Special mineralogical or gemological testing equipment is often needed to positively identify the feldspar minerals. In cavities in basalts; in high-grade metamorphic rocks and as a result of potassic hydrothermal alteration; also authigenic and detrital. A crystal like this would have a much higher price if sold as a mineral specimen than as a piece of facet rough. Large crystals of orthoclase are found in igneous rocks known as pegmatite. Amazonite specific gravity: 2.56-2.57 Index of refraction: 1.53 Birefringence: weak maximum birefringence of 0.008 seen on {001 ... (amazonite, microcline or orthoclase) separate forming lattice type exsolution intergrowths, usually as thin layers of albite roughly parallel to {100} in a host crystal of potassium feldspar. At the time of crystallization, these ions can substitute freely for one another in the mineral’s crystal structure. Specific gravity is the density of a material, in this case a mineral, compared to an equivalent volume of water. Diaphaneity translucent to transparent mohs hardness 6 defining mineral specific gravity 2 55 2 63 crystal system monoclinic tenacity brittle parting. Orthoclase is in the position of extreme potassium content. Specific Gravity (Density) : 2.55 – 2.63 (g/cm3) Refractive Index : 1.518 – 1.525 Biaxial ( – ) Orthoclase is a member of the Potassium Feldspars of the Feldspar Group of minerals that also includes Albite, Amazonite, Andesine, Anorthite, Bytownite, Hyalophane, Labradorite, Moonstone, Oligoclase, Orthoclase, Sanidine and Sunstone. Orthoclase is industrially important in the manufacture of glass and ceramics. Gem crystals from Ampandrandava, Fianarantsoa, and Itrongay, near Antero, Cha®ee Co.; at Crystal Pass, Goodsprings, Origin: Named in 1823 from the Greek orthos, right, and kalo, I cleave, in allusion to the mineral’s perpendicular cleavages. 5. This confirmed the identification that I suspected and eliminated gem albite-orthoclase which was a possible alternate. It shows orthoclase as a major constituent in granites and rhyolites and in some diorites and andesites. The common feldspar of granites, granite pegmatites, and syenites. cleavage angle. Crystal System: Monoclinic. Twin crystals are not uncommon. Member of: Feldspar Group. It can be difficult to An adularescent gem material known as moonstone is an intergrowth of orthoclase and albite. 2KAISi3O8 + 2H+ + 9H2O → H4Al2Si2O9 + 4H4SiO4 + 2K+(orthoclase + water → kaolinite + silicic acid + potassium). It will develop abrasions if used in most types of jewelry, and it can easily cleave upon impact. Watch as I guide you step by step through the process of performing a specific gravity test to identify minerals and crystals. Orthoclase is most widely known as the pink feldspar found in many granites and as the mineral assigned a hardness of "6" in the Mohs hardness scale. Specific Gravity is approximately 2.59 g/cm Streak is (??). In these conditions there is the presence of a large stability field of leucite. Minerals in Igneous Rocks: This chart shows the generalized ranges of mineral abundance in the most common igneous rocks. All feldspar minerals are usually translucent to transparent, display two directions of cleavage that intersect at approximately 90 degrees, have a vitreous to pearly luster on cleavage faces, and have a specific gravity between about 2.5 and 2.6. Orthoclase has a Mohs hardness of 6.0, good right-angle cleavage in two directions, and a specific gravity of 2.5-2.6. Specific gravity is the number of times heavier a gemstone of any volume is than an equal volume of water; in other words, it is the ratio of the density of the gemstone to the density of water. Specific Gravity : 2.55-2.63 Enhancements ... Orthoclase is from Greek words meaning break straight because the cleavages are at 90°. These rocks most often form during regional metamorphism when granitic rocks are subjected to heat and pressure at convergent plate boundaries involving continental crust. At Tanokamiyama, Shiga Prefecture, Specimens are abundant in igneous rocks rich in potassium or silica, in pegmatites, and in gneisses. It may also occur in massive form. Name: From the Greek for straight and fracture, in allusion to the From Guanajuato, Mexico. Specimen Grade vs. Facet Grade Orthoclase: Photo of an orthoclase crystal from the Fianarantsoa Province of Madagascar with a superb crystal form and color. Distinguishing Features: (??). Industrial Usages. This becomes more difficult when their crystals are part of an igneous rock with a grain-size of just a few millimeters or less. Excellent crystals associated with Aegirine come from Mount Malosa, Malawi. Japan. Plagioclase Series Gems. It has a hardness