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What is the difference between Orthoclase and Microcline?

The minerals belonging to this family are of primary scientific interest due to the forms of their crystals and methods of twinning, as well as due to the extremely important role they play in the formation of rocks. But in the world of jewelry they occupy a low position. Of the many varieties of minerals in this family, only one moonstone, which has a beautiful shimmer, is used as a gemstone. Its charm is so great that one can only wonder why this stone is not in great demand. Perhaps he serves as an example of how cheapness prevents success with the public. The family consists of three end members: otoclase and microcline (potassium feldspars), albite (soda feldspars), and anorthite (calcareous feldspars). The fourth member is celsian (barium feldspar), which is very rare. Orthoclase (and microcline) and albite are extreme members of the alkali feldspars. Albite and anorthite make up another very important series of minerals—plagioclases. In addition, there are a number of minerals, the extreme members of which are orthoclase and celsian. The name of the family – “feldspars” (English feldspar) comes from the German word feldspat (h). Orthoclase and microcline have the same chemical composition, being potassium aluminosilicates with the formula KAlSi ; however, they differ in their physical properties. Orthoclase has a monoclinic system, and microcline has a triclinic system, but the angles between the corresponding crystal faces are very close. For example, the angle between two directions of perfect cleavage, which are parallel to the two most common faces, is 90° in orthoclase, and 89°30 in microcline—a difference of only half a degree. Both minerals form twins; the network of polysynthetic twins is so characteristic of microcline that it serves as its diagnostic feature. Orthoclase is sometimes colorless or yellow. Typically, both minerals are white or gray, with the exception of amazonite, which is colored a bright copper-green color. Natural crystals of orthoclase and amazonite are shown in color photo X. Both orthoclase and microcline are, in accordance with the type of optical symmetry, biaxial minerals; The optical sign of both is negative. The values ​​of the smallest and largest principal refractive indices are 1,518–1,522 and 1,526–1,530. It should be noted that pure yellow orthoclase from Madagascar has high refractive indices of 1,522 and 1,527 due to the presence of iron, which is the reason for the color of this mineral. Its density is normal. The density of orthoclase and microcline is 2,56. Mohs chose orthoclase as the standard mineral for his hardness scale and assigned it a score of 6. The hardness of microcline can be slightly higher, ranging from b to 6,5. Both minerals have two directions of excellent cleavage, which are located either at right angles or almost at right angles to each other. Glass shine; The cleavage planes can be pearly. The dispersion of all feldspars is small and for the interval B – G is only 0,012. Orthoclase got its name due to the fact that the directions of its cleavage are located at right angles to each other (from the Greek words 6p0og – straight xHasp? – cleavage). Microcline is the English form of the German word mikroklin, which comes from the Greek words juxpog (small) and xXtveiv (to bend). The name indicates that the cleavage planes in microcline crystals form an angle slightly different from a right angle. Pure or nearly pure orthoclase is known as adularia. This name comes from the Adula massif in Switzerland, where these pure crystals with few facets were first found. In reality, the location of the adularia is the St. Gotthard region, and not the Adula massif, but formerly this latter name was applied to the entire Central Alps, including St. Gotthard. Sanidine is a variety represented by glass-like tabular crystals. The name comes from the Greek word o5vi8, or oavts (tablet). Albite is a soda feldspar that has the formula NaAlSi and is the end member of the plagioclase series. It crystallizes in the triclinic system, but the angles between the faces of its crystals are almost equal to the angles between the corresponding faces of orthoclase crystals. Like other feldspars, albite crystals are often twinned. Albite can be colorless, but usually has a gray color of various shades. Its crystals have two directions of good cleavage, which are inclined at an angle of 86°24. Albite is an optically biaxial positive mineral; the values ​​of the smallest and largest refractive indices are 1,525 and 1,536. Albite density is 2,62, hardness is 6-6/4 on the Mohs scale. The luster is glassy, ​​and on the cleavage planes it is pearly. Albit gets its name from the Latin word albus (white). The name “plagioclase” comes from the Greek words yaKha-yiog (oblique) and xWtoig (cleavage) and indicates that the angle between the cleavage planes of these minerals is noticeably different from straight. Anorthite, another end member of the plagioclase series, is a calcareous feldspar with the formula CaAl . It also crystallizes in the triclinic system, a fact indicated by its name, which is derived from two Greek words: av (not) and op0ig (straight). This name, however, is not entirely apt, because there is another Greek word – avopGog, although rarely used, in which the first syllable has a different meaning and which generally means “straight”, i.e. has the exact opposite meaning. Anorthite has the usual pair of perfect cleavage directions with an angle between them equal to 85°50, and its crystals are often twinned. It is usually white or colorless. Anorthite is an optically biaxial negative mineral. The values ​​of the smallest and largest refractive indices are 1,576 and 1,588. Its density is 2,76, and its hardness is 6-6% on the Mohs scale. The luster is the same as albite. For convenience, the intermediate members of the plagioclase series have been given their own names. If we denote pure albite by the symbol AB, and pure anorthite by the symbol An, then using these symbols we can represent the following intermediate members of the series: Feldspars – a large group of widespread, in particular, rock-forming minerals from the silicate class (Feldspat – from the German “feld” – field and the Greek “spate” – plate, due to the ability to split into plates along cleavage). Most feldspars are representatives of solid solutions of the ternary system of the isomorphic series K[AlSi3O8] – Na[АlSi3O8] – Ca[AlSi2O8], the end members of which are, respectively, orthoclase (Or), albite (Ab), anorthite (An). There are two isomorphic series: albite (Ab) — orthoclase (Or) and albite(Ab) — anorthitis (An). Minerals of the first of them can contain no more than 10% An, and the second – no more than 10% Or. Only in sodium feldspars close to Ab does the solubility of Or and An increase. Members of the first row are called alkaline (K-Na feldspars), the second – plagioclases (Ca-Na feldspars). The continuity of the Ab-Or series appears only at high temperatures; at low temperatures, miscibility breaks with the formation of perthites. Along with sanidine, which is high-temperature, low-temperature potassium feldspars are distinguished – microcline и orthoclase. Feldspars are the most common rock-forming minerals; they make up about 50% of the mass of the Earth’s crust.

