Chemistry Reference
In-Depth Information
At high temperature (above 700°C), the series of alkali feldspars and
of plagioclases are continuous and each of them may contain very limited
quantities of the third end member.
At medium and low temperatures, plagioclase form a complete solid
solution series. But there is a miscibility gap between K-feldspar and
albite, and this gap is more important as the temperature is lower. So
that when alkali feldspar has crystallized at a sufficiently high tempera-
ture and it cools slowly, exsolution of one mineral appears in the other:
perthite is a dominant potassium feldspar with exsolutions of albite;
antiperthite is a plagioclase feldspar with potassium feldspar exsolution,
mesoperthite is a feldspar where the proportions of the two end members
are equivalent.
At low temperatures (below 550-600°C), there is also a gap of sol-
ubility in the plagioclase between An5 and An15 (An1 and An25 below
200°C), these demixed plagioclases, forming very fine lamellar associations,
are called peristerite . There are also two other unmixing solvi: one between
An39-48 and An53-63 (Bøggild exsolutions), the other one between about
An55 and An95 (Hüttenlocher exsolutions). Peristerite, and the other exso-
lutions, cannot be distinguished under the microscope.
In each series, there are high, medium and low temperature forms.
The potassium feldspar has three polymorphs:
1
sanidine : a monoclinic, completely disordered polymorph of high tem-
perature; it occurs only in volcanic rocks.
2
orthoclase : a monoclinic polymorph of intermediate temperature.
3
microcline : a triclinic polymorph of low temperature.
Adularia is a variety of orthoclase, crystallized at low temperature in a
hydrothermal environment.
Albite and plagioclase are triclinic. Albite has a form of high tempera-
ture and one of low temperature. Plagioclases have many polymorphs; they
depend not only on temperature of formation, but also their anorthite con-
tent and their thermal history. Anorthite is completely ordered. The vari-
ous polymorphs can not be distinguished under the microscope and their
distinction requires the use of X-ray diffraction.
3.1.2.2 Stability of feldspars
Due to their open structure feldspars are not stable at high pressures and are
stable at low and medium pressure in a wide temperature range.
K-feldspar is stable at ordinary temperatures. It has an incongruent
melting producing leucite
a liquid at 990°C at ordinary pressure. Its
melting point is significantly lowered by high water pressure. For water
pressures greater than 2.5 kb, the melting becomes congruent. At (water)
+
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