Tourmaline with Stilbite Frosting

Tourmaline
Complex Borosilicates, (Na,Ca)(Mg,Fe2+,Fe3+,Al,Li)3-Al6 (BO3)3Si6O18(OH)4
Crystal System: Hexagonal
Hardness: 7.0
Density: 3.0-3.3

Tourmaline is represented by a group of minerals having complex compositions which are generally represented by the above formula. Elbaite, schorl, buergerite, dravite, uvite and liddicoatite are members of the tourmaline group. The most common member is the black, iron-bearing schorl.

Tourmaline is a mineral of widespread occurrence and is found in granite pegmatites, pneumatolytic veins and granites. It also occurs as a product of metasomatism involving boron.

Tourmaline occurs as transparent to translucent elongated, prismatic crystals in aggregates of parallel or radiating crystals. The two ends of a single crystal are sometimes terminated differently. Enormous crystals have been found. Tourmaline occurs less frequently as stubby prismatic crystals or compact masses. It often exhibits a vitreous luster and is remarkably variable in color, ranging from colorless to blue, pink (rubellite), yellowish green, green, brown (dravite), bluish black, and black. Some crystals are pink at one end and green or black at the other. These different zones of color are due to the time the material has taken to crystallize, with different tinctural agents effecting the crystal at different times during the formative process.

The individual colors are due to the presence of one or more of the following elements acting as chromophores: chrome, copper, iron, manganese, titanium and vanadium. Copper produces blues, chrome and vanadium produce greens, manganese produces pinks and reds, titanium produces browns, while iron in various oxidation states can play a role in producing greens, blues and reds in various tourmaline species. Mixing of these chromophores can produce other colors, such as yellow-green produced by titanium mixed with manganese and low iron.

Stilbite
Sodium Calcium Aluminum Silicate Hydrate, NaCa2Al5Si13O36.14H2O

Stilbite is a widespread mineral of the zeolite group and is most commonly found found in vesicles and cavities in basalt and other igneous rocks.

David Michaels provided this comment:
"The Tourmaline is a piece from the Himalaya Mine. It is unusual because it has a frosting of Stilbite instead of the more common Clevelandite."


Rocks from David Michaels' Collection

Index of Specimen Images

Table of Contents

bkeller@rockhounds.com 7/29/95