Microlite

Microlite is a pale-yellow, reddish-brown, or black isometric mineral composed of sodium calcium tantalum oxide with a small amount of fluorine (Na,Ca)₂Ta₂O₆(O,OH,F). Microlite is a mineral in the pyrochlore group that occurs in pegmatites and constitutes an ore of tantalum. It has a Mohs hardness of 5.5 and a variable specific gravity of 4.2 to 6.4. It occurs as disseminated microscopic subtranslucent to opaque octahedral crystals with a refractive index of 2.0 to 2.2. Microlite is also called djalmaite.

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It occurs as a primary mineral in lithium-bearing granite pegmatites, and in miarolitic cavities in granites. Association minerals include: albite, lepidolite, topaz, beryl, tourmaline, spessartine, tantalite and fluorite.

Microlite was first described in 1835 for an occurrence on the Isalnd of Uto, State of Stockholm, Sweden. A type locality is the Clark Ledges pegmatite, Chesterfield, Hampshire County, Massachusetts. The name is from Greek mikros for "small" and lithos for "stone."

References

• Palache, C., H. Berman, and C. Frondel (1944) Dana’s system of mineralogy, (7th edition), v. I, 748–757
• Webmineral data
• Mindat with location data
• Mineral data publishing PDF