Pink granite, Mohs hardness and moonstone (2024)

What Is Orthoclase?

Orthoclase is a feldspar mineral with a chemical composition of KAlSi₃O₈. It is one of the most abundant rock-forming minerals of the continental crust. Orthoclase is most widely known as the pink feldspar found in many granites and as the mineral assigned a hardness of "6" in the Mohs hardness scale.

Uses of Orthoclase

Orthoclase has several commercial uses. It is a raw material used in the production of glass, ceramic tile, porcelain, dinnerware, bathroom fixtures, and other ceramics. It is used as an abrasive in scouring powders and polishing compounds. It is also cut as a gemstone. An adularescent gem material known as moonstone is an intergrowth of orthoclase and albite.

Geologic Occurrence of Orthoclase

Most orthoclase forms during the crystallization of a magma into intrusive igneous rocks such as granite, granodiorite, diorite, and syenite. Significant amounts of orthoclase are also found in extrusive igneous rocks such as rhyolite, dacite, and andesite.

Large crystals of orthoclase are found in igneous rocks known as pegmatite. They are normally no more than a few inches in length, but the largest reported orthoclase crystal was over 30 feet in length and weighed about 100 tons. It was found in a pegmatite in the Ural Mountains of Russia.

During physical weathering, grains of orthoclase are incorporated into sediments and sedimentary rocks such as sandstone, conglomerate, and siltstone. Chemical weathering alters orthoclase into clay minerals such as kaolinite in reactions similar to the one shown below.

Orthoclase is also a significant constituent of the metamorphic rocks known as gneiss and schist. These rocks most often form during regional metamorphism when granitic rocks are subjected to heat and pressure at convergent plate boundaries involving continental crust. The orthoclase in these metamorphic rocks is inherited from their igneous protoliths.

Orthoclase on the Moon and Mars

Orthoclase is also known in igneous rocks found on the moon and on Mars. Orthoclase is an important constituent of igneous rocks brought back from the moon by astronauts. It has also been detected in the igneous rocks of Mars during analyses done by NASA’s rovers.

Orthoclase as a Feldspar Mineral

Orthoclase is a member of the alkali feldspar series. The alkali feldspars include albite (NaAlSi₃O₈), anorthoclase ((Na,K)AlSi₃O₈), sanidine ((K,Na)AlSi₃O₈), orthoclase (KAlSi₃O₈), and microcline (KAlSi₃O₈).

These feldspar minerals form a solid solution series between NaAlSi₃O₈ and KAlSi₃O₈. The minerals in that series crystallize from melts that usually contain both sodium and potassium ions. At the time of crystallization, these ions can substitute freely for one another in the mineral’s crystal structure. Because of this the alkali feldspars exist in a range of chemical compositions between pure albite (NaAlSi₃O₈) and pure orthoclase (KAlSi₃O₈). A chart summarizing their continuum of compositional relationships is shown.

Because orthoclase is rich in potassium and an end member of the alkali feldspar series, many geologists call it "K-spar," "K-feldspar" or "potassium feldspar."

Physical Properties of Orthoclase

All feldspar minerals are usually translucent to transparent, display two directions of cleavage that intersect at approximately 90 degrees, have a vitreous to pearly luster on cleavage faces, and have a specific gravity between about 2.5 and 2.6. Because of these similarities, the feldspar minerals can be challenging to identify with absolute confidence in the field or the introductory classroom. This becomes more difficult when their crystals are part of an igneous rock with a grain-size of just a few millimeters or less. Special mineralogical or gemological testing equipment is often needed to positively identify the feldspar minerals.

Orthoclase Gemology

As a mineral with a Mohs hardness of 6 and two directions of perfect cleavage, orthoclase is not an especially durable gemstone. It will develop abrasions if used in most types of jewelry, and it can easily cleave upon impact. For these reasons, orthoclase is more of a "collectors gem" than a gem for use in jewelry.

Transparent Orthoclase

Transparent orthoclase with superior clarity is sometimes faceted and sold as a collector’s gem. These gems commonly range from colorless to a bright yellow color. If the specimen is a well-formed crystal, it will likely have a much higher value if sold as a mineral specimen than as a cutting rough.

Moonstone

Moonstone is the most famous orthoclase gem. Moonstone is a translucent to transparent material that consists of alternating layers of orthoclase and albite feldspar. As light penetrates a moonstone cabochon, some of that light scatters at the boundaries between the two interlayered feldspar materials. The scattered light illuminates the stone and produces a phenomenal glow that appears to move beneath the cabochon’s surface. The glow appears to move as the light source is moved, or as the stone is moved, or as the observer chances his angle of observation.

The glow is usually white in color and is the source of the "moonstone" name. The gemological name used for this phenomenon is "adularescence," which is derived from "adularia," an old European name for moonstone.