Monoclinic crystals of Orthoclase that have weathered out of a porphyritic granite. Note the twinning displayed by the bottom crystals.
Orthoclase cleaves readily in two directions as shown in this sample.
Various views of a Carlsbad twinned
crystal.
CHEMISTRY KAlSi3O8 Potassium aluminum silicate
CRYSTALLOGRAPHY Monoclinic
CRYSTAL GROWTH AND HABITS Orthoclase forms prismatic crystals that are typically somewhat stubby or tabular. Crystals are often twinned according to Carlsbad, Baveno or Manebach twin laws. Orthoclase can also be granular or massive.
COLOR AND OTHER OPTICAL PROPERTIES Orthoclase can be colorless, white, tan, light yellow, salmon, buff, and even green. It may show some opalescence and is transparent to translucent.
HARDNESS 6 - 6.5 SPECIFIC GRAVITY 2.5 - 2.6
LUSTER -Vitreous to pearly on cleavage faces STREAK White
BREAKABILITY Very good cleavage in 2 directions producing blocky fragments, brittle
OCCURRENCE Orthoclase is a widespread mineral in felsic igneous rocks like granite, granitic pegmatites and syenites. It is also found in cavities in basalt and in high temperature metamorphic rocks as a result of potassium rich metasomatic fluids.
ASSOCIATED MINERALS Albite, Quartz, Muscovite, Biotite, Hornblende, Schorl, Beryl