Bannisterite
From Wikipedia, the free encyclopedia
CategoryPhyllosilicate minerals
Formula(Ca,K,Na)(Mn2+,Fe2+)10(Si,Al)16O38(OH)8 · nH2O n ~ 5.5
Strunz classification9.EG.75
| Bannisterite | |
|---|---|
 | |
| General | |
| Category | Phyllosilicate minerals |
| Formula | (Ca,K,Na)(Mn2+,Fe2+)10(Si,Al)16O38(OH)8 · nH2O n ~ 5.5 |
| IMA symbol | Ban[1] |
| Strunz classification | 9.EG.75 |
| Dana classification | 74.1.1.4 |
| Crystal system | Monoclinic |
| Crystal class | Prismatic H-M Symbol: 2/m |
| Space group | B2/b |
| Unit cell | 8,955.48 |
| Identification | |
| Color | Dark brown |
| Cleavage | Perfect on {001} |
| Fracture | Micaceous |
| Mohs scale hardness | 4 |
| Luster | Sub-vitreous, resinous, greasy |
| Streak | Creamy white |
| Diaphaneity | Translucent |
| Specific gravity | 2.83 - 2.84 |
| Density | 2.83 - 2.84 |
| Optical properties | Biaxial (-) |
| Refractive index | nα = 1.544 - 1.574 nβ = 1.586 - 1.611 nγ = 1.589 - 1.612 |
| Birefringence | 0.045 |
| Pleochroism | Visible |
| 2V angle | 18°- 28° |
| Dispersion | Weak to moderate r < v |
| Ultraviolet fluorescence | None |
| Common impurities | Zn, Na |
Bannisterite is a phyllosilicate mineral named in honor of mineralogist and x-ray crystallographer Dr. Frederick Allen Bannister (1901–1970). It is chemically similar to tamaite, a calcium-dominant member of the ganophyllite group. It was previously identified as ganophyllite in 1936, but it is more structurally related to the stilpnomelane group. It was approved by the IMA in 1967.[2]
