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Epistilbite is a rare member of the zeolite group. It has a very similar chemical makeup to Stilbite, and can be difficult to distinguish from Stilbite. Epistilbite is named from the Greek Epi which means "similar" and Stilbite, due to the similarity of this mineral to Stilbite. Epistilbite is not dimorphous with Stilbite, despite its prefix Epi which often implies a dimorph. Interestingly, Epistilbite it dimorphous with a different zeolite, the rare Goosecreekite. |
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Chemical Formula |
CaAl2Si6O16 · 5(H2O) |
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Composition |
Hydrous calcium aluminum silicate |
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Color |
Colorless, white, cream, tan, light yellow, light brown, gray. Rarely orange, orange-red, or pink. |
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Streak |
Colorless |
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Hardness |
4 |
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Crystal System |
Triclinic |
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Crystal Forms
and Aggregates |
Crystals are often prismatic, with distinct chisel-topped terminations. Also globular and in hemispherical, ball-like formations. Other habits in include thin, tabular crystals and groups, columnar, in radiating clusters, and in jagged, haphazard crystals.
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Transparency |
Transparent to translucent |
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Specific Gravity |
2.2 - 2.3 |
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Luster |
Vitreous, pearly |
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Cleavage |
1,1 |
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Fracture |
Uneven |
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Tenacity |
Brittle |
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In Group |
Silicates; Tectosilicates; Zeolite Group |
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Striking Features |
Crystal habits and association with other zeolites. |
| Environment |
In air vesicles in volcanic basalt. |
| Rock Type |
Igneous |
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Popularity (1-4)
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3 |
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Prevalence (1-3)
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3 |
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Demand (1-3) |
3 |
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Epistilbite is not a common member of the zeolite group, and good localities are few. Some of the largest crystals come from Berufjördur, Iceland, at Teigarhorn and Fossárfell. Sharp Epistilbite crystal group and aesthetic balls on matrix come from several localities in Maharashtra, India, most notably at Jalgaon, but also at Pune, Nasik, and Ahmadnagar. Small crystals with red inclusions come from Osilo, Sardinia, Italy. In the U.S., colorless micro crystals are found in the Puu o Ehu Quarry, Kailua, Oahu Island, Hawaii.
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Stilbite - Different crystal formation.
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