General properties

Feldspars are silicates with a frame-type crystalline structure; these are openwork structures of silicon-oxygen tetrahedra, in which silicon is sometimes replaced by aluminum. They form rather uniform crystals of monoclinic or triclinic systems, in the form of a few combinations of orthorhombic prisms and pinacoids. Characterized by simple or especially polysynthetic doubles; The laws of twinning found in feldspars are divided into normal (perpendicular), for which the twin axis is perpendicular to any possible crystal face located parallel to the plane of twin intergrowth, parallel, giving which the twin axis is the edge of the crystal, and the plane of twinning intergrowth is parallel to the twin axis, as well as more complex (combined) laws. In this case, the most frequently encountered are the albite (in plagioclase) and Carlsbad (in potassium field pshat) twinning laws. Cleavage is perfect in two directions, along (001) and (010). Crystals without impurities are white or bas-colored, from translucent to translucent and transparent. But more often they contain many impurities and inclusions, giving them any color. Density 2,54-2,75 g/cm³. Hardness 6 (one of the standard minerals on the Mohs scale). All feldspars are easily etched by HF, and plagioclases are also destroyed by HCl.

Subgroups

Plagioclases

  • Albite. (extreme member of the isomorphic series, with the formula: NaAlSi2O6 , contains 0-10% An.)
  • Oligoclase.
  • Andesine.
  • Labrador.
  • Bitovnit.
  • Anorthitis. (extreme member of the isomorphic series, with the formula: CaAlSi2O6, contains 90-100% An)

Plagioclases, mainly salic, are the main rock-forming minerals of igneous and many metamorphic rocks. In igneous rocks, plagioclase, rich in An-molecule, first crystallizes, and then more acidic (rich in silica) is released. In these cases, zoned crystals may develop. Some igneous rocks consist almost entirely of plagioclases (anorthosites, plagioclasites, and others). Albite is often found in pegmatite veins, formed from other plagioclases, and especially from sodium-containing potassium feldspars. Under hydrothermal conditions, plagioclases are altered by weathering into kaolinite minerals and sericite mica. At the same time, plagioclases rich in anorthite component are destroyed faster than acidic ones; albite is more stable during secondary processes.

GeoWiki has an article “Plagioclase”

Potassium feldspars

Potassium feldspars are often collectively simply referred to as “KPS”:

Microcline crystal, Chupinsky district, Karelia. Photo by D. Tonkacheev

All three minerals correspond to the same chemical formula, differing from each other only in the degree of ordering of their crystal lattices.

