Page 54 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS AMPHIBOLE GROUP
The amphiboles will be examined in the order above, i.e. subgroups (a), Ca-poor amphiboles
(b) and (c), but the general optical properties of all amphibole minerals
are given below: Anthophyllite (Mg,Fe),(Mg,Fe),SisOnCOH,F), .... .......... Mg » Fe} orthorhombic
Gedrite (Mg,Fe) 2 (Mg,Fe) 3 Al 2 (Si 6 Al,)On(OH,F), .. Fe » Mg
coLOuR Green, yellow and brown in pale or strong colours. Mg-rich amphiboles 0.967: 1: 0.285
PLEOCHROISM may be colourless or possess pale colours with slight pleochroism,
whereas iron-rich and alkali amphiboles usually are strongly coloured
and pleochroic.
HABIT Amphiboles usually occur as elongate prismatic minerals, often with cummingtonite (2V~ large +ve)
diamond shaped cross sections. and grunerite (2V" large -ve)
*cLEAVAGE All amphiboles have two prismatic cleavages which intersect at 56° are their monoclinic
(acute angle). equivalents, with similar
RELIEF Moderate to high. optical orientation to gedrite
and anthoph.yllite respectively
ALTERATION Common in all amphiboles; usually to chlorite or talc in the presence of
water. A typical reaction is as follows:
Mg,Mg,Si 80,,(0H, F), + H,0--7 Mg 6 Si 8 0 20 (0H) 4 + Mg(OH),
Mg anthophyllite talc brucite
----- b=13
BIREFRINGENCE Low to moderate; upper first order or lower second order interference
colours occur, iron-rich varieties always giving higher interference
colours. The strong colours of alkali amphiboles often mask their
interference colours.
INTERFERENCE Apart from glaucophane and katophorite, most amphiboles have large
FIGURE
2V angles; thus an isotropic section is needed to examine a single optic /
/
axis figure. In the alkali amphiboles dispersion is so strong that inter- a=o.
ference figures may not be seen.
EXTINCTION Orthorhombic amphiboles have parallel (straight) extinction. All other
amphiboles are monoclinic with variable maximum extinction angles
(Fig. 2.5).
n. = 1.596-1.694
ZON ING Fairly common.
TWINNING Common on { 100}; with either single or multiple twins present. np = 1.605-1.710
n y = 1.615-1.722
0 = 0.013-0.028
2V. = 69°-90° (anthophyllite) - ve }
both crystals are length slow
2Vy = 78°-90° (gedrite) +ve
OAP parallel to (010)
D = 2.85-3.57 H = 5'12-6
COLOUR Pale brown to pale yellow.
• 1•1 I 0('11 ROISM Gedrite has a stronger pleochroism than anthophyllite with a and f3 pale
brown, y darker brown. . .
II A BIT Elongate prismatic crystals; basal sections recognised by mtersectmg
cleavages.
Two prismatic { 110} cleavages intersecting at 54o (126°). The two
cleavages are parallel to each other in a prism section and so elongate
prismatic sections appear to have only one cleavage.
42 43