Bannisterite is a calcium potassium manganese aluminosilicate hydroxide hydrate mineral. Franklin samples are remarkably constant in composition, suggesting they might have come from one localized occurrence. Numerous analyses have been published (Dunn et al., 1981b). Bannisterite from Franklin has Fe, Al, and Mg approximately 12, Mn approximately 48, Zn approximately 8, and both Ca and K nearly 4 atoms per unit-cell. Altered Franklin material is enriched in Mg and deficient in Ca and Mn.
Bannisterite was originally described as an uncommon form of ganophyllite (Foshag, 1936) and was redefined as a unique species by Smith and Frondel (1968). A second occurrence, of highly ferroan material from Australia (Plimer, 1977), led to a re-examination of the species by Dunn et al. (1981b). Bannisterite is known from Franklin but not from Sterling Hill. Bannisterite is a layer silicate closely related to ganophyllite.
The crystal structure of bannisterite was determined by Threadgold (1979) using Australian ferroan bannisterite. The structure of Franklin material was further described and refined by Heaney et al. (1990, 1992), who stated that "bannisterite has a modified 2:1 trioctahedral layer structure in which some of the tetrahedra are inverted toward the interlayer region and linked to inverted tetrahedra in the opposite layer. The octahedral sheet is strongly corrugated along b." The tetrahedral sheet consists of 5-, 6-, and 7-fold rings; Al is concentrated into two of the four inverted tetrahedra; and the interlayer cations are highly disordered. A review is given by Guggenheim and Eggleton (1988).
Bannisterite occurs as large massive hand-specimens, up to 250 cm3, composed almost entirely of dark brown platy crystals and slightly offset or curved surfaces. Bannisterite is brittle and the density is 2.83 g/cm3. There is no discernible fluorescence in ultraviolet. It is best distinguished from similar species on the basis of optical properties and/or X-ray powder diffraction data.
Bannisterite was found on the picking table at the Franklin Mine, but nothing is known of its geologic occurrence. The major associated minerals are richterite, barite, quartz, and sphalerite. Several ill-defined manganese layer-silicates are present. (Dunn, 1995)

 Location Found: Franklin (Type Locality)
 Year Discovered: 1967
 Formula: Ca0.5(K,Na)0.5(Mn,Fe,Zn)10(Si,Al)16O38(OH)8 · 5.5H2O
 Essential Elements: Calcium, Hydrogen, Manganese, Oxygen, Potassium, Silicon
 All Elements in Formula: Aluminum, Calcium, Hydrogen, Iron, Manganese, Oxygen, Potassium, Silicon, Sodium, Zinc
 IMA Status: Approved 1967
 To find out more about this mineral at minDat's website, follow this link   Bannisterite

Dunn, Pete J. (1995). Franklin and Sterling Hill New Jersey: the world's most magnificent mineral deposits. Franklin, NJ.: The Franklin-Ogdensburg Mineralogical Society. p.487

Frondel, Clifford (1972). The minerals of Franklin and Sterling Hill, a checklist. NY.: John Willey & Sons. p.44

The Picking Table References
 PT Issue and PageDescription / Comment
View IssueV. 34, No. 2 - Fall 1993, pg. 32Research Report The Crystal Structure of Bannisterite
View IssueV. 23, No. 1 - Spring 1982, pg. 15Mineral Notes Research Reports, Bannisterite
View IssueV. 10, No. 1 - February 1969, pg. 6Mineral Notes - Bannisterite/Ganophyllite/Stilpnomelane
View IssueV. 8, No. 2 - August 1967, pg. 8Mineralogical Data - Bannisterite/Ganophyllite

Bannisterite, minor Willemite and Franklinite from Franklin, NJ.
Bannisterite (dark brown), minor Willemite (red-brown) and Franklinite (black) from Franklin, NJ. Photo by Robert A. Boymistruk.

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