Bannisterite

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Formula: (Ca,K,Na)(Mn²⁺,Fe²⁺)₁₀(Si,Al)₁₆O₃₈(OH)₈.nH₂O
Phyllosilicate (sheet silicate), manganese-bearing mineral
Crystal system: Monoclinic
Specific gravity: 2.83 to 2.84 measured, 2.84 calculated
Hardness: 4
Streak: Creamy white
Colour: Dark brown
Luminescence: Not fluorescent under UV
Common impurities: Zn,Na
Environments

Metamorphic environments

Bannisterite occurs in metamorphosed manganese-zinc orebodies (HOM).

Localities

There are two co-type localities, the Benallt Mine, Rhiw, Aberdaron, Gwynedd, Wales, UK, and the Franklin Mine, Franklin, Sussex County, New Jersey, USA.

At Broken Hill, Broken Hill district, Yancowinna county, New South Wales, Australia, bannisterite is associated with rhodonite, sphalerite, fluorite, galena, apophyllite and quartz (HOM).
Bannisterite from Broken Hill - Image

At the Benallt Mine, Rhiw, Aberdaron, Gwynedd, Wales, UK, ganophyllite was recorded in 1948 in veins cutting manganese ore, but it was noted that the six-sided brown crystals, up to 1 mm in size, had two different optical forms. It was later later demonstrated that these cinnamon-brown crystals could be separated into two different minerals, ganophyllite and a new mineral bannisterite. The bannisterite is associated with calcite and is found on fracture surfaces of the manganese ore (MW).
Bannisterite from the Benallt Mine - Image

At the Franklin Mine, Franklin, Sussex County, New Jersey, USA, two types of so-called "ganophyllite" have been described.
One type occurs as groups of small, pale-brown, lath-like crystals associated with rhodonite, willemite and axinite in thin veinlets that cut the massive ore. This material is ganophyllite.
A second type of so-called "ganophyllite" occurs as aggregates of anhedral dark-brown plates that are as much as 5 cm across cleavage surfaces. It is associated with a greenish black manganese-bearing amphibole, calcite and baryte. This mineral is a layered mineral related to but different from ganophyllite. It is now named bannisterite (Mineralogical Magazine and Journal of the Mineralogical Society, 36 (283) 893-913).
Other minerals associated with bannisterite include rhodonite, sphalerite and quartz (HOM).
Bannisterite from the Franklin Mine - Image

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