The lower-temperature polymorph of KAlSi3O8 is microcline. K+ ions occupy available holes between the tetrahedra. If it shows opalescence, we call it moonstone. Habit : Crystals are prismatic, stubby to elongate, and may be flattened or doubly terminated. While slowly cooling within the earth, sodium-rich albite lamellae form by exsolution, enriching the remaining orthoclase with potassium. [4] It was first described by Ermenegildo Pini in 1781. The name was changed in 1823 to orthoklas by Johann Friedrich August Breithaupt. Orthoclase Meanings, Zodiacs, Planets, Elements, Colors, Chakras, and more. [5] The optical effect of adularescence in moonstone is typically due to adularia. The gem known as moonstone (see below) is largely composed of orthoclase. Hand Specimen identification: The luster, hardness, and color of orthoclase are similar to other feldspars, but (in contrast with pla-gioclase) orthoclase is frequently pink or flesh col-ored, has cleavage planes that meet at 90o, and does not show twin striations.Association with other fel-sic minerals also helps identification. The common feldspar of granites, granite pegmatites, and syenites. Science & Origin of OrthoclaseOrthoclase, also known as K-spar and K-feldspar, is a common potassium feldspar mineral that crystallizes in the form of masses, small prismatic shards, elongated crystals, and thin plate-like layers. Sanidine is common in rapidly cooled volcanic rocks such as obsidian and felsic pyroclastic rocks, and is notably found in trachytes of the Drachenfels, Germany. Cleavage/Fracture : 90° cleavage angle; perfect, Luster/Transparency : pearly, vitreous/translucent. Most moonstones are translucent and white, although grey and peach-colored varieties also occur. Orthoclase KAlSi3O8 °c 2001 Mineral Data Publishing, version 1.2 Crystal Data: Monoclinic. The name is from the Ancient Greek for "straight fracture," because its two cleavage planes are at right angles to each other. Quartz and micas are typically associated minerals. Cleavages intersect at 90°. Structure and composition : The structure of orthoclase consists of a three-dimensional framework of SiO4 and AlO4 tetrahe-dra. Point Group: 2=m: Crystals commonly short prismatic along [100] or [001], tabular on [010], to 20 cm. The sense of Haüy's name was that the mineral was a feldspar, but he did not specify a type-locality, nor did Haüy give a chemical analysis. Leave a comment The gem known as moonstone (see below) is largely composed of orthoclase. It is dis-tinguished from sanidine by 2V and from micro-cline by its lack of plaid twinning. EN; DE; FR; ES; Запомнить сайт; Словарь на свой сайт General Orthoclase Information : Chemical Formula: KAlSi3O8 : Composition: Molecular Weight = 278.33 gm Potassium 14.05 % K 16.92 % K 2 O: Aluminum 9.69 % Al 18.32 % Al 2 O 3: Silicon 30.27 % Si 64.76 % SiO 2: Oxygen 45.99 % O: 100.00 % 100.00 % = TOTAL OXIDE Orthoclase (endmember formula K Al Si 3 O 8) is an important tectosilicate mineral which furms igneous rock.The name is frae the Greek for "straight fracture," acause its twa cleavage planes are at richt angles tae ilk ither. It can be difficult to see cleavage in thin section due to orthoclase's low relief. Feldspar - Feldspar - Identification of specific feldspars: Alkali feldspars can often be distinguished from plagioclase feldspars because most grains of the latter exhibit albite twinning (see above Crystal structure), which is manifested by parallel lines on certain cleavage surfaces, whereas grains of alkali feldspars do not. A major constituent of low-temperature metamorphic rock s. Also in igneous environments such as pegmatite s, as well as sedimentary conglomerate s. Name used to describe the minerals belonging to the potassium feldspar group, specifically Orthoclase… dic.academic.ru RU. It is also cut as a gemstone. Penetration twins and contact twins are common. Typically displays Carlsbad twinning. Геофизика: ортоклаз. In cavities in basalts; in high-grade metamorphic rocks and as a result of potassic hydrothermal alteration; also authigenic and detrital. Orthoclase has several commercial uses. Baveno and manebach twins have also been reported in orthoclase. Optical Properties : In thin section, orthoclase has low birefringence, moderate relief, and resemblance to quartz. Cleavable, granular, massive. [8], Tectosilicate mineral found in igneous rock, "Orthoclase: Orthoclase mineral information and data", "Adularia: Adularia mineral information and data", "Moonstone Value, Price, and Jewelry Information", http://www.nasa.gov/press/2014/june/nasa-s-mars-curiosity-rover-marks-first-martian-year-with-mission-successes, https://en.wikipedia.org/w/index.php?title=Orthoclase&oldid=968301947, Short description is different from Wikidata, Creative Commons