Origin of Name
From the Greek word orthos (right angle) and klasis (to break), referring to this mineral’s perpendicular cleavages.
Hand Specimen Identification
The luster, hardness, and color of orthoclase are similar to other feldspars, but (in contrast with plagioclase) orthoclase is frequently pink or flesh colored, has cleavage planes that meet at 90°, and does not show twin striations. Association with other felsic minerals also helps identification. Distinguishing orthoclase from its polymorphs, microcline and sanidine, can be very difficult without X-ray or optical data. It is sometimes confused with calcite or corundum but can be distinguished by its hardness.
Physical Properties
| Hardness | 6 |
| Specific Gravity | 2.56 |
| luster/transparency | Pearly, Vitreous/translucent |
| Color | White, Pink, Turbid |
| streak | white |
| Fracture | Uneven |
| Cleavage | 90⁰ cleavage angle; perfect (001), good (010), poor (110) |
Optical Properties
In thin section, orthoclase has low birefringence, moderate relief, and resemblance to quartz. However, it has negative relief, is biaxial, and is often clouded by fine-grained alteration. It is distinguished from sanidine by 2V and from microcline by its lack of plaid twinning. Biaxial, Alfa = 1.525, Beta = 1.528, Gama = 1.528, Dealta = 0.007, 2V = 60° to 65°
Crystallography
Monoclinic, a = 8.56, b = 12.99, c= 7.19, Beta = 116.01°, Z = 4; space group C2/m; point group 2/m.
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Habit
Crystals are prismatic, stubby to elongate, and may be flattened or doubly terminated. Penetration twins and contact twins are common.
Structure and Composition
The structure of orthoclase consists of a three dimensional framework of SiO4 and AlO4 tetrahedra, K+ ions occupy available holes between the tetrahedra. Most orthoclase contains some Na replacing K; complete solid solution between orthoclase and albite (NaAlSi3O8) is possible only af high temperature. Some orthoclase contains small amounts of CaAl replacing NaSi.
Occurrence and Associations
Orthoclase is common in many kinds of silicic igneous rocks, sediments such as arkoses, and a variety of metamorphic rocks. Quartz and micas are typically associated minerals.
Related Minerals
The principal K-feldspar polymorphs are sanidine (high-temperature form), orthoclase (moderate temperature form), and microcline (low-temperature form). They differ in the way SiO4 and AlO4 tetrahedra are arranged in their structure. Several other related minerals are known: Adularia is a colorless, transparent form of K-feldspar that forms prismatic crystals. If it shows opalescence, we call it moonstone. Perthite is a form of K-feldspar containing exsolved patches or lamellae of albitic feldspar.