Section M 5.1.2.
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Alkali (K) Feldspar

Alkali feldspars have the composition (K,Na) [AlSi3O8]. Densities are in the range 2.55-2.63g/cm3. Colors are normally colorless or white, but pink, yellow, red, and green do occur. In thin section, alkali feldspars are colorless.

The crystal system is temperature dependant. At high temperatures, it is monoclinic, but switches at about 500oC to triclinic. This temperature varies with composition. Most natural alkali feldspars exsolve into Na-rich and K-rich phases. This can be complicated, so a variety of twinning and exosolution textures can be seen. These textures can be useful in recognition and in petrological diagnosis.

Common replacement ions in terrestrial alkali feldspars include Ba, Ti, Fe2+, Fe3+, Mg, Sr, and more rarely Mn. At 1 bar pressure, K-feldspar melts at 1150oC to leucite and liquid, under anhydrous conditions. See DHZ for reaction details. By adding water, the melting point can be reduced to about 950oC. Terrestrially, all feldspars are very susceptible to aleration due to weathering and hydrothermal activity. Although very common on Earth, the alkali feldspars are rare on the moon.

This is a late stage mineral taking up the relatively large K ions which will not fit into the main lunar rock-forming minerals (olivine, pyroxene, and plagioclase feldspar). Hence, it is found in the "KREEPy" (K, REE, P -rich) late stage basalts. Such examples (especially from Apollo 11) are non-stoichiometric in composition. High-silica highland breccias of KREEPy compositions (cf. sample 12013) have been found to have K-feldspar compositions approaching 50%. Sample 12013 contains no basalt component, but appears to have at least one component from a (currently unknown) high silica, high KREEP highland rock. Barium contents can approach 0.9 mol% for K:Na=1:1, and 2.9 mol% for K:Na=3:1 in lunar samples.


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