Humite

(mineralogy) A humic coal mineral. A series of magnesium neosilicate minerals closely related in crystal structure and chemical composition.

A homologous series of magnesium nesosilicate minerals having the general composition Mg_2n+1(SiO₄)_n(F, OH)₂. The known species include norbergite (n = 1), chondrodite (n = 2), humite (n = 3), and clinohumite (n = 4). All are based on hexagonal close-packed oxygen and fluorine atoms, the Mg atoms occupying octahedral interstices and the Si atoms occupying tetrahedral interstices. Forsterite, norbergite, and humite are orthorhombic; chondrodite and clinohumite are monoclinic; brucite is trigonal. Manganese analogs of these minerals occur as pink grains in metamorphosed manganese ores derived from preexisting siliceous carbonates and sedimentary manganese oxides. Other cations which can occur as substituents are Fe²⁺, Ca²⁺, Al³⁺, and Ti⁴⁺.

The minerals of the humite group have similar physical properties. The luster is resinous, and the color usually light yellow, brown, orange, or red. The pure synthetic Mg end members are colorless. Hardness is 6–6½ on Mohs scale, specific gravity is 3.1–3.2. They are very difficult to distinguish visually, and x-ray diffraction, electron microprobe, or optical techniques are required. They are found in regionally crystallized marbles, usually the skarn minerals associated with iron ores. See also Silicate minerals.

Humite specimen with spinel (purple) intergrowth

Humite is an orthorhombic mineral of a transparent vitreous brown to orange color. It is found in the volcanically ejected masses of Vesuvius. It is a nesosilicate of iron and magnesium which also contains fluorine and hydroxide. The formula is: (Mg,Fe)₇(SiO₄)₃(F,OH)₂.

It was first described in 1813 and named for Abraham Hume (1749-1838).

Humite is also used as the name of a group of closely related minerals -- humite, clinohumite, chondrodite, and norbergite. These minerals are similar in structure and in chemistry. Deer et al. (1997) note that a general formula for the humite-group minerals can be simply expressed as nM₂SiO₄ . M(OH,F)₂ , where M is Mg, Fe, Ti etc. and n = 1 (norbergite), 2 (chondrodite), 3 (humite), and 4 (clinohumite). Deer et al. (1997) also note that although the formula above correctly describes the chemistries of these minerals, structural formulas are more complex.

See also

• Jerrygibbsite

References

• Deer, W. A., Howie, R. A., and Zussman, J., 1997, Rock-forming minerals: Volume 1A, Second Edition, Orthosilicates. The Geological Society, Bath, UK. ISBN 1-897799-8-8

External links

• Webmineral
• Mindat w/ locations

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