Chemistry Reference
In-Depth Information
Post-skarn hydration stages
: epidote replaces the garnet; amphibole
(actinolite) replaces pyroxene; alteration of periclase into brucite, talc, serpen-
tine on the olivine, chlorite, carbonate. Crystallization of hydrous minerals
like pectolite. It is at this stage that the main mineralization (W, Sn, Be, Mo,
B, Cu, Pb, Zn, Au, Ag, U, REE, etc.) associated with skarn, are formed.
The only known occurrence of charoite is rocks which have suffered
a potassium metasomatism at the contact of a nepheline-aegyrine syenite
of the Murun massiv, between the Chara and Olekma rivers in Yakutia,
Russia.
Metamorphism and alterations of the ultrabasic
rocks - rodingites
The moderate temperature metamorphism of ultramafic and ultrabasic
(peridotite, serpentine) leads to the formation of rocks composed of talc,
chlorite, amphibole (tremolite, anthophyllite), called steatite or soapstone.
Hydration and carbonation of serpentine form a paragenesis talc
mag-
nesite. Brucite also appears in veins in serpentinite. Other carbonates (cal-
cite, dolomite, and aragonite) may also appear.
Rodingites
are rocks formed of grossular (or hydrogrossular), a pyroxene
of the diopside-hedenbergite-aluminous diopside series and, in lesser quanti-
ties, vesuvianite, epidote, scapolite, ect, prehnite, magnetite/hematite. These
rocks form veins and masses in massive serpentine. They likely originated
from veins of gabbro or dolerite that underwent calcic metasomatism.
+
Very high pressure metamorphism
White schist facies, of very high pressure and low temperature is defined by
the paragnenesis talc
+
kyanite:
chlorite
+
quartz
talc
+
kyanite
+
H
2
O
3.4.2.3 Igneous rocks
Some of the minerals discussed in this chapter are found in various igne-
ous rocks: allanite is common accessory mineral in intermediate to acidic
igneous rocks rich enough in calcium (granite, granodiorite, quartz diorite,
syenite, monzonite); scapolite exceptionally appears in some pegmatites.
Other calcic and magnesian minerals that occur in igneous roks, are
specific to alkaline rocks, many of them strongly under-saturated in silica.
Alkaline rocks are under-saturated in aluminum mainly in respect to alkalis
(sodium and/or potassium), there are also rocks under-saturated in alumi-
num in respect to calcium.
