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The structure of a feldspar crystal is based on aluminosilicate tetrahedra. Each tetrahedron consists of an aluminium or silicon ion surrounded by four oxygen ions. Each oxygen ion, in turn, is shared by a neighbouring tetrahedron to form a three-dimensional network.
The crystal structure of ideal potassium feldspar has four sets of tetrahedral sites, each capable of accepting either an aluminum or a silicon ion. These are labeled the T 1 o, T 1 m, T 2 o, and T 2 m sites.
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.
Orthoclase crystal twinning from the Organ Mountains in New Mexico. Orthoclase is a common constituent of most granites and other felsic igneous rocks and often forms huge crystals and masses in pegmatite. Typically, the pure potassium endmember of orthoclase forms a solid solution with albite, the sodium endmember (NaAlSi 3 O 8), of plagioclase.
Phenocrysts of clear glassy rounded quartz and white orthoclase feldspar are set in a fine-grained matrix. Sample is just over 10 cm long. Quartz-porphyry, in layman's terms, is a type of volcanic rock containing large porphyritic crystals of quartz. [1] [2] These rocks are classified as hemi-crystalline acid rocks.
Labradorite occurs in mafic igneous rocks and is the feldspar variety most common in basalt and gabbro. The uncommon anorthosite bodies are composed almost entirely of labradorite. [ 4 ] It also is found in metamorphic amphibolites and as a detrital component of some sediments.
Celsian is an uncommon feldspar mineral, barium aluminosilicate, Ba Al 2 Si 2 O 8.The mineral occurs in contact metamorphic rocks with significant barium content. Its crystal system is monoclinic, and it is white, yellow, or transparent in appearance.
The mineral anorthoclase ((Na,K)AlSi 3 O 8) is a crystalline solid solution in the alkali feldspar series, in which the sodium-aluminium silicate member exists in larger proportion. It typically consists of between 10 and 36 percent of KAlSi 3 O 8 and between 64 and 90 percent of NaAlSi 3 O 8. [5] An anorthoclase crystal from Mount Erebus ...