Page 57 - A Practical Introduction to Optical Mineralogy

AMPHIBOLE GROUP
RELIEF Moderate.
• ALTERATION Common (see introduction).
III~EF RINGENCE Low to moderate.
IN rE RFERENCE Bxa figure seen on a (100) prismatic face (anthophyllite) or a basal face
FIGU RE (gedrite) but crystals are usually so small that figures may be impossible
to obtain. Best results will be obtained from a single optic axis figure.
EXT IN CTION Straight; crystals are length slow.
OCCU RRENCE Unknown in igneous rocks, the orthorhombic amphiboles occur widely
in metamorphic rocks, with anthophyllite found in association with
cordierite.
c 'urnmingtonite Cummingtonite and grunerite are the monoclinic equivalents of
Grunerite anthophyllite and gedrite. Cummingtonite (the Mg-rich form) is posi-
tive, whereas grunerite (the Fe-rich form) is negative. 2V is large, and
density and hardness are similar to anthophyllite-gedrite. Birefringence
is moderate to high (grunerite) and each mineral has oblique extinction
with ·{cleavage = lOa to 21 ° on the (010) prism face (see Fig. 2.5).
Cummingtonite occurs in metamorphosed basic igneous rocks, where
it is associated with common hornblendes. Grunerite occurs in
metamorphosed iron-rich sediments, where it is associated with either
magnetite and quartz or with almandine garnet and fayalitic olivine, the
latter minerals being common constituents of eulysite bands.
Amosite (brown asbestos) is asbestiform grunerite.

Figure 2.5 Extinction angles of amphiboles. Note that c"{3 for katophorite will
be cleavage"slow ray, since the other component in this orientation is a.
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