Isotropic Minerals

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Isotropic Minerals
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fluorite
A
B
C
D
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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
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B
A
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.
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The garnet minerals
Garnets A large group of cubic minerals with
general formula A32B23(SiO4)3 A Ca2,
Mg2, Fe2 or Mn2 B Al3,Fe3 or Cr3 There
are also many other synthetic compositions
possible
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Garnet compositions
 Ugrandite group Grossular Ca3Al2(SiO4)3
Andradite Ca3Fe32(SiO4)3
Uvarovite Ca3Cr2(SiO4)3   Pyralspite group
Pyrope Mg3Al2(SiO4)3 Almandine Fe23Al2(SiO
4)3 Spessartine 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
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Garnet
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• 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

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quartz inclusions in garnet
plane polarized light
same grain (X-polars
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high relief pronounces fractures and grain
margins