Page 61 - A Practical Introduction to Optical Mineralogy

AMPHIBOLE GROUP

(a)
71 I Common hornblende
+ !
II-- 2V" variable, usually large, but
;---r-- 2Va1
.............. 2 V" for hastingsite is very small :
I 010
............ pargasite has -y as acute
~ bisectri x and is +ve
<U )( I
Oil "'-..
c ~
~
:::. :11 I I-- ]"-.,
N
II I
1.7 4
~ ------b = [3
1.7 2
~
~
./.
.Q 1.7( I v v
u 11/ ~ v
..t 1.6 X v
....;:;-~
~ / ~ v
0 1.66 [/:: v v
X n"
u
,V v
1.6
v c 4v ]:7
1.6 -v
n. = L615- L70S} T_he large variati?n in RI is due to comp.osi.tional
1.61 I n = L6 S-L7Z 9 differences, particularly the Mg : Fe ratiO m the
1
1.5X n: = L632- L o hornblende. Ferric iron and aluminium in the Z
73
\lXI 90 XO 70 60 50 40 30 20 I 0 0 = o.o _0.02B sites will also affect both Rls and 2V
0 14
100 x Mg:(Mg + Fe2+ + FeH + Mn)
'Common' hornblende 2V. = 15°-90° - ve (Mg hornblendes are +ve with 2V, almost 90°)
OAP is parallel to (010)
D = 3.02-3.50 H = 5-6
coLouR Variable, light brown or green but much darker colours for iron-rich
Key
(b) varieties.
Na atoms P a parga site •1•J EOCHROISM Variable with a pale brown or green, {3 andy brown green. Iron-rich
Ed cdcni te
Pa Ed Ed Pa Ts tsche nnakitc varieties have a yellow brown or green, {3 deep green or blue green, and
1 1'-. !-Ia hasti ngsitc y very dark green.
"' I HABIT Prismatic crystals common, usually elongate.
""
""' I { 100} and { 001} may also be present.
hornblende ""'-hornblende""'- •n EAVAGE Usual amphibole cleavages (see introduction). Partings parallel to
Ts !-Ia "" "'.li-l a "" Ts RELIEF Moderate to high.
() """ AI 1 ERATION See introductory section.
JII KJ I·KINGENCE Moderate: maximum interference colours are low second order blues,
~-- AI atoms (replacing Si) AI atoms (replacing Mg) --..
but these are frequently masked by the body colour, especially if the
hornblende is an iron- or sodium-rich variety.
IN II RII,RENCE
HGURE Similar to tremolite-actinolite with extinction angle y'cl up to 30° (see
Figure 2.6 (a) Variation of 2V angle and indices of refraction in the 'common' I IINC'TION . .
hornblende series (after Deer, Howie & Zussman 1962) (b) field of common ANGLE IntroduCtiOn).
hornblende compositions. I WINNING 1
49