Microcline

Microcline is a potassium feldspar with the same chemical composition as orthoclase (KAlSi3O8) but a more ordered, lower-temperature crystal structure (triclinic rather than monoclinic). The name means "small slope," referring to its cleavage angle being slightly off 90 degrees.

It is one of the most widespread minerals in granitic rocks and is best known for amazonite, its blue-green to green gem variety colored by trace lead and structural water. Microcline can form enormous crystals in pegmatites, among the largest single crystals known.

It commonly contains perthitic intergrowths of albite that appear as streaks or wisps within the crystal, a clue to its origin from slow cooling and exsolution.

Microcline forms from the slow cooling and ordering of potassium feldspar in plutonic igneous rocks such as granite and syenite, and especially in granite pegmatites, where it can grow into crystals meters across. The ordered triclinic structure develops at relatively low temperatures over long timescales, so microcline is favored in slowly cooled deep-seated rocks.

It is also stable in many metamorphic rocks like gneiss. Amazonite-bearing pegmatites occur in Colorado (Pikes Peak), Virginia, Brazil, Madagascar, Russia, and Ethiopia. Perthitic albite lamellae within microcline form by exsolution as the host cools and potassium and sodium feldspar unmix.

Microcline is a hard (Mohs 6-6.5) feldspar with two good cleavages meeting near 90 degrees, a vitreous luster, and a white streak. Color ranges from white and cream to pink and salmon, with the green to blue-green amazonite variety being instantly recognizable.

Under magnification microcline often shows a characteristic cross-hatched (tartan) twinning, diagnostic when seen under a microscope. Perthitic streaking from exsolved albite may be visible to the eye.

It is essentially indistinguishable from orthoclase in hand specimen except by the tartan twinning or the amazonite color. Distinguish it from quartz by cleavage and from plagioclase by the absence of simple parallel albite striations on common faces.

Microcline is an important industrial feldspar, used in ceramics, porcelain, glass, glazes, and as a mild abrasive, where it provides potassium and alumina and acts as a flux.

Its amazonite variety is widely used in jewelry as cabochons, beads, and carvings, valued for its soothing green-blue color, often with white albite streaks. Large, well-formed pegmatite crystals are prized by mineral collectors.

Amazonite is popular in metaphysical circles as a stone associated with communication, calm, and balance, though these are traditional beliefs and not scientifically demonstrated. As a major rock-forming mineral, microcline is also significant in geology and petrology.