A Brief Introduction To The Tourmaline Group

   All collectors and much of the public has heard of tourmaline, but not all realize that it isn’t actually a mineral, but a group name, presently applied to thirteen distinct species. This is a brief introduction to the group; for those desirous of more in-depth information a list of references is provided.

   This is an excellent group for a collector to specialize in. Crystal habit isn’t particularly diverse, but there is an endless variety of colors and color combinations available. In fact, tourmaline shows a greater range of colors -over one hundred!- than any other gem. Individual crystals are often color-zoned, with either different colored cores and exteriors, like elbaite 'watermelon' tourmaline with a red interior and green exterior; or with bands of color perpendicular to the length of the crystal, like pink elbaite crystals with blue caps. It is a popular gemstone, with global annual sales of millions of dollars; and a single mine can produce literally tons of crystals, so there are always lots of specimens on the market from around the world. Elbaite is the most popular member of the group, and schorl, dravite, and uvite are also plentiful. Specimens from the old classic localities, like Elba, Italy; or Mount Mica, Maine, while seldom available, are fun to hunt for. Color-zoned crystals, 'bent' (broken and healed) crystals, and doubly terminated crystals showing hemimorphism are particularly desirable. Brazilian crystals with brilliant blue or red color are among the most valuable and sought after, and huge crystals worth hundreds of thousands of dollars have been found.

Basic Physical Properties of the Tourmaline Group

Tourmaline group members are borosilicates of the general formula:

WX3Y6(BO3)3Si6O18(O,OH,F)4, where

W= Ca, K, Na

X= Al, Fe2+, Li, Mg, Mn2+

Y = Al, Cr3+, Fe3+

Dana's New Mineralogy breaks the group into five subgroups:

Alkali-Deficient Tourmaline Subgroup: Foitite, Magnesiofoitite (?)*, Rossmanite*
Calcic Tourmaline Subgroup: Liddicoatite, Uvite, Feruvite
Ferric Tourmaline Subgroup: Buergerite, Povondraite
Lithian Tourmaline Subgroup: Olenite, Elbaite
Sodic Tourmaline Subgroup: Dravite, Schorl, Chromdravite
* "unnamed" in the book

Species of the tourmaline group crystallize in the Hexagonal system, Trigonal Division. They are usually elongated prisms, occasionally stubby, and hemimorphic (with different terminations on each end of the crystal). Crystals are often triangular in cross-section.

Physical properties: Hardness of 7 to 7.5, brittle, conchoidal fracture, no cleavage, specific gravity a little over 3, transparent to translucent, colorless streak, vitreous to resinous luster. Occasionally pleochroic or chatoyant. Strongly piezoelectric and pyroelectric. Insoluble in acids.

Geological Occurrence: Common in igneous and metamorphic rocks, and in sedimentary rocks as detrital or authigenic grains. Very common in pegmatites and greisen, occasionally in marbles. Dietrich states that "... one or another of the tourmaline group minerals occurs in essentially all kinds of rocks..."

The nomenclature of the tourmaline group is a mess. In this list you’ll not find the popular varietal names, like "rubellite" or "indicolite", these names are useless and confusing, and should be avoided. Formulae cited are ideal end-member composition, from the Caltech site.

Buergerite   Na Fe³⁺₃Al₆(BO₃)₃Si₆O₁₈(O)₃(OH)

Dark brown to black crystals, in divergent sprays, to 4 cm long. Uncommon, from Mexquitic, San Luis Potosi, Mexico, where it occurs in hydrothermally altered tuffs in clay seams in rhyolite. Panczner (1987) in Minerals of Mexico states: "This is the type location for this mineral, but the exact location is unknown. The man who discovered and mined all the buergerite that reached the mineral world has died and told no one the exact location." This appears to be the only locality for buergerite.

Chromdravite   NaMg₃[Cr,Fe³⁺]₆(BO₃)₃Si₆O₁₈(OH)₄

Very rare, found as dark green crystals and grains to perhaps a few mm, in micaceous metasomatic clay-carbonate rocks at the Velikaya Guba uranium occurrence, Zaonezhskii Peninsula, Onezhkii basin, central Karelia, Russia.

Dravite   NaMg₃Al₆(BO₃)₃Si₆O₁₈(OH)₄

Crystals are stubby to long prismatic, often striated vertically. Colors include brown, black, greenish-black, dark red, and pale bluish-green to emerald green. Found in many places, includint many in the US, and Austria, Kazakhstan, Kenya, and Brazil, but the best and most famous crystals are probably those from Yinnietharra, Western Australia, where sharp complete doubly-terminated brown crystals to the size of a softball (surprisingly nonhemimorphic!) have been found. Not hard to obtain.

