Chlorastrolite

  Chlorastrolite is a green or bluish green stone, usually with finely radiated or stellate masses. Stellate masses tend to be chatoyant. It can be subtranslucent to opaque. It is a variety of pumpellyite: Ca₂(Mg,Fe)Al₂(SiO₄)(Si₂O₇)(OH)₂·H₂O. Chlorastrolite was once thought to be an impure variety of prehnite or thomsonite.

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Occurrences

It occurs as amygdaloids and fracture fillings in basalt and as beach pebbles and granules in loose sediments, derived from those rocks.

It was first described from Isle Royale, Lake Superior by C. T. Jackson and J. D. Whitney in 1847. Chlorastrolite, also known as greenstone, is the official state gem of Michigan.

Other Names

• Green starstone as found especially in Michigan's Upper Peninsula.
• Greenstone or Isle Royale greenstone and Lake Superior greenstone.
• Turtle back is a name sometimes given because of the overall pattern, particularly in polished pieces, which roughly resembles that of a turtle shell.
• Uigite is a variety found on the island of Skye off the coast of Scotland.
• Zonochlorite is a misnomer sometimes given rounded masses of chlorastrolite within or weathered out of amygdaloidal cavities, especially those from the area around Nipigon Bay (on Lake Superior), Ontario, Canada. Confusion may derive from the fact that zonochlorite is sold as faux chlorastrolite.

References

• R.V.Dietrich: Gemrocks - chlorastrolite
• Norman King Huber, 1979, The geologic story of Isle Royale National Park, USGS Bulletin 1309
• Mindat w/ locations