Chemistry Reference
In-Depth Information
3.2.5.4 Alterations of olivine
Iddingsite is an alteration product of olivine of volcanic rocks; it appears as
a reddish (orange, red brownish, brown, black) fringe located at the periph-
ery of the crystals or along their fractures. Iddingsite is an aggregate made
of cryptocrystalline goethite, hematite, smectite (saponite, montmorillonite)
and sometimes quartz, calcite and magnetite. It is more or less pleochroic
depending on the more or less ordered orientation of goethite and the sheet
minerals. The transformation of olivine into iddingsite involves oxidation,
hydration and some leaching of magnesium. It is most often a deuteric alter-
ation due to the effect of fumaroles on the lava.
Olivine is also altered into greenish cryptocrystalline aggregates (called
bowlingite), made of chlorite, smectite (saponite), and possibly serpentine,
talc, brucite, mica. etc. This may be a deuteric alteration, like iddingsitiza-
tion, but also a supergene alteration.
Chlorite, more or less largely crystallized, often with Prussian blue inter-
ference colors, may replace olivine.
Serpentinization is a very common and spectacular alteration of ultra-
mafic rocks.
3.2.6 Serpentine and serpentinization
Serpentines are phyllosilicates composed of kaolinite-type TO layers with a
tetrahedral layer and octahedral layer. The formula of serpentine is:
Mg 3 Si 2 O 5 (OH) 4
with very low amounts of iron replacing magnesium and, traces of Cr, Mn,
Co, Ni.
The most common variety of serpentine is antigorite . The bastite is an
antigorite that appears as a more or less complete pseudomorph of orthopy-
roxene. Chrysotile is the fibrous variety, used once to asbestos. Lizardite
is a very fine grained, scaly variety. Some varieties have qualities of semi-
precious stones imitating jade (“bowenite”, “serpophite”).
Experimental data indicate temperatures of transformation of for-
sterite into antigorite (with talc or brucite) in the range from 450-600°C,
depending on water pressure. But the temperatures of serpentinization in
natural systems are apparently lower: antigorite is formed at temperatures
of about 220-480°C, lizardite and chrysotile at temperatures of about
85-115-250°C.
Serpentinization is the transformation of ultramafic rocks, with oliv-
ine and orthopyroxene (dunite, harzburgite, lherzolite) in rocks formed of
serpentine, magnesian chlorite, talc, brucite, magnetite, magnesite and/or
dolomite. This transformation involves hydration and is likely accompanied
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