Page 105 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS MICA GROUP
*INTERFERENCE A basal section gives a perfect Bxa figure with a very small 2V, although
coLouR Pale brown, colourless. FIGURE
*PLEOCHROISM Weak, with pale colours; a yellow, f3 andy brownish red, green, deeper the strong body colour may tend to mask the colour that determines the
sign of the mineral.
yellow.
*HABIT Small tabular crystals common, frequently subhedral. EXTINCTION Nearly straight on cleavage. A speckled effect is seen near the extinction
*cLEAVAGE Perfect { 001} cleavage. position, which is very characteristic of micas.
RELIEF Low to moderate. TWINNING Extremely rare, similar to phlogopite twins.
*BIREFRINGENCE High, third order colours commonly present; weak body colours only OCCURRENCE Common mineral in a variety of rocks. Biotite occurs in most
slightly mask the interference colours. metamorphic rocks which have formed from argillaceous sediments. It
*INTERFERENCE A basal section shows a good Bxa figure with a very small 2V. forms after chlorite and is a constituent of regional metamorphic rocks
FIGURE existing up to very high grades, although its composition changes with
the metamorphic grade. These changes may be accompanied by changes
EXTINCTION Usually straight, but slight angle {3' cleavage = 5° (max) on (010) face
in colour, due to Mg and Ti increasing in amount in the mineral.
may occur.
Biotites are primary crystallising minerals in acid and intermediate
TWINNING Rare on { 310}, seen on cleavage section.
plutonic igneous rocks, and in some basic rocks. Biotite is not common
OTHER FEATURES Reaction rims may occur in phlogopites found in kimberlite intrusions.
in acid and intermediate extrusive and hypabyssal rocks.
•occuRRENCE Found in metamorphosed impure magnesian limestones where phlogo-
Biotite is a common mineral in clastic arenaceous sedimentary rocks
pite forms by reactions between the dolomite and either potash feldspar
but is very prone to oxidation and degradation.
or muscovite. Phlogopite is a common constituent of kimberlite, occurs
The type of mica depends on its Mg: Fe ratio. Thus:
in many leucite-bearing rocks, and is a minor constituent of ultramafic
rocks.
2
phlogopite is a mica with Mg + between 100 and 70%
biotite is a mica with Mg'+ between 60 and 20% (and increasing R +)
3
3 monoclinic siderophyllite is a mica with Mg'+ < 10% (R'+ is AP+)
Biotite K,(Mg,Fe )6_.(Fe + ,Al,Ti)o- 2 Si6-sA1,_,0 20 (0H,F) 4
3
3
(identical to phlogopite) lepidomelane is a mica with Mg'+ < 10% (R + is Fe +)
annite is a mica with Mg2+ 0% (R + is zero).
3
n. 1.565-1.625
n p 1.605-1.696
n > = 1.605-1.696
0 = 0.040-0.080
2V. = 0-25° - ve
OAP is parallel to (010)
D = 2.7-3.3 H = 21fz
coLouR Brown or yellowish ; occasionally green.
*PLEOCHROISM Common and strong with a yellow, f3 and y dark brown. Note that
pleochroism cannot be detected in a basal section, and that a prism
section showing a cleavage is best.
*HABIT Tabular, subhedral hexagonal plates.
*CLEAVAGE { 001} perfect.
RELIEF Moderate.
*ALTERATION Common in rocks which have undergone hydrothermal alteration. The
biotite alters to chlorite with potash being released in the reaction. The
reverse reaction occurs during progressive metamorphism, with chlorite
changing in composition as biotite forms at higher temperatures.
*BIREFRINGENCE High to very high but masked by body colour. Note that the birefrin-
gence of basal sections of all micas is virtually zero since the n p and n
1
values are almost equal, and n p and n 1 lie in the (001) plane (i.e. the
basal section).
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