Structural features and nomenclature

Approximate scheme of isomorphism in alkali feldspars

The microcline is of triclinic system (pseudomonoclinic), the angle between the cleavage planes differs from the straight line by 20°. Adularia – with an ordered structure and the same formula, but with a cleavage inclination of 30°. Sanidine is monoclinic, with a completely disordered structure (K(AlSi)4O8), is stable at temperatures above 500 °C, and orthoclase, also strictly monoclinic, has a partially ordered structure K(A1,Si)Si2O8 and is stable at temperatures between 500° and 300°C. Below this temperature, the stable form is microcline. Orthoclases almost always contain a certain amount of Na2Oh, the intermediates between orthoclase and albite are called anorthoclases. The orthoclase-albite series is usually stable at high temperatures; lowering the temperature leads to the precipitation of albite in orthoclase (perthite) or orthoclase in albite (antiperthite). The solid solution with sanidine is a monoclinic modification of Na[AlSi308] containing some potassium and is known as barbierite; another modification of the same composition, but triclinic, forms a solid solution with high-temperature albite. Varieties: adularia (named after an area in the Alps), low-temperature orthoclase with or without weakly developed facets (010), sometimes opalescent and used as a semi-precious stone (moon rock). Amazonite – light green microcline. The crystallographic forms of pseudomonoclinic triclinic representatives (microclines and some adularia) are similar to those of orthoclase. Orthoclase is characterized by a right angle between the cleavage planes.

A staining method is used to distinguish plagioclases from potassium feldspars. To do this, the rock surface or mineral plate is etched with HF and then placed in a K-rhodizonate solution; — plagioclases, with the exception of albite, are painted brick-red.

Potassium feldspars are the main rock-forming minerals of acidic igneous rocks (granites, syenites, granodiorites, etc.), as well as some widespread metamorphic rocks (gneisses). The latter are dominated by low-temperature microcline, while igneous rocks of the plutonic type contain orthoclase, and volcanic rocks contain sanidine. Anorthoclase is a typical mineral of sodium-rich igneous rocks.

Orthoclase and microcline, together with quartz and muscovite, are the main minerals of pegmatites. If beryl is present in them, the microcline can be enriched in beryllium, which, like aluminum, can replace silicon atoms. Pegmatites are characterized by intergrowths of orthoclase (microcline) with quartz, known as “written granite” pegmatite and which are a product of crystallization of a eutectic magmatic melt. Adularia is a typical feldspar in alpine-type hydrothermal veins.

Compared to plagioclases, K-feldspars are more resistant to destruction, but they can be replaced by albite, giving rise to “metasomatic perthite”. Under hydrothermal conditions and weathering, they change into minerals of the kaolinite group.

Deposits of potassium feldspars are well known in Norway, Sweden, Madagascar, on the territory of the Ilmensky Nature Reserve and in many other pegmatite occurrences of the Southern Urals. Also in Maine, USA, and elsewhere.

Potassium-barium feldspars (Hyalophanes)

Potassium-barium feldspars (hyalophanes) are rare in nature. They are isomorphic mixtures of K[AlSi3O8] – Ba[Al2Si2O8].

Quite a rare mineral. Individual cream-colored crystals have exclusively collection value.

Application

Feldspars are widely used in the ceramics industry as fillers, light abrasives (for example, in the production of toothpastes), and also as raw materials for the extraction of rubidium and some other impurity elements they contain. Some varieties of translucent and transparent plagioclase, which have an opalescent effect or silvery-bluish and golden iridescence, are used as ornamental stones in jewelry.

From publications

  • Alekseev V.I., Sokolova N.G. Evolution of order and composition of alkali feldspars of the Northern granite massif (Chukotka). – Zap. RMO, 2007, part 136, issue 2, p. 62-74
  • Dolzhanskaya T.Yu. Use of typomorphic features of feldspars to identify the internal structure of the alkaline massif of the Cherry Mountains in the Urals. – Appl. and ecology aspects of minerals: abstract. report Godich. ses. All mineral. islands, Zvenigorod, March 19 – 21, 1990. Book 2. – M., 1991. – P. 61 – 63. Rus.
  • Kupletsky V.I. Feldspars in the Kem region. – KEPS materials. L., 1924. Issue. 48. Stone building materials. pp. 29-46.
  • Kurbatov S.S. Feldspars of the USSR and the possibility of using them in the ceramic industry. – Tr. State research ceramic in-ta. 1928. Issue. 2. P. 40.

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