Orthoclase

KAlSi₃O₈, is an important tectosilicate mineral which forms igneous rock.

Its two cleavage planes are at right angles to each other. The gem known as moonstone is largely composed of orthoclase.

Orthoclase is a common constituent of most granites and other felsic igneous rocks and often forms huge crystals and masses in pegmatite.

Typically, the pure potassium endmember of orthoclase forms a solid solution with albite.

While slowly cooling within the earth, sodium-rich albite lamellae form by exsolution, enriching the remaining orthoclase with potassium. The resulting intergrowth of the two feldspars is called perthite.

The higher-temperature polymorph of KAlSi₃O₈ is sanidine. The lower-temperature polymorph of KAlSi₃O₈ is microcline. Adularia is a low temperature form of either microcline or orthoclase originally reported from the low temperature hydrothermal deposits. The optical effect of adularescence in moonstone is typically due to adularia.

In general, keys to identifying K-feldspar are its (lack of) color, its low birefringence, and its twinning. In thin section, microcline, orthoclase and sanidine are distinguished by their twinning, optical sign, and 2V^o.

Important properties

·Appearance and habit - Microcline and orthoclase are generally anhedral; sanidine is more likely to form euhedral phenocrysts. K-feldspar crystals are typically tabular and often elongate. Exsolved Na-plagioclase may give them a wormy or ribbony appearance (perthite). Although there are two good cleavages, they may not be well displayed.

·Color - colorless, but may be altered giving it a grainy or grayish appearance. Orthoclase is frequently cloudy due to incipient alteration; sanidine is generally clear.

·Interference colors - birefringence is low, typically only white to gray interference colors.

·Twinning common - Microcline is often characterized by a distinctive plaid (cross-hatched) twinning with twin domains that pinch and swell. Orthoclase and sanidine may display simple twins that divide grains into two domains. When untwinned, distinguishing the different K-feldspar varieties may be difficult.

·Intergrowths - Perthite is created when an original high temperature feldspar unmixes (exsolves) into two compositions. Perthites grains consist of zones of Na-rich plagioclase alternating in complicated patterns with zones of K-feldspar.

·Interference figure - Obtaining an interference figure for microcline is difficult due to twinning. Orthoclase is biaxial (-) with a large 2V. Sanidine is biaxial (-) with a very small 2V (pseudouniaxial sometimes).

Similar minerals:

·K-feldspar is similar to quartz, but quartz lacks cleavage, lacks twinning, does not alter, exhibits undulatory extinction, and is uniaxial.

·K-feldspar can be difficult to distinguish from plagioclase. None of the K-feldspars develop parallel sided polysynthetic twins, but sanidine and orthoclase both may show simple twinning that appears similar to plagioclase. Microcline often shows distinctive "tartan plaid" twinning, but plagioclase may have two sets of twins yielding a similar appearance. In plagioclase, the contacts between twins tend to be sharper, and the twins do not pinch and swell.

·K-feldspar is similar to quartz, but quartz lacks cleavage, lacks twinning, does not alter, exhibits undulatory extinction, and is uniaxial.