Search results
Results From The WOW.Com Content Network
Plagioclase displaying cleavage. (unknown scale) In volcanic rocks, fine-grained plagioclase can display a "microlitic" texture of many small crystals. Plagioclase (/ ˈ p l æ dʒ (i) ə ˌ k l eɪ s, ˈ p l eɪ dʒ-,-ˌ k l eɪ z / PLAJ-(ee)-ə-klayss, PLAYJ-, -klayz) [4] is a series of tectosilicate (framework silicate) minerals within the feldspar group.
The most common mineral assemblage of granulite facies consists of antiperthitic plagioclase, alkali feldspar containing up to 50% albite and Al 2 O 3-rich pyroxenes. Transition between amphibolite and granulite facies is defined by these reaction isograds: amphibole → pyroxene + H 2 O biotite → K-feldspar + garnet + orthopyroxene + H 2 O.
Since they are primarily composed of plagioclase feldspar, most of Proterozoic anorthosites appear, in outcrop, to be grey or bluish. Individual plagioclase crystals may be black, white, blue, or grey, and may exhibit an iridescence known as labradorescence on fresh surfaces. The feldspar variety labradorite is commonly present in anorthosites.
Feldspar is present as plagioclase (typically oligoclase or andesine) with alkali feldspar making up less than 10% of the total feldspar content. Quartz (SiO 2 ) is present as more than 20% of the total quartz-alkali feldspar-plagioclase-feldspathoid ( QAPF ) content of the rock.
Feldspar is present as plagioclase (typically oligoclase or andesine) with 10% or less potassium feldspar. Quartz is present at between 5 and 20% of the rock. Biotite, amphiboles and pyroxenes are common dark accessory minerals. [1] Quartz diorite occurs in association with other granitic rock such as granodiorite, and with volcanic rock. In ...
Micrographic differs from graphic granite in being so much finer-grained that the texture can only be seen in a petrographic thin section with a microscope. The feldspar is usually orthoclase , but can also be albite , oligoclase or microcline .
Labradorite ((Ca, Na)(Al, Si) 4 O 8) is a calcium-enriched feldspar mineral first identified in Labrador, Canada, which can display an iridescent effect . Labradorite is an intermediate to calcic member of the plagioclase series. It has an anorthite percentage (%An) of between 50 and 70. The specific gravity ranges from 2.68 to 2.72.
Feldspar (/ ˈ f ɛ l (d) ˌ s p ɑːr / FEL(D)-spar; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. [3] The most common members of the feldspar group are the plagioclase (sodium-calcium) feldspars and the alkali (potassium ...