Chemistry Reference
In-Depth Information
Potassium feldspar, orthoclase and microcline, are altered into musco-
vite by a leaching of the alkalis. The microscopic appearance of this mus-
covite is very different from the alteration of the plagioclase into sericite:
muscovite shows a habit of large crystals with very irregular outlines, called
mottled muscovite, or sometimes «spongy» or «skeletal» muscovite, it is
developed in the fractures or at the rim of the feldspar, the rest of which
remains unaltered.
Sericitization is an alteration of plagioclase in a fine muscovite, sericite.
In comparison with muscovite, sericite often has a lower content in potas-
sium and a higher one in silica and thus is close to illite. Sericite forms very
small dispersed crystals, oriented or not, affecting the whole feldspar. Serici-
tization often develops being guided by the structure of plagioclase: cleav-
ages, zoning, etc.; the more calcic parts of plagioclases, especially the core of
crystals, are more extensively sericitized.
Saussuritization in a replacement of calcic plagioclase by albite
epidote
Ca
2
(Al,Fe
3+
)Al
2
(Si
2
O
7
)(SiO
2
)(OH) (and sometimes calcite, sericite, zeolites).
Saussuritization reflects a leaching of calcium, addition of water (and ferric
iron), oxidizing conditions and a fall in temperature. It is a retromorphosis
into greenschist facies. This hydrothermal alteration is often associated with
ore deposits.
The albitization is a metasomatic transformation that develops both on
potassium feldspar (chessboard texture) and plagioclase.
Feldspars can also be altered into chlorite, pumpellyite, prehnite, some
zeolites, etc.
+
3.1.3 Feldspathoids
Feldspathoids are tectosilicates close to the feldspars but differ by a lesser
amount of silica. They are thus incompatible with the quartz, orthopyrox-
ene and pigeonite. Some feldspathoids contain salts: chlorides, sulfates, car-
bonates, sulfides. Analcime is not a feldspathoid but contains zeolitic water,
it is close to feldspathoids by its chemical composition and its occurrences.
3.1.3.1 Chemical composition
Feldspathoids are classified according to the Si/Al ratio:
Si/Al
=
2: leucite group, minerals equivalent to a feldspar
minus 1
SiO
2
:
Leucite
KAlSi
2
O
6
(tetragonal pseudocubic)
Analcime
NaAlSi
2
O
6
· H
2
O (zeolitic water) (cubic)
Si/Al
=
1: minerals equivalent to a feldspar
minus 2
SiO
2
:
without additional salts (hexagonal minerals):