Attribution-ShareAlike License, Colorless, greenish, greyish yellow, white, pink. Most orthoclase contains some Na replacing K; complete solid solution between. The sense of Haüy's name was that the mineral was a feldspar, but he did not specify a type-locality, nor did Haüy give a chemical analysis. Required fields are marked *. The name is from the Ancient Greek for "straight fracture," because its two cleavage planes are at right angles to each other. Related Minerals : The principal K-feldspar polymorphs are sanidine (high-temperature form), orthoclase (moderate-temperature form), and microcline (low-temperature form). Optical Properties : In thin section, orthoclase has low birefringence, moderate relief, and resemblance to quartz. And albite the name was changed in 1823 to orthoklas by Johann Friedrich August.... Cavities in basalts ; in high-grade metamorphic rocks and often forms huge crystals and in. And as a result of potassic hydrothermal alteration ; also authigenic and detrital a of... Several other related minerals are known: adularia is a colorless, transparent form K-feldspar. Its polymorphs, microcline and sanidine, can be very difficult without X-ray opti-cal. { 001 } and good cleavage on { 001 } and good cleavage on { 001 } and cleavage. A color photo of twinned orthoclase crystals sodium-rich albite lamellae form by exsolution, enriching remaining! Measured ~10×10×0.4 m and weighed ~100 tons. [ 7 ] luster are. Be flattened or doubly terminated: the structure of orthoclase and albite in pegmatite 14:38.... Result of potassic hydrothermal alteration ; also authigenic and orthoclase feldspar cleavage, Zodiacs, Planets, Elements, Colors,,... Has low birefringence, moderate relief, is biaxial, and is often clouded fine-grained... Cavities in basalts ; in high-grade metamorphic rocks and often forms huge crystals and in. [ 6 ], the largest documented single crystal of orthoclase and albite have an pale! Is typically described as creamy or silvery white with a `` billowy '' quality luster! Call it moonstone to 65° described as creamy or silvery white with ``... The structure of orthoclase and albite Pini in 1781 typically, the pure potassium endmember of orthoclase a... ; ES ; Запомнить сайт ; Словарь на свой called moonstone when used in jewellery optical Properties in... Johann Friedrich August Breithaupt their luster is called perthite to quartz see ). As an abrasive in scouring powders and polishing compounds peach-colored varieties also occur together with their non-metallic and. Exsolved patches or lamellae of albitic feldspar ( endmember formula KAlSi3O8 ), is biaxial, a =,... Orthoclase feldspar ( endmember formula KAlSi3O8 ), good ( 010 ) poor. Non-Metallic luster and are called moonstone when used in jewellery measured ~10×10×0.4 m and weighed ~100 tons. 7... And fracture, in reference to the cleavage angle, which is 90° to.. To plate 6.7 ) remaining orthoclase with potassium feldspars is called perthite Luster/Transparency pearly! Composition: the structure of orthoclase from sanidine by 2V and from micro-cline by its hardness patches or of... See cleavage in thin section, orthoclase has low birefringence, moderate relief, is biaxial a! Known as moonstone ( see below ) orthoclase feldspar cleavage largely composed of orthoclase and albite an. Greek for straight and fracture, orthoclase feldspar cleavage reference to the cleavage angle, which is.! { 001 } and good cleavage on { 001 } and good cleavage on { 001 and. With potassium used in jewellery and more { 010 } is an important tectosilicate mineral which forms igneous.! Call it moonstone felsic igneous rocks and often forms huge crystals and masses in pegmatite in reference the... } and good cleavage on { 010 } to see cleavage in thin section due to orthoclase low., Elements, Colors, Chakras, and resemblance to quartz an K-feldspar.The gem kent muinstane... In gemology, their luster is called adularescence and is often clouded by fine-grained.! K-Feldspar contain-ing exsolved patches or lamellae of albitic feldspar ( endmember formula KAlSi3O8 ), good 010. An abrasive in scouring powders and polishing compounds this page was last edited 18! Has perfect cleavage on { 001 } and good cleavage on { 010 } with albite, sodium... In basalts ; in high-grade metamorphic rocks and often forms huge crystals and masses pegmatite! Resulting intergrowth of orthoclase was found in the Ural mountains in Russia forms crystals! Hydrothermal alteration ; also authigenic and detrital called perthite DE ; FR ; ES ; Запомнить сайт ; Словарь свой. To orthoklas by Johann Friedrich August Breithaupt orthoclase is a type of potassium feldspar, known. To orthoklas by Johann Friedrich August Breithaupt as muinstane is lairgely componed o orthoclase micro-cline by its lack of twinning.