Page 108 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS OLIVINE GROUP
80
coLOUR Usually colourless, but may appear pale yellow if Fe + content very high. + ~
2
PLEOCHROISM Extremely rare; only Fa olivines may show a and {3 pale yellow, withy 90 --- ~ -- -- --- 1--- --- --
-- - -
yellow.
~ ~
HABIT In igneous rocks olivine ranges from anhedral (plutonic rocks) to sub- V> (!) r-....
(!) 80 .......
....
hedral (extrusive rocks). A rough six-sided crystal shape can be seen in on
<Vo:
(!) ~
basalts where the olivine occurs as phenocrysts. :s 70
(!)
cLEAVAGE {010} poor with rare {100} imperfect fracture. o;, ..........
RELIEF Variable depending upon composition; forsterite is moderate to high c: 60 !'..........
"'
;:,. ...........
and fayalite very high.
N ~'-.......
• ALTERATION Olivine is very susceptible to hydrothermal alteration, low grade
50
metamorphism and the effects of weathering. Mixtures of products are ...........
produced, the most common being serpentine, chlorite, talc, carbonates, 40
various iron oxides and iddingsite and bowlingite (which appear to be
mixtures of hydrated iron oxides including goethite and hematite). The 1.90
most common reaction is that of olivine altering to serpentine:
1.85 ---~
3 olivine + water + silica ~ serpentine ~
c: ny ~ ~
.2 ,....,.....
ti 1.80 v
serpentine may break down further to talc and carbonates. ~ ~ v--
v
Some alteration of olivine is usually present along irregular basal e 1.75 L..---:::: ~
.......
fractures and poor cleavages, and complete alteration can occur with the 0 ~ v v n"
X
olivine crystal totally replaced by serpentine, often with a release of iron (!) 1.70 ./ ./
'0 v
ores. c: t:::::: B
*BIREFRINGENCE High, with maximum interference colour lower third order (for iron-rich 1.65 v
olivines).
*INTERFERENCE 2V is generally very large, and a single isogyre should be examined from 60
FIGURE an isotropic section. Such a grain will not appear black on rotation but 1. 100 90 80 70 60 50 40 30 20 10 ()
Fo Fa
will appear greyish, owing to the high relief and dispersion of olivine.
The optic axis figure will show an isogyre in which the direction of
Figure 2.19 Variation of 2V angle and indices of refraction for the forsterite-
curvature is difficult to determine if 2V is very large (> 80°). Olivines
fayalite (olivine) series (after Bowen & Schairer 1935).
from most basic igneous rocks have a compositional range of Fo 85 Fa 15 to
Fo, 0 Fa 50 , which represents a 2V range of from 90° to 75° (Fig. 2.19).
EXTINCTION Straight on poor {010} cleavage or prism face. Olivines may show reaction rims against plagioclase crystals (called
TWINNING Rare in most olivines. Sometimes broad deformation lamellae parallel kelyphitic margins or corona structures) in metamorphosed basic
to (100) occur in ultramafic igneous rocks. igneous rocks or in some ultrabasic troctolites, where an olivine or
OTHER FEATURES Zoning is occasionally present but is not a diagnostic feature. Mg-rich olivine plus serpentine kernel is surrounded by successive rims of pyrox-
olivine may contain minor amounts of trivalent ions which may be ene and garnet or spinel and amphibole. In basic igneous rocks contain-
exsolved as tiny inclusions of chromite or magnetite. ing Mg-rich olivine, quartz is never present since it would have com-
OCCURRENCE Mg-rich olivine is an essential mineral in most ultra basic igneous rocks - bined with the olivine in the crystallising magma to give more
dunites, peridotites and picrites. Olivine is also common in many basic orthopyroxene; however, in some later stage iron-rich igneous rocks
igneous rocks such as gabbros, dolerites and basalts, and more Fe-rich (syenogabbros, ferrogabbros) , iron-rich olivine coexists with quartz.
olivine may occur in some undersaturated igneous rocks such as larvi-
kites, teschenites and alkali basalts. Iron-rich olivine occurs in some
ferrogabbros, trachytes and syenites. Olivine can occur in meta-
morphosed basic igneous rocks and metamorphosed dolomitic lime-
stones, in which the olivine is nearly pure forsterite .
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