Page 62 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS
AMPHIBOLE GROUP
•occuRRENCE Common hornblendes are primary minerals, particularly in intermedi- Alkali amphiboles
ate plutonic igneous rocks, although they can occur in other types. In Glaucophane N a 2 ( Mg,Alz)SisOzz( OH)z monoclinic
intermediate rocks, the hornblende has a Fe: Mg ratio of about 1: 1, Riebeckite Naz(Fe~•Fe~•)Sis022(0H)z 0.54 : 1:0.29
whereas hornblendes are more Mg rich in basic rocks and very iron rich {3 = 104°
c
in acid rocks ( - 20: 1). Hornblende may occur in some basic rocks (e.g.
troctolites etc.) as a corona surrounding olivine crystals, caused by 13 l Crossite
I I
reaction between olivine and plagioclase. Hornblende is stable under a II
wide range of pressure and temperature (PT) conditions in metamor- a mineral inte rmediate
phism, being an essential constituent of the amphibolite facies. Horn- in composition between
blendes become more alumina rich with increasing metamorphic grade. glaucophane and
riebeckite
Pure tschermakite occurs in some high grade metamorphic rocks (often
with kyanite) and pure pargasite occurs in metamorphosed dolomites.
Secondary amophiboles in igneous rocks are usually tremolites or cum-
mingtonites, but may be hornblendes.

'I
I
I
I
II III

c
al
I I
I I
I '
----b = 13
010

,,

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