Microcline is the stable form below about 400 C. Orthoclase and sanidine are stable above 500 C and 900 C, respectively. It is a type of potassium feldspar, also known as K-feldspar.The gem known as moonstone (see below) is largely composed of orthoclase. Feldspar - Feldspar - Crystal structure: Sanidine and orthoclase are monoclinic or nearly so; the plagioclase feldspars are triclinic. Click on any alkali feldspar specimen image below to enlarge it. Although petrologists had used twinning as a method of distinguishing orthoclase from microcline, Wright and Stewart (1958) used the difference in certain d values to calculate a degree of disorder of a sample and thus have a quantitative method to identify the structural state of a potassium feldspar. The high potassium content and high … Code: micm102 Microcline Specimen Dimensions: 3.5" x 2.4" x 1.2" Found at Itinga, Brazil Price: $41.50 This natural … Cell Parameters: a = 8.6Å, b = 13.03Å, c = 7.17Å. Orthoclase, or orthoclase feldspar (endmember formula K Al Si 3 O 8), is an important tectosilicate mineral which forms igneous rock.The name is from the Ancient Greek for "straight fracture," because its two cleavage planes are at right angles to each other. Exsolution lamellae (Orthoclase and Microcline) Common minerals that might be confused with K-feldspar and occur in similar rocks: Quartz (Clear, no twinning, forms subgrains), Plagioclase (multiple twins) K-feldspar (microcline) in gneiss . Microcline and Orthoclase Mineral Specimens & Crystals Try our Potassium Feldspar Specimens Satisfaction Guarantee - compare our prices and our superior quality. Specimens are abundant in igneous rocks rich in potassium or silica, in pegmatites, and in gneisses. Potassium feldspar (orthoclase and microcline) is the typical variety of feldspar and, because of this, many arkosic deposits are red to pink. The color is from the presence of lead. Arkoses are typically poorly to moderately sorted and contain many subrounded to highly angular grains. orthoclase (medium temperature polymorph) monoclinic Colorless, white, gray, flesh pink, simple twinning sometimes visible (Carlsbad), hardness 6-8 3. microcline (low temperature polymorph) triclinic White to pale yellow, common in coarse cleavable masses, short prismatic crystals commonly twinned (Carlsbad), hardness 6-7 Orthoclase and microcline are very difficult to tell apart in hand specimen. Sanidnine: Crystal System: Monoclinic . Microcline with Green Tourmalines. Name: From the Greek for straight and fracture, in allusion to the cleavage angle. Analyser out: Analyser in: Note: Polysynthetic twins and un-twinned quartz K-feldspar cleavage in detrital orthoclase grains in sandstone . Pure orthoclase is rare some sodium is usually present in the structure. This mineral is important in ceramics, to make the item itself and as a glaze. 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