Microcline

Information about Microcline

Microcline is one of the most common rock-forming minerals on earth – a potassium aluminium silicate feldspar found in granites, pegmatites, and a wide range of other igneous and metamorphic rocks worldwide. As a plain grey or white mineral in a piece of granite it is familiar without being exciting; but microcline also includes one of the most celebrated and striking gemstone varieties in mineralogy – amazonite – and produces some of the largest individual mineral crystals ever recorded anywhere on earth.

As a plain mineral specimen, microcline most commonly forms as blocky, prismatic crystals or as massive material in pale grey, white, cream, or salmon-pink tones, with a vitreous lustre and two good cleavages.

Amazonite – the vivid blue-green to green variety of microcline – is in an entirely different category as a collector and lapidary material. Its striking colour, caused by trace amounts of lead and water in the crystal structure, makes it one of the most instantly recognisable feldspar varieties, and fine crystallised amazonite specimens from Colorado or Russia command considerable prices. Despite its name, amazonite is not found anywhere in the Amazon Basin; the name arose from a confusion by early Spanish explorers between this green mineral and other green stones they had encountered in that region.

Microcline is the fully ordered, lowest-temperature form of potassium feldspar – the end member of a structural series that also includes orthoclase (partially ordered, the most common form) and sanidine (disordered, found in rapidly cooled volcanic rocks). The name comes from the Greek mikros (“small”) and klinein (“to slope”), referring to the very slight deviation from a right angle between its two cleavage directions – a feature that distinguishes it structurally from orthoclase, whose cleavages meet at exactly 90 degrees, though the difference is too small to see with the naked eye.

Uses and History

Microcline and the feldspars more broadly are among the most industrially important minerals in the world. Feldspar is the primary raw material for porcelain and ceramic manufacture, and is also used extensively in glass production, as a mild abrasive, and as a filler in paints and rubber. The world feldspar mining industry produces tens of millions of tonnes per year, much of it microcline or closely related orthoclase.

Amazonite has been used as a decorative and lapidary material since antiquity. It was used in ancient Egypt for carvings and amulets – beads of amazonite have been found in the tomb of Tutankhamun – and it has been mined continuously from the Ural Mountains of Russia for centuries. It is still widely used today in jewellery, carvings, and decorative objects.

Microcline was formally described as a mineral species in 1830, and the name was coined in reference to its crystallographic properties. Its structural relationship to orthoclase and sanidine – and the fact that all three are essentially the same chemical compound in different states of atomic ordering – was only fully understood in the 20th century with the development of X-ray diffraction.

One of microcline’s most extraordinary distinctions is in the record books for crystal size. The largest documented single mineral crystals ever recorded anywhere on earth were microcline – a single crystal from the Devil’s Hole Beryl Mine in Colorado, USA, measured approximately 50 by 36 by 14 metres and weighed an estimated 14,000 tonnes. Even setting aside such extreme examples, pegmatite microcline crystals weighing several tonnes are not uncommon.

Notable localities for collectable microcline include the Pikes Peak and Crystal Park districts in El Paso County, Colorado, USA, which produce the finest amazonite specimens known – vivid blue-green crystals to 15 cm or more, often combined with smoky quartz and albite; Konso in Ethiopia, a more recent source of excellent amazonite; Ilmen and the Ural Mountains in Russia, classic sources of both plain microcline and amazonite; Baveno in Italy, the type locality for the Baveno twin law; Kragerø in Norway; Itinga in Minas Gerais, Brazil; and Papachacra in Catamarca, Argentina. In Wales, the Museum of Wales records microcline from the Precambrian Coedana Granite of central Anglesey, where crystals up to 40 mm long occur in the porphyritic facies of the granite; from the Sarn Complex on the Llŷn Peninsula in Gwynedd, where both perthitic and non-perthitic microcline occurs in the Sarn Granite; from north-east Anglesey, where metamorphic microcline occurs in calc-silicate rocks in the area around Bryn Fuches farm; and from the St David’s granophyre in Pembrokeshire.

Mineralogy

Hazards and Warnings

No specific health risks have been recorded for microcline. Mineral collectors should wash their hands after handling specimens as a matter of good practice.

Almost all rocks, minerals (and, frankly, almost all other substances on earth) can produce toxic dust when cutting, which can cause serious respiratory conditions including silicosis. When cutting or polishing rocks, minerals, shells, etc, all work should be done wet to minimise the dust, and a suitable respirator or extraction system should be used.

Translations

Further Reading / External Links

• https://en.wikipedia.org/wiki/Microcline
• https://www.mindat.org/min-2704.html
• https://museum.wales/mineralogy-of-wales/database/?mineral=320&name=Microcline
• https://www.handbookofmineralogy.org/pdfs/microcline.pdf
• https://www.britannica.com/science/microcline