Chemistry Reference
In-Depth Information
pressures over 10 kb, K-feldspar is replaced by muscovite (actually by
phengite).
At ordinary temperatures, in the presence of water and under low pres-
sures, albite is metastable; the stable form is analcime (analcime
+
quartz
=
>
albite
+
water). Towards high pressures, albite is replaced by jadeite (albite
= >
quartz). An albite melts in dry conditions at temperatures of
about 1100°C. The presence of water significantly lowers its melting point:
748°C under 5 kb of water pressure.
Anorthite is not stable at low temperature (below 500°C). It is replaced
at low pressure, in the presence of water, by zeolites: in particular heulan-
dite. (CaAl
2
Si
7
O
18
jadeite
+
⋅
6H
2
O), laumontite (CaAl
2
Si
4
O
12
⋅
4H
2
O) and wairakite
(CaAl
2
Si
4
O
12
2H
2
O) At higher pressure (5-6 kb), it is replaced by lawsonite
(CaAl
2
Si
2
O
7
(OH)
2
⋅
⋅
2H
2
O), and at even higher pressure (and temperature)
by the association kyanite (Al
2
SiO
5
)
+
zoisite (Ca
2
Al Al
2
Si
3
O
12
(OH)) or
kyanite
grossular (Ca
3
Al
2
Si
3
O
12
). It melts at 1550°C in dry conditions; its
melting point is also greatly reduced by water pressure.
The different end members of the feldspars are rarely alone in a rock and
their melting is actually that of the liquidus in the KAlSi
3
0
8
- NaAlSi
3
0
8
-
CaAl
2
Si
2
0
8
system (or in derived binary systems), these systems are strongly
dependent on the pressure water.
+
3.1.2.3 Occurrences of feldspars
Feldspars are the most abundant minerals in the Earth's crust, forming
about 60% of it.
Sedimentary rocks
Although it is a fragile mineral and do not support long transport, feld-
spar is a constituent of certain clastic rocks (arkose, greywacke). K-feld-
spar is stable in sedimentary environment. It forms either clastic fragments
or authigenic crystals (often crystallizing from clastic fragments) Albite is
metastable but appears frequently as authigenic crystals. It is also described
in some pedogenic alterations. Plagioclases are all the more rapidly altered
as they are more calcic.
Hydrothermal rocks
Albite and adularia are common in hydrothermal veins. Calcic plagioclase
never occurs in such rocks.
Albite is also formed by metasomatism of various rocks and minerals:
the result of such process is a rock formed by more than 95% of albite, albi-
tite. The albite formed at the expense of K-feldspar present, the particular
habit of chessboard albite.
