be coloured in thin section (blue and purple are common colours) and in images B and D the purple colour is unevenly distributed. In D the intensity of the colour increases towards a small inclusion of uraninite (partially

plucked out during thin section preparation) and the origin of colour of the fluorite from this locality is thought to be due to radiation damage. A is from the Noyes Mine near Madoc, Ontario and B, C and D are from the Richardson (Fission) Mine near Wilberforce, Ontario. View A is 5.5 mm across and B, C and D are 2.2 mm across. A, B and D ppl and C x-nicols A B

Sodalite Na8(AlSiO4)6Cl2 Sodalite is a rather uncommon mineral restricted to alkaline igneous rocks, and is commonly associated with other feldspathoids. It is colourless to pale blue in plane light and is isotropic, with a low refractive index (n = 1.48-1.49), which gives it negative relief relative to nepheline or feldspars with which it commonly occurs. In these images, sodalite occupies much of the lower left corner (arrow), and has an albite inclusion. The sodalite is pale blue in A, compared to the colourless plagioclase and the pale yellow cancrinite that occupies most of right side of these images. Except for its blue colour, sodalite is easily confused with analcime. Sample from a cancrinite-nepheline syenite from Bancroft, Ontario. Images are 2.2

mm across. A ppl, B x-nicols. The garnet minerals Garnets: A large group of cubic minerals with general formula A32+B23+(SiO4)3 A Ca2+, Mg2+, Fe2+ or Mn2+ B Al3+,Fe3+ or Cr3+ There are also many other synthetic compositions possible

Garnet compositions Ugrandite group Grossular Andradite Uvarovite Ca3Al2(SiO4)3 Ca3Fe3+2(SiO4)3 Ca3Cr2(SiO4)3

Pyralspite group Pyrope Almandine Spessartine Mg3Al2(SiO4)3 Fe2+3Al2(SiO4)3 Mn3Al2(SiO4)3

Solid solutions within each group are typical. At high T (above 700oC) there is also solid solution between the 2 groups. Note: Solid solutions are always more extensive at higher Temperatures Garnet Plane-Polarized Light High relief, rough surface Clear, occasionally very light pinkish, reddish, greenish, pale brown No cleavage

Euhedral crystals of inclusions common (A) Retrograde metamorphism common (B). Garnet will revert to greenish amphiboles or chlorite, still retaining the original outline of the crystal. Remnants of garnet are often preserved in the interior Internal fracturing of grains common Crystals commonly contain inclusions, incorporated as the garnet grew quartz inclusions in garnet plane polarized light

same grain (X-polars high relief pronounces fractures and grain margins

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