The gem variety of pumpellyite, chlorastrolite, is best known from the Lake Superior district of the United States. It typically forms aggregates of packed fibers that are mixed with other minerals, resulting in a green and white pattern reminiscent of tortoise shell. The effect is best observed when the fibers are in radial clusters that yield circular markings.
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|Crystallography||Monoclinic; Crystals fibrous, ﬂattened plates, in clusters or dense mats of random fibers.|
|Colors||Green, bluish green, brown.|
|Luster||Vitreous: silky when fibrous.|
|Hardness||6; chlorastrolite: 5-6.|
|Specific Gravity||3.18-3.33; chlorastrolite: 3.1-3.5.|
|Cleavage||Distinct in 2 directions. No cleavage in massive material.|
|Stone Sizes||Chlorastrolite is cut as cabochons up to 1 to 2 inches long. Very fine deep green material occurs in small sizes, yielding stones less than 1 inch long.|
|Formula||Ca2MgAl2(SiO4)(Si2O7)(OH)2 · H2O|
Distinct, as follows:
a: colorless/pale greenish yellow/pale yellowish green.
β: bluish green/pale green/ brownish yellow.
γ: colorless/ pale yellowish brown/ brownish yellow.
Note: Pale-colored pumpellyite has low birefringence, weak dispersion, and lower indices. Dark-colored pumpellyite has higher values for all these properties.
PUMPELLYITE (= CHLORASTROLITE) Also note:
Ferropumpellyite: contains ferrous iron. Julgolditet: contains both ferric and ferrous iron.
Optics: a=1.674-1.702; β=1.675-1.715; γ= 1.688-1.722.
Biaxial (+) and (-), 2V: 26—85°. May show anomalous birefringent colors. Mean index ~ 1.7.
Occurrence: Pumpellyite occurs in a wide variety of igneous and metamorphic rocks and environments.
Scotland; Austria; Finland; USSR; New Zealand; South Africa; other localities,
Calumet, Michigan: in copper ores; also at Isle Royale.
Lake Superior, Michigan (non-gem) and on the Keweenaw Peninsula, Michigan (non-gem).
Lake Superior: basic igneous rocks on the periphery of the lake contain spherical aggregates of green fibers of chlorastrolite, in masses. This material is sometimes cut but is not homogeneous.
California: in glaucophane schists.
New Jersey: in basalts (traprocks).
Comments: The gem variety of pumpellyite, chlorastrolite, is best known from the Lake Superior district of the United States. It typically forms aggregates of packed fibers that are mixed with other minerals, resulting in a green and white pattern reminiscent of tortoise shell. The effect is best observed when the fibers are in radial clusters that yield circular markings. The turtle-back pattern is considered most desirable and. because of the chatoyancy of the fibers, seems to move within the stone as the lighting is changed. Pumpellyite is a common mineral in many parts of the world, but fine green material is scarce and greatly prized by collectors. The best color is a very intense green resembling the color of fine emerald or Imperial jade. Good-quality chlorastrolite with strong pattern and color is now difficult to obtain.
Name: Pumpellyite after Raphael Pumpelly, Michigan geologist who did pioneering studies of the Keweenaw Peninsula copper district of Michigan. Chlorastrolite is from Greek words meaning green star stone.