Orthoclase





Authors: Dominic Humlie, Kelli Graham, Dallas Olejniczak

Mineral Name:

Orthoclase



Chemical Composition:

Orthoclase is composed of potassium aluminum silicates (KAlSi3O8). Orthoclase is a non-ferromagnesian mineral, meaning it doesn’t contain high density iron or magnesium elements.

Color:

Colorless to white, grayish yellow, pale pink, and green. The crystals may also appear transparent in their pure crystal formation.

Streak:

Colorless/ white

Hardness:

6 on Mohs scale

Cleavage/Fracture:

Two uneven cleavage planes, with an angle between cleavage planes that is about 90 degrees.

Crystal Form:

Orthoclase crystals can be composed in prismatic or tabular shapes. This means they can appear very blocky, extending in one plane of direction and can be very thin. The crystals may produce striations on any of their parallel sides, as can be seen closely on Figure 2. (5a.) Orthoclase can also undergo a phenomenon called twinning, specifically Carlsbad twinning. In general, crystals twin when they are under intense amounts of stress and/or high temperature. For most twins, crystals form symmetrically over a plane called the composition surface and are relatively identical, like siamese twins. A Carlsbad twin forms a crystal which has rotated 180 degrees to form an interesting shape. Although Orthoclase can only form Carlsbad twins, Carlsbad twins are not limited to just Orthoclase. Even so, Carlsbad twins are usually indicative of Orthoclase. (13.)

Luster:

Vitreous, Sub-Vitreous, Resinous, pearly.

Special Features:

Orthoclase may fluoresce a dull white or red under UV light. (12.)

Varieties:

Noted varieties of orthoclase can be translucent like in adularia or valencianite. A gemstone variety of orthoclase called moonstone, shows a color sheen due to adularescence. Adularescence is the appearance of “ghost-like” white and sometimes blue colored whisps within the stone due to structural anomalies or even an excessive amount of water inside the mineral itself. (1.) There are also yellowish varieties of noble orthoclase found in parts of Madagascar. (5a.) The most common variety, however, is associated with the pink potassium feldspars that is most likely found in granite compositions. (3.)

Mineral Group:

Orthoclase belongs to three groups: the silicates, the tectosilicates, and the feldspar group. Orthoclase contains a high volume of silicon and oxygen formed in a specific tetrahedra, which puts this mineral within the silicates. The specific structure of the oxygen atoms make it a tectosilicate. This means the oxygen atoms of the tetrahedrons, considered the foundation of the silicates, are shared between each other adjacently. Orthoclase very clearly belongs to the feldspar group based on its composition of aluminum with silica. Orthoclase belongs to the potassium feldspars along with sanidine, and microcline. (10. & 12.)

Environment:

Orthoclase is mainly found in felsic igneous rocks. Its formation can be described by Bowen’s reaction series. As melt in a magma chamber cools, silica increases and the mineral crystallizes into the next consecutive mineral of the series. Feldspars begin forming as calcium-rich minerals in hotter magmas, then sodium-rich plagioclase feldspars form as the magma cools, and finally potassium-rich feldspar forms in cooler granitic-type rocks from felsic magmas. Orthoclase is found mostly metamorphic rocks like gneiss, but can also be spotted in felsic igneous rocks, like pegmatites, which are very coarse intrusive rock formations. Orthoclase and other minerals can grow substantially in pegmatites because water and other volatile content is concentrated in the last portion of magma, lowering the mineral-forming temperatures and allowing magma to cool exceptionally slow. This slow cooling creates very large crystals. The mineral can also be found cemented in sedimentary rock formations after weathering initially from igneous rocks. (5a. & 12. & 9)

Potassium feldspars, and other feldspars, make up most of the planet’s crust. They’re a major component to the building of sandstones because of their resistance, like quartz. This eventually breaks down to clay and then to mud, which is very common.

Associated Rock types:

Orthoclase mostly forms during magma crystallization into intrusive igneous rocks. Rocks such as granite, diorite, granodiorite, and syenite. Large amounts of orthoclase has also been found in extrusive igneous rocks such as rhyolite, dacite, and andesite. Orthoclase is also an extraterrestrial mineral since reports from NASA confirm the the presence of orthoclase as a primary mineral found in igneous rocks on the moon and on Mars. (10.)

Occurrence in North America:

In the United States, crystals were found in New Mexico, Sandia mountain, Bernalillo Co, New Mexico. Organ Mountains, Dona Ana Co., New Mexico Colorado, Clark Co.,Nevada. The French Creek mine, St.Peters, Chester Co., Pennsylvania. (5a.)

Economic Uses:

Orthoclase is a common raw mineral for some ceramics such as porcelain and is often used for decorative stone in buildings. (12.) Many find the distinct cleavage and appearance to be appealing in orthoclase feldspars. Although orthoclase has perfect cleavage planes, manufacturer’s are able to cut orthoclase material that tends to be more transparent into facets and more opaque pieces are cut “en cabochon,” or polished into an oval form (without facets). (6.) These gems are then used for stones in fine jewelry. (11.) A common example of a orthoclase gemstone would be Moonstone. Moonstone is a desired gem for its bluish, white sheen and “moonlight” glow. Structural anomalies throughout give moonstone it adularescence and is polished in cabochons to accentuate its glimmer. (5b.)

