zoisite

(zoi'sīt')
n.
A gray, brown, or pink mineral, Ca₂Al₃(SiO₄)₃(OH), used in ornamental stonework.

[German Zoisit, after Baron Sigismund Zois von Edelstein (1747-1819), Slovenian noble.]

Ca
Orthorhombic -- Rhombic bipyramidal

Environment

Metamorphic rocks; also in quartz veins, pegmatites, and some ore deposits.

Crystal description

Usually as isolated prismatic single crystals, sometimes to 4 in. (10 cm) long, but with tips generally poorly developed. Best crystal shapes are found when zoisite is embedded in quartz or metal sulfides, from either of which it breaks cleanly. Also in interlocking masses of needles.

Physical properties

Almost colorless, gray, brown, pink (thulite), blue, violet (tanzanite hues), and green (smaragdite). Luster glassy; hardness 6; specific gravity 3.3-3.4; fracture subconchoidal to uneven; cleavage perfect side pinacoid. Translucent to transparent. Pink thulite may fluoresce yellow-orange in longwave ultraviolet. Tanzanite is very pleochroic in three directions: yellow-orange, blue, and purple when it has not been heated. Heating destroys the yellow-orange color direction.

Composition

Alkaline calcium aluminum silicate (24.6% CaO, 33.7% Al ₂ O ₃ , 39.7% SiO ₂ , 2.0% H ₂ O). Some Fe may replace the Ca, and when enough is present -- over 5% Fe ₂ O ₃ -- it lowers the symmetry, and examples grade into clinozoisite and epidote.

Tests

Grows "worms," swells, and fuses in a dark bubbly mass that then does not easily melt down to a sphere. Light-colored varieties color blue on second melting after touch with drop of cobalt nitrate solution.

Distinguishing characteristics

Differing from the amphiboles by the single plane of cleavage (and often a pearly luster on the cleavage face). Tourmaline has no cleavage. Zoisite is much paler in hue than other members of group. Pink tourmaline is not fluorescent like some thulite, which is seldom in crystals of any appreciable size. Strong directional coloring differences (pleochroic), especially tanzanite.

Occurrence

Not uncommon in metamorphic rock areas in the U.S., as in New England, and most easily recognized as gray prismatic crystals in quartz veins. The fluorescent pink variety, thulite, is found in Mitchell Co., North Carolina, in a pegmatite with albite feldspar. Good brown crystals are found embedded in the sulfide ores at Ducktown, Tennessee. In California, zoisite occurs in green schists near Sulphur Bank.

Remarks

With the discovery of a vanadium-pigmented and very pleochroic blue phase in an impure, high-lime-content micaceous gneissic rock in Tanzania, promoted in the gem trade as tanzanite, zoisite has assumed much more importance as a species. It is also the green matrix material that often accompanies ruby corundum. Thulite derives its name from the ancient one for Norway (Thule), where it occurs associated with blue cyprine at Telemark in attractive specimens. The transparent, now costly, gemmy variety from Tanzania has been profitably promoted as tanzanite into popularity. It is a vanadium-pigmented and very pleochroic blue phase in an impure, high-lime-content, micaceous gneissic rock. Its ready cleavage makes it far less wearable than the sapphires it resembles, but it has given zoisite much more importance as a species. Zoisite is also the green matrix material that often accompanies ruby corundum. A gemmy green has been found in Pakistan.

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