of 6, a specific gravity of 2.56-2.58, and a vitreous to pearly lustre. Adularia and orthoclase are similar, but adularia is pseudo-orthorhombic. Cleavages intersect at 90°. Widespread. For additional information, see the gemstone section on Orthoclase and Moonstone. Specific density (also called specific gravity) Density is the mass of an object divided by its volume. good cleavage on {010}. Specific gravity: 2.56 Index of refraction: 1.52 Birefringence: weak maximum birefringence of 0.005 Pleochroism: colorless in section ... Orthoclase (KAlSi 3 O 8) is the medium temperature pure potassium end member of the alkali feldspar group. These feldspar minerals form a solid solution series between NaAlSi3O8 and KAlSi3O8. It is a major component of granite its pink crystals give granite its typical color. & Series: Dimorphous with microcline; forms a From Cornwall, The glow appears to move as the light source is moved, or as the stone is moved, or as the observer chances his angle of observation. Fine examples from St. Pure orthoclase is rare some sodium is usually present in the structure. Occurrence. The lowest values are found in rather pure potassium feldspars; the highest values pertain to anorthite. Because of this the alkali feldspars exist in a range of chemical compositions between pure albite (NaAlSi3O8) and pure orthoclase (KAlSi3O8). see cleavage in thin section due to orthoclase’s low relief. Orthoclase is a feldspar mineral with a chemical composition of KAlSi3O8. Moonstone is a variety of orthoclase that exhibits a schiller effect. Pink Granite: A specimen of coarse-grained granite with pink crystals of orthoclase. Transparent orthoclase with superior clarity is sometimes faceted and sold as a collector’s gem. Cleavage/Fracture : 90° cleavage angle; perfect (001), good (010), poor {110}/uneven. In California, from the Pala and Mesa Grande districts, San Diego Widely distributed as a prominent rock constituent, occurring in all types of rocks; igneous, in granites, syenites, porphyries, etc. In Russia, from the Mursinka-Alabashka area, near Yekaterinburg Moonstone is the most famous orthoclase gem. Co.; in Colorado, on Mt. An adularescent gem material known as moonstone is an intergrowth of orthoclase and albite. During physical weathering, grains of orthoclase are incorporated into sediments and sedimentary rocks such as sandstone, conglomerate, and siltstone. If the specimen is a well-formed crystal, it will likely have a much higher value if sold as a mineral specimen than as a cutting rough. Density, or specific gravity, is a bulk property of a mate­ rial that is independent of direction and is uniform within a mass of material under ideal circumstances. Luster/Transparency : pearly, vitreous/translucent. CLASSIFICATION: Dana System # 76.1.1.1. As a mineral with a Mohs hardness of 6 and two directions of perfect cleavage, orthoclase is not an especially durable gemstone. Microcline is an extremely common mineral, and only remarkable occurrences are mentioned here. (Sverdlovsk), Ural Mountains. Occurrence, Localities and Origins: Orthoclase is one of the most common of minerals. It was found in a pegmatite in the Ural Mountains of Russia. The glow is usually white in color and is the source of the “moonstone” name. Orthoclase is also known in igneous rocks found on the moon and on Mars. The ideal value for the potassium-rich alkali feldspars is 2.56, which is less than the lowest value for the plagioclases (namely, 2.62 for albite). Orthoclase is found in silica-rich igneous rocks such as granite, and in high grade metamorphic rocks. Slight differences in refractive indices, specific gravity, temperature of their conversion to sanidine (a high-temperature form of potassium feldspar), and axial angle, however, indicate the existence of two different species. Chemical weathering alters orthoclase into clay minerals such as kaolinite in reactions similar to the one shown below. This is known as perthitic texture. Streak : white. England. These gems commonly range from colorless to a bright yellow color. Feldspar Mineral Classification: This ternary diagram shows how feldspar minerals are classified on the basis of their chemical composition. Specimen and photo by Arkenstone / www.iRocks.com. White, gray, pink, reddish, yellow, green, colorless, Perfect in two directions intersecting at 90 degrees, Ceramics, glass, abrasives, gemstones, Mohs scale mineral. Most orthoclase forms during the crystallization of a magma into intrusive igneous rocks such as granite, granodiorite, diorite, and syenite. 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orthoclase specific gravity

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