Geigerite

Geigerite is a mineral, a complex hydrous manganese arsenate with formula: Mn₅(AsO₃OH)₂(AsO₄)₂·10H₂O. It forms triclinic pinacoidal, vitreous, colorless to red to brown crystals. It has a Mohs hardness of 3 and a specific gravity of 3.05.[1][2]

Geigerite
General
Category	Arsenate minerals
Formula (repeating unit)	Mn5(AsO3OH)2(AsO4)2·10H2O
Strunz classification	8.CE.05
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H-M symbol)
Space group	P1
Unit cell	a = 7.94 Å, b = 10.69 Å c = 6.77 Å; α = 80.97° β = 84.2°, γ = 81.85°; Z = 1
Identification
Formula mass	1,012.54 g/mol
Color	Red, colorless
Crystal habit	Platy, partly triangular crystals to 0.5 mm
Cleavage	Perfect
Tenacity	brittle
Mohs scale hardness	3
Luster	Vitreous - pearly
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3.05
Optical properties	Biaxial (-)
Refractive index	nα = 1.601 nβ = 1.630 nγ = 1.660
Birefringence	δ = 0.059
Pleochroism	Very weak, colorless to rose-red
2V angle	89°
References	[1][2][3][4]

It was discovered in Grischun, Switzerland in 1989. It was named in honor of Thomas Geiger (1886–1976), Wiesendangen, Switzerland, who studied the Falotta manganese ores.[4][5]

Composition

The chemical composition of geigerite is hydrous manganese arsenate (Mn₅(AsO₃OH)₂(AsO₄)₂·10H₂O).[3] The chemical composition was found by using an electron microprobe in the Falotta mines in Switzerland.[3]

Geologic occurrence

Geigerite can be found in the abandoned manganese mine in Oberhalbstein, Switzerland. It is mainly found in cavities in adiolarites, which are a form of igneous rock that have either a radial or fanlike texture of crystals. Geigerite is then formed by metamorphism of manganese oxide ores.[3] Recently, geigerite has been found in Fukushima Prefecture, Japan.[6] Geigerite has also been found in Mt. Nero Mine, Borghetto Di Vara, La Spieza, Italy.[7]

Structure

Geigerite’s crystal system is triclinic with perfect cleavage on the {010}. The Herman Mauguin symbol for geigerite is 1 and its space group is P1.[1] Geigerite contains two arsenate ions which are independent of one another. The first is the AsO₃OH group, and the second is the AsO₄. In the acidic AsO₃OH group, the As-O bonds are much shorter than the As-OH bonds. Similarly, in the AsO₄ group, As-O bonds are also shorter than the As-O bonds. The remaining bonds within both arsenate groups have nearly equal distances. In the ions there are three manganese atoms, these three links to six oxygen atoms to form a normal octahedral formation.[3]

Special characteristics

One interesting structural feature of geigerite is the presence of a complicated network of hydrogen bonds, which exceed the number of the hydrogen atoms.[3] Geigerite is classified under a group of metal copper (II) arsenates called the Lindackerite group. Minerals within this group have a formula where M equals either copper, calcium, manganese, zinc, or cobalt.[8]

See also

• List of minerals named after people

References

1. Geigerite data on Webmineral.com
2. Geigerite on Mindat.org
3. Graeser, S.; Schwander, H.; Bianchi, R.; Pilati, T.; Gramaccioli, C. M. (1989): Geigerite, the manganese analogue of chudobaite: Its description and crystal structure. American Mineralogist: 74, 676-684.
4. Geigerite in the Handbook of Mineralogy
5. Geiger, Thomas (1948). Manganerze in der Radiolariten Graubündens (PDF) (Ph.D.). ETH Zürich.
6. Kato et al. (1990) Ganseki-Koubutsu-Koshogaku Zasshi, 85, 184.; Mineralogical Journal Vol. 18 (1996), No. 4 pp 155-160
7. Cabella, R. (2000) Geigerite from Mt Nero manganese mine (Northern Apennines, La Spezia, Italy). Neues Jahrbuch fur Mineralogy-Monatshefte, 570-576.
8. Hybler, M. (2003) Crystal structure of Lindackerite, (Cu,Co,Ni)CU4(AsO4)(2)(AsO3OH)(2)center dot 9 H2O from Jachymov, Czech republic. European Journal of Mineralogy, 1035-1042.

• Bergknappe (2000) Verein der Freunde des Bergbaus in Graubunden Stiftung Bergbaumuseum Graubunden Schelzboden-Davos. 31-32.