Datolite

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From The Gemology Project
Datolite
Chemical composition CaB[OH|SiO4]
Crystal system Monoclinic
Habit Short prisms, porcelaneous cryptocrystalline aggregates
Cleavage None
Fracture Conchoidal to uneven
Hardness 5 - 5.5
Optic nature Biaxial -
Refractive index 1.622 - 1.670
Birefringence 0.044
Specific gravity 2.95 - 3.0
Lustre Vitreous to porcelainous
Datolite from the Caledonia Mine, Mass City, Michigan


Datolite was named 1806 by Professor Jens Esmark from the Greek “to divide,” referring to the granular structure of the first specimens studied from Arendal, Norway.

Contents

Chemical composition

Calcium boron silicate hydroxide, CaBSiO4(OH) with minor to trace amounts of iron, manganese, and aluminum.

Diagnostics

Color

Crystals are colorless to white, often with a pale green or greenish yellow tint. Massive datolite is translucent to opaque grayish, reddish, pink, brown or yellow.

Refractive index

Optical nature and sign: biaxial negative
Refractive Index: nα = 1.622 – 1.626 nβ = 1.649 – 1.654 nγ = 1.666 – 1.670

Magnification

Massive datolite from Michigan cut en cabochon sometimes exhibits microscopic inclusions of native copper. The copper has sometimes oxidized to give the rough a copper-green color.

Occurence

Small quantities of greenish yellow to colorless datolite faceting rough have been produced at Dal'negorsk (Dalnegorsk), Primorskiy Kray, Russia. In addition to faceting rough, there was a largely unsuccessful attempt about 2002 to introduce a skarn rock composed of wollastonite and datolite to the US lapidary market; the material is marked by alternating bands light and dark olive-green bands and swirls. The skarn was apparently also produced in Primorskiy Kray (Dalnegorsk?). Compact, porcelain-like masses of datolite, some as large as 30cm in diameter and of a pleasing brown, red, yellow, malachite-green, white or grey color,, have been recovered since the mid-19th century from dozens of mines in Houghton, Keweenaw, and Ontonagen counties in Michigan’s famed “Copper Peninsula.” Rough is still being recovered by reworking abandoned mine dumps as well as scuba diving off the coastline.

Sources

  • Anthony, John W., Bideaux, Richard A., Bladh, Kenneth W., and Nichols, Monte C. (1990): Handbook of Mineralogy: Mineral Data Publishing, Tucson, Arizona
  • Palache, Charles, Harry Berman & Clifford Frondel (1944): The System of Mineralogy of James Dwight Dana and Edward Salisbury Dana Yale University 1837-1892, Seventh edition
  • Rosemeyer, T. (2003) The occurrence of porcelaneous datolite in michigan's lake superior copper district Part I: Northern Keweenaw County and Isle Royale National Park Rocks & Minerals, Vol. 78 No. 3 pp. 170-88
  • Rosemeyer, T. (2005) The occurrence of porcelaneous datolite in Michigan's Lake Superior Copper District: part 2: Southern Keweenaw, Houghton, and Ontonagon Counties, Michigan. Rocks & Minerals, Rocks & Minerals Vol 80 No. 3 pp154-177

External links

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