Page 65 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS AMPHIBOLE GROUP
*COLOUR Glaucophane is lavender blue or colourless, whereas riebeckite is dark I 1 ht1 rite Na Ca(Mg,Fel+,FeZ+,Mn),SisOn(OH,F), monoclinic
2
blue to greenish.
*PLEOCHROISM Common in both minerals, with a colourless, f3 lavender blue, and
y blue in glaucophane, and a blue, f3 deep blue, andy yellow green in Richterite
riebeckite. Oxyhornblende
HABIT Glaucophane occurs usually as tiny blue prismatic crystals whereas Kaersutite
(" c
riebeckite occurs as either large subhedral prismatic crystals or tiny all with a as Bxa, I Katophorite
crystals in the ground mass of some igneous rocks such as alkali and large 2 V"' 13 I
I I
microgranites. I I
CLEAVAGE See introduction. I I
RELIEF Moderate to high. Ill()
ALTERATION Rare in glaucophane; more common in riebeckite, which may alter to a
fibrous asbestos (crocidolite). Riebeckite is often found in intimate
association with sodic pyroxenes (aegirine), in alkali granites and
syenites for example.
BIREFRINGENCE Low to moderate; riebeckite interference colours are usually masked by
the mineral colour.
INTERFE RENCE The optic axial angles of both minerals may vary considerably in size. In ----b=f3 -----b =-y
FIGURE riebeckite the strong colour of the mineral may make the sign very
difficult to obtain.
*EXTIN CTION Glaucophane is length slow with a small extinction angle of y'cleavage
ANGLE
(slow'cleavage) of 6-9°. Riebeckite is length fast with an extinction
angle of a (fast tel = 6-8°. An (010) section in each mineral will give a
maximum extinction angle. The variation in extinction angles· is caused I
I
by the replacement of AP• by FeJ+ in glaucophane and FeZ+ in I
I
riebeckite. (/
TWINNING Can be simple or repeated on { 100}.
DISTINGUISHING The lavender blue colour of glaucophane and the fact that it is almost 11 " 1.605-1.685
FEATURES length slow, and the deep blue colour of riebeckite and that it is nearly ll p 1.618-1.700
length fast, are important identification points. Where a mineral has a n, 1.627-1.712
strong body colour, a mineral edge should be obtained which must be 8 0.022-0.027
wedge shaped. At the very edge the mineral is so thin that the body 2V" = 66°-90° -ve
colour has a limited effect. Then, using a high powered lens (e.g. x 30), OAP is parallel to (010)
whether the mineral is length fast or length slow can be obtained using a D = 2.97-3.45 H = 5V2
first order red accessory plate.
1111 ouK olourless, pale yellow.
*ocCU RRENCE Glaucophane is the essential amphibole in blueschists, which form llltiiMOI\M Weak, in pale colours, yellow, orange and blue tints. f3 is usually darker
under high Plow T conditions in metamorphosed sediments at destruc- in colour than a andy, which are very pale.
tive plate margins and are commonly found in association with ophiolite
111\1111 ee introduction.
suites. Riebeckite occurs in alkali igneous rocks, especially alkali gran-
, 11 VA <. I' Normal, see introduction.
ites where it is associated with aegirine. Fibrous riebeckite (crocidolite, Ml 1 11'1' Moderate to high.
blue asbestos) is formed from the metamorphism at moderate T and P or
1
"" ' 1 ~I Nil IN< ' Moderate.
massive ironstone deposits. 1 11111111 N< H Large 2V on (100) face, but an isotropic section perpendicular to a
11 111 1 KJ1 single optic axis should be obtained and the sign and size of 2V deter-
mined from it.
I II IliON Larger than normal with y' cleavage 15 to 40° measured in an (010)
Nl II' prism section.
52 5