High-temperature, sodium-rich igneous environments. Also as dense aggregates, grainy, and massive.įor more detailed information on the crystal structure of the Feldspars.Ģ,1 - basal 2,1 - prismatic 3,1 - pinacoidal. And, as those crystals grow within the manga chamber, they sometimes trap a little piece of liquid magma. Similar crystals have also been reported on Mount Kenya and Mount Kilimanjaro. This type of feldspar is rich in sodium, potassium, and aluminium silicate. So you can imagine, inside of a magma chamber, deep within the Earth, there is a liquid molten vat of magma and crystals. An erebus crystal is a crystal of anorthoclase found in the immediate area surrounding Mount Erebus near McMurdo Station, Antarctica. Unfortunately the crater is pretty inaccessible, so specimens are only recovered. Erebus, which is only a short distance from the US Antarctic base, McMurdo Station. These Anorthoclase crystals are quite plentiful in the crater atop Mt. In well formed crystals, usually prismatic. So there are these things called melt inclusions, which are little blobs of liquid magma. Anorthoclase Up for sale is a 6x2x1.4 cm crystal of Anorthoclase from Mt. Sodium potassium silicate, with a varying ratio of sodium to potassium.Ĭolorless, white, cream, yellow, pink, brown, reddish, greenish, gray, black ( Click here for more detailed information on the crystal structure of the Feldspars.) Localities produce known examples of these particular Feldspars, in which case the mineral type can be determined withoutĬomplex analysis. The scientific classification of Sanidine, Anorthoclase,Īnd Albite can only be determined with x-ray analysis. (For this reason, AnorthoclaseĬrystals can either be triclinic or monoclinic, although the tendency is usually toward triclinic.) Triclinic system, it leans toward Albite. Sanidine in regard to its crystal structure, and if it forms in the If it forms a monoclinic crystal, it leans toward ThisĪrrangement of the atoms forms the crystal structure of the mineral.ĭepending on the environment it forms in, it will either form a monoclinic or When Anorthoclase forms, its atoms form in a specific arrangement. Albite crystallizes in the triclinic system, whereas Sanidine crystallizes in the monoclinic system. It is interesting to note that this mineral series contains two end members crystallizing in different crystal systems. The name is from the Greek αν-, ορθός and κλάσις (not cleaving at right-angles), for its oblique cleavage.Anorthoclase is the intermediary member of the Albite- Sanidine series. It was first described in 1885 for an occurrence on Pantelleria Island, Trapani Province, Sicily. The mineral is typically found as a constituent of the fine grained matrix or as small phenocrysts which may occur as loose crystals in a weathered rock. Occurrence Īnorthoclase occurs in high temperature sodium rich volcanic and hypabyssal (shallow intrusive) rocks. With a summit elevation of 3,794 metres (12,448 ft), it is located in the Ross Dependency on Ross. It is the sixth-highest ultra mountain on the continent. If the high temperature intermediate composition alkali feldspars are allowed to cool slowly, exsolution occurs and a perthite structure results. Mount Erebus is the second-highest volcano in Antarctica (after Mount Sidley), the highest active volcano in Antarctica, and the southernmost active volcano on Earth. Anorthoclase and high albite exhibit triclinic symmetry, whereas sanidine and the low temperature orthoclase have monoclinic symmetry. d Cerberus Moulin, part of Pure Imagination Cave on Sandy Glacier, Mount Hood, 2015. c Crystals inside Hubert’s Nightmare Cave, Mount Erebus, 2010. Below 400–600 ☌ (800–1,100 ☏) only very limited solution exists (less than about 5% on both the low albite and microcline ends). Mount Erebus (Antarctica) is famous for its ice chimneys (Fig. Intermediate members of this series, high albite, anorthoclase and high sodium sanidine are stable at temperatures of 600 ☌ (1,100 ☏) and above. An anorthoclase crystal from Mount Erebus, Antarctica Structure and stability Īnorthoclase is an intermediate member of the high albite – sanidine alkali feldspar solid solution series. It typically consists of between 10 and 36 percent of KAlSi 3O 8 and between 64 and 90 percent of NaAlSi 3O 8. The mineral anorthoclase ((Na,K)AlSi 3O 8) is a crystalline solid solution in the alkali feldspar series, in which the sodium-aluminium silicate member exists in larger proportion. Polysynthetic twinning produces a grid pattern on Short prismatic crystals tabular, rhombic, flattened along These Anorthoclase are some of the best examples of the species known, and come from the crater of Mt. Doubly terminated anorthoclase crystal from Kinki Region, Honshu, Japan. A 6.5x3.4x2 cm loose crystal of Anorthoclase with small Aegirine crystals from Mt.
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