Silicates: Alkali Feldspars

Composition: (K,Na)AlSi₃O₈; minor calcium may substitute for potassium and sodium

Crystal system: Monoclinic or triclinic; crystals prismatic to tabular, also as irregular grains or cleavable masses

5.1.1
pink microcline
Ontario Canada

Properties:

• Vitreous to silky lustre
• Variable color; colorless, white or pink, also green
• White streak
• Knife does not scratch (Mohs Hardness 6)
• Average density
• Two perfect cleavages at close to 90^o
• Cleaves rather than fractures
• Not ferromagnetic
• Does not react with hydrochloric acid (HCl)

Occurrence:

Feldspars are one of the most common minerals in the earth's crust. Sanidine and orthoclase, the high temperature alkali-feldspar varieties, occur in alkali-rich igneous rocks such as rhyolite and granites. Microcline, the lower temperature alkali-feldspar variety, occurs in granites, pegmatite dikes and hydrothermal vein deposits. Orthoclase and microcline also occur in some metamorphic rocks and as discrete grains in immature sandstones

5.1.2 pink microcline Ontario Canada	5.1.3 perthite structure in microcline

Minor colorless or white plagioclase intergrown with pink microcline is termed perthite. The alkali feldpars are distinguished from the plagioclase feldspars by the presence of lamellar twinning in the latter and the common presence of perthite structure in the former. Specialised characterisation is often required to distinguish the various alkali feldspars from each other.

5.1.4 white microcline cleavage rhomb Londonderry WA Australia	5.1.5 orthoclase crystals Qld Australia

Feldspars are important industrial minerals used mainly in the manufacture of glass and ceramics.

5.1.6
orthoclasecrystals
Qld Australia