The Mineral epistilbite
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.
Chemical Formula
CaAl2Si6O16 · 5(H2O)
Color
Colorless, white, cream, tan, light yellow, light brown, gray. Rarely orange, orange-red, or pink.
Properties
Streak
Colorless |
Hardness
4 |
Transparency
Transparent to translucent |
Specific Gravity
2.2 - 2.3 |
Luster
Vitreous, pearly |
Cleavage
1,1 |
Fracture
Uneven |
Tenacity
Brittle |
Crystal Habits
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.
Noteworthy Localities
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.
Common Mineral Associations
Quartz, Calcite, Heulandite, Mordenite, Stilbite, Apophyllite, Celadonite
Distingushing Similar Minerals
Stilbite - Different crystal formation.