Orthoclase (alkali feldspar)

Two cleavages at ninety degrees

Feldspar is a framework silicate, and its crystal structure contains two planes of weakness at right angles. Break a crystal and you get a stepped surface with square corners — not the curved glassy fracture of quartz and not the three-direction rhombus of calcite. In practice this is the observation that resolves most feldspar identifications. Add the hardness: at 6 (per National Park Service), orthoclase is soft enough to take a white powder line from a steel knife, which immediately separates it from the quartz it is usually found next to.

Recognising the alteration state

Feldspar is chemically unstable at the surface, and the degree of alteration is genuinely useful information about how long a rock surface has been exposed.

• Fresh: glassy, slightly pearly on the cleavage face, with the cream or pink colour clearly visible.
• Partly altered: the surface turns opaque and cloudy white, and a hand lens shows tiny flakes of white mica (sericite) sitting on the cleavage planes.
• Fully kaolinised: soft, chalky, white, and it will rub off on your fingers. At this stage the feldspar is now largely kaolinite clay.

Twins, and why they are worth recording

Feldspar twins more readily than almost any other common mineral, and the twin laws are diagnostic. Carlsbad twins produce a single crystal divided into two halves with different orientations, giving a blocky specimen with an internal seam. Baveno twins share a single plane. Manebach twins give a characteristic flattening. In thin section, the alternating light and dark stripes of albite twinning are the standard way plagioclase is identified, and the geometry of the twinning is what lets petrologists determine the composition of a plagioclase grain without a chemical analysis.

Where it comes from and what it becomes

Orthoclase crystallises from silica-rich melts and is one of the two main feldspars in granite, granodiorite and rhyolite, along with plagioclase. The largest and best-formed crystals come from pegmatites, where water-rich residual melt allows crystals to grow for a long time in open cavities; crystals over a metre across occur. When orthoclase is heated above about 500 degrees Celsius it becomes disordered and is properly called sanidine, the high-temperature form found in volcanic rocks; cool it slowly and it becomes microcline, which is what most orthoclase in ancient granites actually is, and which shows a distinctive cross-hatched twinning pattern under the microscope. The green variety, amazonite, gets its colour from lead and water in tiny amounts within the crystal structure — a colour that fades if the crystal is heated.

A mineral that matters economically

Feldspar is quarried in large quantities for the glass and ceramics industries: it acts as a flux, lowering the melting temperature of the silica mix and providing aluminium at the same time. Most commercial feldspar comes from pegmatites, which can be mined for feldspar, mica and quartz simultaneously. As a field collector, the useful habit is to note the twin law, the alteration state and the associated minerals — a pegmatite with large orthoclase, muscovite and quartz is a classic combination, and the presence of beryl, tourmaline or tantalite in the same body is what makes such outcrops worth reporting.

Sources

Hardness, streak, cleavage and specific-gravity values quoted on this sheet are checked against the references below.

• Geology.com Mineral Encyclopedia — mineral property database — hardness, cleavage, streak, specific gravity and chemical composition per mineral
• Geology.com Mohs Hardness Scale — reference article by a PhD geologist — Mohs scale, field hardness tests, hardness of common minerals
• National Park Service — U.S. National Park Service geology pages — mineral definition, physical properties and common minerals
• Tulane University Silicate Structures — university mineralogy course notes — crustal element abundance and silicate structural groups