Chemistry Reference
In-Depth Information
0.50 wt% nickel - it is the origin of New Caledonian type lateritic nickel
deposits. Chromium levels are generally below 0.1 wt% except in olivines
from komatiites where they reach 0.33 wt%. The manganese contents are
lower than 0.50 wt% (except in certain alkaline acidic rocks where they
reach 2 wt%). Peridot is a gem-quality forsterite.
Forsterite is the major constituent of the upper mantle. At greater
depths, it is replaced by two polymorphs: beyond 410 km by wadsleyite (a
sorosilicate) and beyond 520 km by ringwoodite (with spinel structure).
Magnesian olivines are an important constituent of mafic and ultrama-
fic rocks and metamorphic impure dolomitic limestones. The iron-bearing
olivine is rarer and occurs in differentiated igneous rocks of the alkaline and
tholeiitic series.
Minerals of the fayalite-
knebellite
FeMnSiO
4
-
tephroite
Mn
2
SiO
4
series
are rare and occurs almost only in the metamorphosed (iron) - manganese
deposits.
Monticellite
CaMgSiO
4
is a mineral of metamorphic limestones, skarns
and some rare alkaline rocks, carbonatites, alnoites (melilite lamprophyre).
It will be treated with calcic and magnesian minerals.
3.2.5.2 Stability of olivines
Melting point of forsterite is at 1890°C, that of fayalite at 1205°C. The
phase diagram is that of the simple, continuous series type: early olivines
are rich in magnesium and they are progressively enriched in iron during
fractional crystallization.
The Mg
2
SiO
4
-SiO
2
system shows a peritectic point at 1560°C with reaction
between olivine and the liquid to form enstatite (under a pressure of 1 atmos-
phere). At higher pressure there is a eutectic point between forsterite and
(proto)-enstatite. The system Fe
2
SiO
4
-SiO
2
shows a eutectic point at 1177°C.
The boundary between the two types of phase diagrams is poorly known.
Forsterite is therefore incompatible with quartz, whereas fayalite is.
Towards lower temperatures, stability of olivine is limited by the appear-
ance of hydrated phases: ferro-magnesian amphiboles (Mg, Fe)
7
Si
8
O
22
(OH)
2
,
serpentine Mg
3
Si
2
O
5
(OH)
4
, talc Mg
3
Si
4
O
10
(OH)
2
, brucite Mg(OH)
2
and/or
carbonate (dolomite, magnesite). Formation of these phases is controlled
not only by temperature but by the fugacities of H
2
O and CO
2
and chemical
potential of SiO
2
.
3.2.5.3
Occurrences of olivines
Meteorites
Olivine is a major constituent of chondrites. Its composition varies depending
on the type of chondrite: Fo 100-95 in enstatite E chondrites, Fo 84-80 in
