Microcline with Green Tourmalines. Specimens are abundant in igneous rocks rich in potassium or silica, in pegmatites, and in gneisses. Sanidnine: Crystal System: Monoclinic . It is a type of potassium feldspar, also known as K-feldspar.The gem known as moonstone (see below) is largely composed of orthoclase. 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 . 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. Click on any alkali feldspar specimen image below to enlarge it. The color is from the presence of lead. This mineral is important in ceramics, to make the item itself and as a glaze. 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. Potassium feldspar (orthoclase and microcline) is the typical variety of feldspar and, because of this, many arkosic deposits are red to pink. Cell Parameters: a = 8.6Å, b = 13.03Å, c = 7.17Å. Arkoses are typically poorly to moderately sorted and contain many subrounded to highly angular grains. Outside the geological community, only dedicated mineral collectors can tell these apart. But a deep green variety of microcline called amazonite stands out in a pretty homogeneous field. 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. 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. Microcline is the stable form below about 400 C. Orthoclase and sanidine are stable above 500 C and 900 C, respectively. Code: micm102 Microcline Specimen Dimensions: 3.5" x 2.4" x 1.2" Found at Itinga, Brazil Price: $41.50 This natural … Feldspar - Feldspar - Crystal structure: Sanidine and orthoclase are monoclinic or nearly so; the plagioclase feldspars are triclinic. The high potassium content and high … Microcline and Orthoclase Mineral Specimens & Crystals Try our Potassium Feldspar Specimens Satisfaction Guarantee - compare our prices and our superior quality.