5.7 Alkali Feldspar Diagram

Alkali feldspar is a solid solution mineral ranging in composition from Na-rich (albite) to K-rich (sanidine). The alkali feldspar melting diagram (Figure 5.07) has an azeotropic point. Upon cooling, melts rich in potassium crystallize alkali feldspar (sanidine) that is even more K-rich.
Water-Ethanol Phase Diagram

Figure 5.07. Alakli Feldspar Phase Diagram. The liquidus and solidus for the alkali feldspar solid solution meet at an azeotropic (minimum) point. This relationship is typical for minerals that have a solvus at lower temperatures. Click on the diagram to see a larger, interactive version .

Melts rich in sodium crystallize alkali feldspar (albite) that is even more Na-rich. Solid solution minerals with a minimum on the liquidus are also likely to have a solvus that bounds a region at lower tempera-tures where two feldspars are stable. This is true for alkali feldspar and the two-feldspar region is shown on (Figure 5.07).

When an alkali feldspar of intermediate composition is cooled slowly into the two-felspar region, the single feldspar will unmix (exsolve)and form a perthite. A perthite is an intergrowth of orthoclase and albite (or microcline and albite) formed from a single alkali feldspar crystal during cooling. Examples of perthite in thin section are shown in Figure 5.08 .
NaI-RbI Phase Diagram

Figure 5.08. Perthite. Microperthite in a thin section of an alkali feldspar granite from Cape Ann, Massachusetts. In the crossed polarized light (xpl) image, the albite portion of the crystal is at extinction showing clearly the complex intergrowth texture.

Perthite intergrowths may have compli-cated textures ranging from patchy to anastamosing lamellae, depending on the cooling history of the hosting rock. A perthite has more K-felspar (orthoclase or microcline) than albite in the intergrowth. If there is more albite than K-feldspar, the intergowth is an antiperthite.

Water under pressure (PH2O) changes the melting temperature for alkali feldspar. This effect is shown on the large version of Figure 5.07 if you use the pressure slider. PH2O has little effect on the alkali feldspar solvus, so the lowered solidus intercepts the solvus creating a solid solution eutectic.