| Geigerite | |
| Category: | Arsenate minerals |
| Formula: | Mn5(AsO3OH)2(AsO4)2·10H2O |
| Imasymbol: | Gga[1] |
| Molweight: | 1,012.54 g/mol |
| Strunz: | 8.CE.05 |
| System: | Triclinic |
| Class: | Pinacoidal (same H-M symbol) |
| Symmetry: | P |
| 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 |
| Color: | Red, colorless |
| Habit: | Platy, partly triangular crystals to 0.5 mm |
| Cleavage: | Perfect |
| Tenacity: | brittle |
| Mohs: | 3 |
| Luster: | Vitreous – pearly |
| Opticalprop: | Biaxial (−) |
| Refractive: | nα = 1.601 nβ = 1.630 nγ = 1.660 |
| Birefringence: | δ = 0.059 |
| Pleochroism: | Very weak, colorless to rose-red |
| 2V: | 89° |
| Streak: | White |
| Gravity: | 3.05 |
| Diaphaneity: | Transparent to translucent |
| References: | [2] [3] [4] |
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.[3]
It was discovered in Grischun, Switzerland in 1989. It was namedin honor of Thomas Geiger (1886–1976), Wiesendangen, Switzerland, who studied the Falotta manganese ores.[4] [5]
The chemical composition of geigerite is hydrous manganese arsenate (Mn5(AsO3OH)2(AsO4)2·10H2O). The chemical composition was found by using an electron microprobe in the Falotta mines in Switzerland.[6]
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.[6] Recently, geigerite has been found in Fukushima Prefecture, Japan.[7] Geigerite has also been found in Mt. Nero Mine, Borghetto Di Vara, La Spieza, Italy.[8]
Geigerite’s crystal system is triclinic with perfect cleavage on the . The Herman Mauguin symbol for geigerite is and its space group is P.[2] 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.[6]
One interesting structural feature of geigerite is the presence of a complicated network of hydrogen bonds, which exceed the number of the hydrogen atoms.[6] Geigeriteis 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.[9]