Bannisterite
General
Category	Mineral
Formula (repeating unit)	(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 mineral named in honor of mineralogist and x-ray crystallographer Dr. Frederick Allen Bannister (1901-1970). It is a calcium-dominant member of the ganophyllite group, and was previously identified as ganophyllite in 1936, but otherwise it is structurally related to the stilpnomelane group. It was approved by the IMA in 1967. ^[2]

Properties

Bannisterite grows in flat platy aggregates. It has smooth cleavages without ruling. It is a pleochroic mineral, which is an optical phenomenon. Depending on which axis the mineral is being inspected on, it appears as it changes colors. on the x axis, the mineral is seen in a colorless to pale yellow color. Common impurities include zinc and sodium. It occurs in sulfuric zinc deposits, as well as at silicate-carbonate hosted zinc deposits. It is associated with quartz, baryte, fluorite, rhodonite, sphalerite, galena, calcite, the apophyllite group and the amphibole supergroup. It doesn't have luminescent properties, under long, or shortwave ultraviolet light. It has a type locality in the Benallt mine in Wales, United Kingdom, and in the Franklin mine, New Jersey, United States. ^[2] It mainly consists of oxygen (44.64%) silicon (21.43%) and manganese (17.95%), but otherwise contains iron (5.23%), zinc (3.68%), aluminum (2.17%), magnesium (1.89%), hydrogen (1.09%), calcium (0.97%), potassium (0.84%) and sodium (0.12%). It is bladed, meaning it has thin lath-like crystals, and is prismatic, meaning the shape of the crystals are slender prisms. It has a barely detectable radioactivity due to it containing potassium. It has a 11.93 radioactivity measured in Gamma Ray American Petroleum Institute units. The concentration of the mineral measured per GRapi units is 8.39%. ^[3]

References