THE MINERAL STILBITE
- Chemistry: NaCa2Al5Si13O36 -14H2O, Hydrated sodium calcium aluminum silicate
- Class: Silicates
- Subclass: Tektosilicates
- Group: Zeolites
- Uses: mineral specimen and chemical filter
Stilbite is a common and perhaps the most popular zeolite mineral for collectors.
Stilbite crystals can aggregate together to form a structure resembling wheat sheafs.
This hourglass structure looks like several crystals stacked parallel to each other with the tops and bottoms of this structure fanning out while the middle remains thin.
Stilbite's hallmark crystal habit is unique to stilbite and a rarer but related zeolite called stellerite
Whether in the wheat sheafs or not, stilbite can be a hansome specimen with its pearly luster and often colorful pink tints.
Stilbite commonly forms nice crystals inside the petrified bubbles (called vesicles) of volcanic rocks that have undergone a small amount of metamorphism.
Stilbite's structure has a typical zeolite openness about it that allows large ions and molecules to reside and actually move around inside the overall framework.
The structure 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 stilbite can act as a chemical sieve.
Stilbite's structure contains rings of alumino-silicate tetrahedrons oriented in one direction and this produces the prominant pinacoid faces, the perfect cleavage and the unique luster on those faces.
- Color is pink or white; also tinted yellow and red.
- Luster is vitreous to pearly especially on the prominant pinacoid and cleavage surfaces.
- Transparency: crystals are transparent to mostly translucent.
- Crystal System is monoclinic; 2/m
- Crystal Habits include platy often thin crystals that can aggregate together into a wheat sheaflike structure.
The prominant pinacoid is sometimes but rarely modified by other pinacoid and prism faces.
Cruciform (cross-like) twins can also be found.
Also forms radiating nodules.
- Cleavage is perfect in one direction parallel to the prominant pinacoid.
- Fracture is uneven.
- Hardness is 3.5 - 4.
- Specific Gravity is approximately 2.2 (very light)
- Streak is white.
- Associated Minerals are quartz, calcite, babingtonite, apophyllite, heulandite, natrolite and other zeolites.
- Notable Occurrences include Poona, India; Scotland; Iceland; New Jersey and Nova Scotia, Canada.
- Best Field Indicators are crystal habit, luster, density and associations.