BANNISTERITE


Ca0.5(K,Na)0.5(Mn,Fe,Zn)10(Si,Al)16O38(OH)8 · 5.5H2O,
calcium potassium manganese silicate oxyhydroxide hydrate




Franklin is one of the two type localities for bannisterite, the other being the Benallt mine in Rhiw, Gwynneth, Wales.  The species was unknown in Palache’s time, but shortly after his monograph was published in 1935, a paper by Foshag (1936) described the mineral, which was then regarded as a variety of ganophyllite.  The specimens used in describing it had been found by R.B. Gage on the picking table at Franklin.  More than three decades later, bannisterite was recognized as a species new to science (Smith and Frondel, 1968).  Bannisterite is dark brown, with vitreous luster, and has a perfect cleavage that gives it a pronounced platy to sheeted habit visually resembling that of hendricksite or biotite mica.  In bannisterite, however, the cleavage plates are brittle rather than elastic.

Bannisterite occurs in coarse-grained masses of intergrown grains commonly 4-6 cm across, and in some specimens more.  Quartz is a commonly associated mineral, as is dark green richterite; these minerals are helpful in identifying it.  Dunn (1995, p. 487) mentions barite and sphalerite as additional minerals in some specimens.  Many specimens, however, are small fragments devoid, or nearly so, of associated species; good hand specimens are quite rare and held in high regard. On a global scale, Franklin bannisterite ranks as one of the finest localities for the species, the other notable collector locality being Broken Hill, New South Wales, Australia.  It should also be mentioned that bannisterite remains a rare mineral on the world scene, having been reported to date (October 2026) from only 30 localities in fewer than a dozen countries.

References

Dunn, P.J., Leavens, P.B., Norberg, J.A., and Ramik, R.A. (1981), Bannisterite: new chemical data and empirical formulae. American Mineralogist, vol. 66, nos. 9-10, p. 1063-1067. [Proposes a new empirical chemical formula for bannisterite based on numerous new analyses.  Includes 15 specimens from Franklin]

Dunn, P.J. (1995), Franklin and Sterling Hill, New Jersey:  the world’s most magnificent mineral deposits.  Privately published in five volumes, 755 pp.   [Bannisterite described, with one photo, on p. 487]

Foshag, W.F. (1936), Ganophyllite and zincian amphibole from Franklin Furnace, New Jersey. American Mineralogist, vol. 21, p. 63-67.  [First description of bannisterite from Franklin, then confused with ganophyllite.  Provides physical, optical, and chemical data]

Frondel, Clifford (1972), The minerals of Franklin and Sterling Hill, a check list.  Wiley-Interscience, 94 pp. [Brief overview of bannisterite on p. 44]

King, V.T., King, N.E., Betancourt, P.P., Bostwick, R.C., Chin, Peter, Hecht, T.J., Kuitems, S.M., Moritz, Harold, Nemetz, J.D., Nikischer, A.J., Sanford, Stephen, Van Fleet, J.A., and Verbeek, E.R. (2021), The Mineralogy of Franklin and Ogdensburg, New Jersey—A Photographic Celebration. Privately published, 1400 pp. [Photos of bannisterite specimens on pp. 449, 624, 660, and 787]

Smith, M.L. and Frondel, Clifford (1968), The related layered minerals ganophyllite, bannisterite and stilpnomelane. Mineralogical Magazine, vol. 36, p. 893-913. [Bannisterite recognized as a new mineral species distinct from ganophyllite]



Images




Dark brown, lustrous, coarse-grained mass of bannisterite with minor white calcite
Earl R. Verbeek
A dark brown, lustrous, coarse-grained mass of bannisterite with minor white calcite. The resemblance to mica is well shown in this specimen, but cleavage sheets of bannisterite are brittle rather than flexible. Franklin Mineral Museum specimen FMM-1133; ex SPEX/Gerstmann collection.

Identifier: BNS1a
Locality: Franklin mine, Franklin
Specimen size: 12.5 x 7.5 x 6 cm





Bannisterite in broad, intergrown, tabular, dark brown crystals of micaceous habit
Earl R. Verbeek
A fine bannisterite specimen, catalog number R6724, in the collection of the National Museum of Natural History. The bannisterite is in broad, intergrown, tabular, dark brown crystals of micaceous habit, some of which are 4-5 cm across. The white mineral at left is calcite, and the dark green mineral embedded in the calcite is an amphibole, probably richterite.

Identifier: BNS2a
Locality: Franklin mine, Franklin
Specimen size: 9 x 8 x 6 cm



Bannisterite in broad, intergrown, tabular, dark brown crystals of micaceous habit
Earl R. Verbeek
This is the same specimen as shown in BNS2a, but here viewed from a different angle to better show the diverse orientation and tabular shape of the bannisterite grains. The micaceous habit of the mineral is well shown near the center of the photo, where some of the grains are seen on edge.

Identifier: BNS2d
Locality: Franklin mine, Franklin
Specimen size: 9 x 8 x 6 cm





Large, dark brown, intergrown, platy grains of bannisterite
Earl R. Verbeek
Large, dark brown, intergrown, platy grains of bannisterite, showing pronounced micaceous habit, on a matrix of granular franklinite ore. This specimen, formerly no. 334 in the collection of former Sterling Hill miner John Kolic, is now (2021) on display in the Sterling Hill Mining Museum.

Identifier: BNS3a
Locality: Franklin mine, Franklin
Specimen size: Not measured; ca. 10 cm across



Large, dark brown, intergrown, platy grains of bannisterite
Earl R. Verbeek
The reverse side of the specimen shown in photo BNS3a, showing the label by Lawson Bauer (former chief chemist for New Jersey Zinc Company) identifying the mineral as ganophyllite, which it was then supposed to be. It was not until 1968 that bannisterite was described as a new species.

Identifier: BNS3b
Locality: Franklin mine, Franklin
Specimen size: Not measured; ca. 10 cm across





A cleavage mass of lustrous, dark brown bannisterite, white calcite, fine-grained franklinite-willemite ore.
Earl R. Verbeek
A cleavage mass of lustrous, dark brown bannisterite, bordered on the left by nearly nonfluorescent white calcite and then by fine-grained franklinite-willemite ore. A good example, particularly because it is in matrix (many are not). The attractive, dark brown color of the bannisterite, as revealed by multiple internal reflections from cleavage planes, is well shown in this photo. This specimen is labeled as ganophyllite on the back, as many are. Formerly owned by Fred Parker, it later became Mark Leger specimen ML356.

Identifier: BNS4a
Locality: Franklin mine, Franklin
Specimen size: 7.5 x 4.5 x 3 cm





Coarse-grained, intergrown, platy masses of dark brown bannisterite
Earl R. Verbeek
Coarse-grained, intergrown, platy masses of dark brown bannisterite, nearly free of other minerals except for local, almost microscopic, patchy films of a gray mineral, probably calcite. This specimen of bannisterite, unlike most others, is color-mottled in shades of medium to dark brown, and the cleavage faces have a subvitreous to locally dull luster, possibly due to alteration. This is specimen FMM-1132 of the Franklin Mineral Museum and came to that institution through donation of the SPEX/Gerstmann collection.

Identifier: BNS5a
Locality: Franklin mine, Franklin
Specimen size: 9 x 6.5 x 3 cm