Systematic mineralogy - Mineralogy for Petrologists - page 105

varies from Fo 90/89 to Fo 82/73. It is missing in the intermediate terms.
Iron-rich olivines only appear in very differentiated terms, if any.
In mafic-ultramafic stratiform plutonic complexes of Bushveld/Skaer-
gaard type, olivine appears in the poorly differentiated and cumulative basal
parts of the complexes: Fo 90-86 in bronzitites, harzburgites, dunites of the
basal zone of the Bushveld complex; Fo 67-53 in the gabbros of the lower
part of the Skaergaard intrusion. It is lacking in the middle parts of these
complexes. Iron-rich olivines appear in the upper parts: Fo 45-5 in the fer-
rogabbros, ferrodiorites, hortonolites (=iron-rich olivine dunite) from the
upper zone of the Bushveld complex; Fo 40-0 ferrogabbros and ferrodior-
ites from the upper zone and border group of Skaergaard.
In the calc-alkaline series, olivine only appears in very little evolved
terms (basalt, basaltic andesite).
In alkaline volcanic rocks , there are two generations of olivine: early
phenocrysts and microlites.
The most common alkaline series are of basalt/basanite-hawaiite-
mugearites-benmoreites-trachyte-phonolite association type. Most times
olivine is important in basalts/basanites and hawaiites and ceases to crys-
tallize in more evolved terms. Its composition varies in these rocks from
Fo 85-80 to about Fo 55-50. In some series, olivine is also present in the
most evolved terms, trachytes and even up to phonolites. Compositions are
up to Fo 35-27. Fayalite (Fo 15-5) is present in some alkaline rhyolites
(pantellerites, comendite). Fayalite (For 17-0) also appears in differentiated
alkaline plutonic rocks , silica-saturated or not (nepheline syenite, quartz
syenite, alkali granite); it is associated with sodic amphiboles, rich in iron
(riebeckite-arfvedsonite) or not so rich (hastingsite) and with hedenbergite.
Magnesian olivines are also important in the peralkaline basic rocks of
the nephelinite family (nepheline basalts, leucitites, melilite basalts) (Fo 92
to 70 in differentiated terms) and in the kimberlites (For 95-85) and lam-
proites (ultrapotassic hypabyssal and extrusive rocks with forsterite, leucite,
sanidine, phlogopite).
Olivine is a common constituent of some rocks of alkaline mafic-ultra-
mafic complex . Compositions range from Fo 93-77 in carbonatites, alnoites
(melilite lamprophyres) and melilitites.

Metamorphic rocks
Forsterite appears in high grade regional or contact metamorphism of
impure dolomitic limestones, dolostones and in skarns.
Fayalite is common in metamorphosed iron formations, especially in
the banded iron formations (BIF).
Prograde metamorphism of serpentine produces olivine (with talc,
chlorite, tremolite, magnesian amphibole). Hydration of pyroxenites
(Cpx + Opx) produces the paragenesis olivine + calcic amphibole.