Microcline
Microcline is a member of the potassium (K) Feldspars of the Feldspar Group of minerals that also includes Albite, Andesine, Anorthite, Bytownite, Hyalophane, Labradorite, Moonstone, Oligoclase, Orthoclase, Sanidine, and Sunstone. Microcline is a potassium-rich alkali Feldspar and is an important igneous rock-forming tectosilicate mineral. Microcline forms some of the largest crystals known, including one believed to weigh over 17,500 tons (15,908,890 kg) and measuring 162 x 118 feet (49 x 36 m) found at the Devils Hole Beryl Mine, Fremont Couty, Colorado, U.S.A (Peter C. Rickwood, “The Largest Crystals”, American Mineralogist, Volume 66, pages 885-907, 198I).
Microcline was named in 1830 by Johann Friedrich August Breithaupt from the Greek words μικρός (mikron) meaning little and κλινειν (klinein) meaning to incline, in allusion to the slight deviation of the cleavage planes from 90 degrees. Microcline is found as three varieties; Amazonite, Chesterlite and Ferruginous Microcline. Another “type” of Microcline is Perthite. Amazonite is, by far, the most well known and popular variety of Microcline and is readily available as mineral specimens and gemstones.
Amazonite is the pale green to a bluish green variety of Microcline. Some of the most beautiful Amazonite crystals come from the Pike’s Peak and Crystal Peak areas of Colorado, USA. Amazonite is almost always opaque and makes for beautifully colored cabochons. However, a recent find in Mogok, Myanmar (Burma) has produced a small number of extremely rare transparent crystals that have been faceted into gems. Another recent find in Vietnam has produced vivid green crystals that have been faceted into beautiful gems of amazing clarity and color.
Chesterlite is a minor variety of Microcline that is white to pale tan and found only at the Poorhouse Quarry, West Bradford, Chester County, Pennsylvania, USA. Ferruginous Microcline is a rare iron (Fe) the rich variety of Microcline that is found in shades of red, red-brown and pinkish-red. Mindat.org does not list any localities for Ferruginous Microcline.
Perthite is not actually a variety of Microcline but an intergrowth of Albite or Oligoclase within a Microcline host, occasionally also within an Orthoclase host. The intergrowths are visible as white ribbons of Albite running through a tan to pale pink Microcline host. Perthite was originally described from near Perth, Lanark County, Ontario, Canada. Perthite is also found at a few localities in Norway and the USA.
Microcline distribution: a widespread mineral. Notable occurrences include: at Fredriksvärn, Arendal, and Larvik, Norway. In the Ilmen Mountains, Ural Mountains, and on the Kola Peninsula, Russia. At St. Gotthard, Ticino, Switzerland. On Mt. Greiner, Zillertal, Tirol, Austria. At Baveno, Piedmont, Italy. In the USA, at Amelia, Amelia County, Virginia; Haddam, Middlesex County, Connecticut; and Magnet Cove, Hot Spring County, Arkansas. In Colorado, in the Pikes Peak area, El Paso County, Crystal Peak, Teller County, with large crystals from the Devil’s Hole beryl mine, Fremont County; in the Black Hills, Pennington and Custer Counties., South Dakota. At Bancroft, Ontario, Canada. From Klein Spitzkopje, Namibia. In Brazil, from Minas Gerais, at Fazenda do Bananal, Salinas, Urucum, and Capelinha. At Ambositra, Madagascar. From Kimpusan, Yamanshi Prefecture, and Tanakamiyama, Otsu, Shiga Prefecture, Japan. At Broken Hill, New South Wales, Australia.
| Category: | Tectosilicate |
| Chemical Formula: | KAlSi3O8 |
| Potassium Aluminum Silicate | |
| Molecular Weight: | 278.33 gm |
| Composition: | Potassium | 14.05 % | K | 16.92 % | K2O |
| Aluminum | 9.69 % | Al | 18.32 % | Al2O3 | |
| Silicon | 30.27 % | Si | 64.76 % | SiO2 | |
| Oxygen | 45.99 % | O | |||
| 100.00 % | 100.00 % | = TOTAL OXIDE |
| Crystallography: | Triclinic – Pinacoidal |
| Crystal Habit: | Crystals are prismatic and elongated, perhaps the largest of any species, to 50 m and 13,500 tons. Cleavable to granular, massive. |
| Twinning: | Carlsbad, Baveno, and Manebach laws very common; polysynthetic twinning on the Albite and Pericline laws give an orthogonal grid pattern. |
| Cleavage: | Perfect on {001} and {010}, intersecting at ~90º; partings on {100}, {110}, {110}, and {201}. |
| Fracture: | Uneven/Irregular |
| Tenacity: | Brittle |
| Moh’s Hardness: | 6.0 – 6.5 |
| Density: | 2.54 – 2.57 (g/cm3) |
| Luminescence: | Possibly Fluorescent, cherry red under SW UV |
| Radioactivity: | Barely Detectable; GRapi = 200.97 (Gamma Ray American Petroleum Institute Units) |
| Color: | Pale green to dark green, aqua green, bluish green |
| Transparency: | Translucent to Opaque |
| Luster: | Vitreous, Pearly on cleavages |
| Refractive Index: | 1.514 – 1.539 Biaxial ( – ) |
| Birefringence: | 0.007 |
| Dispersion: | Relatively weak; r > v |
| Pleochroism: | None |
