- Chemistry: CaAl2Si6O16
- 5H2O, Hydrated Calcium Aluminum Silicate
- Class: Silicates
- Subclass: Tectosilicates
- Group: Zeolites
- Uses: Mineral specimen and chemical filter.
Epistilbite is one of the rarer zeolites, a popular group of minerals
to collect. Epistilbite commonly forms in the petrified bubbles (called
vesicles) of volcanic rocks that have had a slight amount of exposure to
metamorphism. It also forms in some pegmatites. Epistilbite's name means
over stilbite perhaps in allusion to crystals of epistilbite
that form over crystals of
stilbite, a closely related
Epistilbite has similar crystal habits to stilbite although it rarely
forms the large, impressive clusters that has made stilbite so famous.
Epistilbite's structure has a typical zeolite openness that allows large
ions and molecules to reside and actually move around inside the overall
framework. The structure actually contains open channels that allow water
and large ions to travel into and out of the crystal structure. The size
of these channels controls the size of the molecules or ions and therefore
a zeolite like epistilbite can act as a chemical sieve, allowing some ions
to pass through while blocking others.
- Color is colorless, white, pink and reddish.
- Luster is vitreous.
- Transparency: Crystals are transparent to translucent.
- Crystal System is monoclinic; 2/m
- Crystal Habits include tabular to prismatic crystals, sometimes
fibrous and radiating. Twinning
is common with the interpenetration twin forming a cruciform (cross) shaped
- Cleavage is perfect in one direction.
- Fracture is uneven.
- Hardness is 4.
- Specific Gravity is approximately 2.2 - 2.3 (very light)
- Streak is white.
- Associated Minerals are quartz,
and other zeolites.
- Notable Occurrences include Poona, India; Berufjord, Iceland;
Osilo, Sardinia, Italy; Isle of Skye, Scotland and in pegamtites at Bedford, New York and in the basalts of Hawaii, USA
- Best Field Indicators are crystal habit, low density, locality