Industrial Uses:

Orthoclase is versatile and has a variety of uses. Economically, adding the presence of potassium feldspar (orthoclase) as a component of glass will lower the melting point of quartz. This mix then is used to manufacture glass. Lowering the melting point, in turn, makes the glass have a lower viscosity and inevitably becomes less expensive to distribute. Adding kaolin, as well as quartz, aids in the manufacturing of some electric and dental tools as well as commonly known porcelain products. (7.)

Orthoclase , as well as sanidine and microcline, is used to make glass and clay materials like porcelain, ceramic tiles, and ceramic glazes. It’s mostly used as aggregate for concrete and asphalt, accompanied by many other minerals. Potassium feldspars are also used in abrasives and hard-surface cleaners. This placement of aluminum into sediment deposits can then be leached in extremely wet weather conditions. Leaching of aluminum-rich soils can form the common mineral bauxite, which is the leading source of industrial aluminum on Earth (12.)

First Notable Identification:

The word orthoclase originates from greek terminology for “straight klasis” or “straight breaking.”(7.) This reference comes from its perfect cleavage planes within the structure of the mineral. The first noted sighting of orthoclase was by Rene Just Hauy in 1801. It was noted among high-grade metamorphic rocks. (8.)

How We Identified It:

Common distinguishing attributes associated with identifying orthoclase do not come from the color of the sample. Instead, first establishing hardness on Moh’s hardness scale, then by noticing the linear, “vein-like” structures inside the sample were useful. The luster is vitreous to pearly and its cleavage is notably “perfect” at 90 degrees. Some samples may have contained small dark dots and even more defined lines throughout its cleavage planes.

Don’t Confuse It With:

Orthoclase is easily confused with sanidine, microcline, plagioclase, as well as spodumene and calcite. Sanidine can be distinguished through complex methods, but can also be identified simply by its lack of opacity: it is never opaque. Similarly, microcline has a unique dark green color. (12.) Plagioclase and can be differentiated from orthoclase by the very thin striations that form on the surface of a cleavage plane that are parallel and Spodumene has a splintery fracture. Calcite can be differentiated from orthoclase by its softness (5a. & 4.)



Bibliography:

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  2. "Bowen's Reaction Series." Schieber, Jurgen. Indiana University Bloomington, 2007, www.indiana.edu/~geol105/images/gaia_chapter_5/bowen.htm. Accessed 12 Mar. 2018.

  3. “Industrial Use of Minerals.” Industrial Uses of Minerals, Carleton College, 2005, www.people.carleton.edu/~cdavidso/Geo110/MinUse05.htm.

  4. “K-Feldspar and Plagioclase Feldspar.” Edited by University of Pittsurgh, K And Plagioclase Feldspar, Dept. of Geology and Planetary Sciences, www.pitt.edu/~cejones/GeoImages/1Minerals/1IgneousMineralz/Feldspars.html.

  5. (a)Friedman, Hershel. “Minerals.net.” Orthoclase: The mineral Orthoclase information and pictures, 1997, www.minerals.net/minerals/orthoclase.aspx
    (b)Friedman, Hershel. “Minerals.net.” Moonstone Gemstone, Minerals.net, 1997, http://www.minerals.net/gemstone/moonstone_gemstone.aspx

  6. “Moonstone Gemstone Information.” Moonstone Gemstone and Jewelry Information: Moonstone Sheen - GemSelect, 11 Nov. 2006, www.gemselect.com/gem-info/moonstone/moonstone-info.php

  7. Classicgems.net. “Orthoclase.” Orthoclase : ClassicGems.net, 2004, classicgems.net/gem_orthoclase.htm.

  8. "Orthoclase." Mindate.org, Hudson Institute of Mineralogy, Jan. 2018, https://www.mindat.org/min-3026.html. Accessed 4 Feb. 2018.

  9. Ralph, Jolyon, and Katya Ralph. “Orthoclase.” Orthoclase Gemstone Information, Gemdat.org, 1993, www.gemdat.org/gem-3026.html.

  10. “Orthoclase Feldspar.” Orthoclase, Geology.com, 2005, www.geology.com/minerals/orthoclase.shtml.

  11. (12)“Orthoclase Gemstone Information.” Buy Gemstones: Semi Precious & Precious Stones, Natural Gems at Wholesale; GemSelect, Gemselect.com, 2005, www.gemselect.com/gem-info/orthoclase/orthoclase-info.php.

  12. (11)"Potassium Feldspar." University of Minnesota, edited by Joshua Feinberg, https://www.esci.umn.edu/courses/1001/minerals/potassium_feldspar.shtml. Accessed 4 Feb. 2018.

  13. Nelson, Stephen A. “Twinning, Polymorphism, Polytypism, Pseudomorphism.” Tulane University - Mineralogy, 16 Sept. 2013, www.tulane.edu/~sanelson/eens211/twinning.htm.