Elbaite   Na(Li_1.5Al_1.5)Al₆(BO₃)₃Si₆O₁₈(OH)₄

This is the most famous tourmaline group member, occurring in many colors, including red, green, blue, yellow, and rarely colorless. Crystals are short to long prismatic, vertically striated, occasionally acicular or other habits. Elbaite most often occurs in granite pegmatites, and is found in fine crystals at numerous localities all over the world. Famous localities include the Himalaya Mine in California; Mount Mica in Maine; Elba Island in Italy; and many mines in Brazil. Crystals to 1.6 meters long have been found. Click here to see a couple of nice elbaites from CA and Brazil.

Feruvite   CaFe²⁺₃[Al₅Mg](BO₃)₃Si₆O₁₈(OH)₄

Rare, dark brown-black grains to 2 millimeters, usually zoned with feruvite, dravite, and schorl intergrowths. First found on Cuvier Island, New Zealand; also known from Red Cross Lake, Manitoba; and the Sullivan deposit, British Columbia. Formed by hydrothermal replacement of silicates in a pegmatitic rock.

Foitite   [Fe²⁺₂(Al,Fe³⁺)]Al₆(BO₃)₃Si₆O₁₈(OH)₄

Dark indigo to black, typical prismatic striated crystals with triangular cross-section, found in crystals to a few cm in California's White Queen Mine. Known from a few other localities in Montana, Italy, Czech Republic, and Australia. Click here to see a foitite from Italy.

Liddicoatite   Ca(Li₂Al)Al₆(BO₃)₃Si₆O₁₈(OH)₄

Found in stout crystals, up to 25x10 centimeters, striated along their lengths. Astonishing and complex multi-colored color-zoning, in triangular patterns, is exposed when crystals are cut perpendicular to their elongation. The best are found in pegmatites in various parts of Madagascar, also some from the Czech Republic, Russia, and Brazil. The polished slabs are beautiful but very expensive. Click here for a beautiful photo; or here for a cool Caltech animation taking you thru the slices of a zoned crystal.

Magnesiofoitite   [Mg²⁺₂(Al³⁺)]Al₆(BO₃)₃Si₆O₁₈(OH)₄

New and rare, found as very small acicular greyish crystals on matrix, from the Kyonasawa area, Mitomi-Mura, Yamanashi-Ken, Honshu, Japan.

Olenite   NaAl₃Al₆(BO₃)₃Si₆O₁₈(O)₃(OH)

Rare, vitreous pale pink. Found in granitic pegmatites as zones in elbaite crystals to a few cm, from the Decembrist pegmatite field, Olenek River Basin, Russia; and Elba Italy. Also from Maine, California, and North Carolina.

Povondraite   NaFe³⁺₃Fe³⁺₆(BO₃)₃Si₆O₁₈(O)₃(OH)

Very rare, black crystals to 3 mm, in clusters to 1 cm. Found in fractures and lining cavities in schist metamorphosed from sedimentary rocks, at Cristalmuya gorge, 200 m downhill from the San Francisco Mine, 50 km from Villa Tunari, Alto Chapare Province, Cochabamba Dept., Bolivia.

Rossmanite   (LiAl₂)Al₆(BO₃)₃Si₆O₁₈(OH)₄

New and rare, pale pink prismatic striated crystals 5 mm thick x 25 mm long, associated with elbaite in lepidolite, from the Rozna pegmatite, Hradisko Quarry, near Rozna, western Moravia, Czech Republic

Schorl   NaFe²⁺₃Al₆(BO₃)₃Si₆O₁₈(OH)₄

Usually black, rarely dark brown, blue, green.. Superb lustrous sharp crystals, from granite pegmatites, quartz veins, greisens, schists, gneisses, and phyllites. Localities for fine xls are numerous, including California, Maine, Brazil, Pakistan and Afghanistan, and others. Crystals to 1.5 meters have been found.

Uvite   CaMg₃[Al₅Mg](BO₃)₃Si₆O₁₈(OH)₄

Crystals are often equant, as well short to long prismatic. Colors include black, red, green and brown. Lustrous sharp equant black crystals found at several localities in New York State, as well as Burma, Sri Lanka, Kenya, and many other places. Finest specimens are bright red or green crystals on quartz or magnesite crystals from Brumhado, Brazil. Click here to see a typical specimen from NY.

Internet References

Hard Copy References

- Mineralogy, John Sinkankas, 1964
- The Tourmaline Group, R.V. Dietrich, 1985
- Tourmaline, special issue of the Mineralogical Record, V16 #5, 1985
- Minerals of Mexico, William Panczner, 1987
- Dana's New Mineralogy, Eighth Edition, Gaines,Skinner,Foord,Mason, and Rosenweig, 1997
- Fleischer's Glossary of Mineral Species, Mandarino, 1999
- Handbook of Mineralogy, Volume II, Parts 1 & 2, Anthony,Bideaux,Bladh,Nichols, 1995
- Minerals First Discovered on the Territory of the Former Soviet Union, Pekov, 1998