BREITHAUPTITE


NiSb,
nickel antinomide




Breithauptite is a rare component of the nickel arsenide assemblage from the Trotter mine at Franklin.  It occurs in microscopic grains and had passed unrecognized until 1990, when Hoyt B. Sutphin of the U.S. Geological Survey (Denver) noticed it in polished sections.  In reflected light, as shown in the photographs below, breithauptite shows a violet tint and is distinctly pleochroic.  The only mineral likely to be confused with it is nickeline, also present in these same samples, but the color of nickeline inclines more toward coppery red rather than violet, and the two are clearly different when viewed together.

In the polished sections so far examined, all derived from the same two specimens, breithauptite occurs either embedded within pararammelsbergite near its contact with nickeline or lies along the contact between these two minerals.  Nowhere was breithauptite observed to be fully embedded in nickeline, nor was it observed in contact with any of the other arsenide minerals present except for the two mentioned.  Trotter mine nickeline (nominally NiAs) contains a small amount of antimony as an impurity and in some specimens is partly replaced along its outer margins by pararammelsbergite (NiAs2), as discussed in detail by Oen et al. (1984).  Remnant patches of nickeline within pararammelsbergite (see photo BTH1f below) are a visible expression of this process.  The restriction of breithauptite to the transition zone between nickeline and pararammelsbergite suggests that it formed from antimony freed from the nickeline during replacement.

Trotter mine breithauptite occurs as pinkish lavender to violet, anhedral blebs, most of them 0.04 mm or less in maximum dimension, and thus can be seen and studied only at magnifications of 150× or greater.  Its small grain size precluded study by X-ray camera methods during the study of Verbeek and Sutphin (1990), and it has not yet been subjected to electron probe microanalysis.  It is most conveniently identified by reflected light microscopy, where its lavender to violet color and strong reflection pleochroism (see photo BTH1c) distinguish it from the other minerals present.

To date, breithauptite has been observed and studied in only two specimens:  the one shown here, and another, derived from the same parent mass, presently (2023) in the collection of Steven Phillips of Hamburg, NJ.  Two other sawn slabs are known from the same specimen; one is at Harvard University (catalog no. 117576) and the other is on display in Baum Hall of the Franklin Mineral Museum (specimen FMM-121).  All likely contain breithauptite.

References

Dunn, P.J. (1995), Franklin and Sterling Hill, New Jersey:  the world’s most magnificent mineral deposits:  Privately published in five volumes, 755 pp. [Short description of breithauptite on p. 544]
 
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 breithauptite—same specimen as shown here—appear on p. 799]

Oen, I.S., Dunn, P.J., and Kieft, Cornelis (1984), The nickel arsenide assemblage from Franklin, New Jersey: description and interpretation. Neues Jahrbuch für Mineralogie Abhandlungen, vol. 150, no. 3, p. 259-272. [No mention of breithauptite, but this paper offers the definitive description, with a genetic model, of the textural relations and compositions of the various nickel arsenide minerals from the Trotter shaft occurrence at Franklin]

Verbeek, E.R., and Sutphin, H.B., 1990, Breithauptite from the nickel‑arsenide assemblage at Franklin, New Jersey:  The Picking Table (Journal of the Franklin-Ogdensburg Mineralogical Society), vol. 31, no. 1, p. 2-5. [First description of breithauptite from Franklin]



Images




Dendritic mass of nickeline and associated arsenide minerals, tiny, pale violet blebs breithauptite
Earl R. Verbeek
This specimen, a sawn and polished slab from a dendritic mass of nickeline and associated arsenide minerals from the Trotter mine, is the one in which Franklin breithauptite in microscopic grains was first discovered. Nickeline (NiAs) is the coppery red mineral along the lower edge of the slab. As shown in the photograph, the nickeline dendrites toward their bulbous outer margins are partly replaced by pararammelsbergite (NiAs2, the pale gray mineral). The replacement texture is particularly evident at lower left, where only “ghost” remnants of nickeline are seen, but their original dendritic texture is still clearly visible. Succeeding the pararammelsbergite in outward growth of the dendrites are rammelsbergite (also NiAs2, darker gray) and small amounts of gersdorffite (NiAsS), skutterudite (CoAs3), and safflorite ((Co,Ni,Fe)As2). The black mineral in thin rims around the margins of the dendrites is stilpnomelane. Associated minerals, in addition to those mentioned, include fluorite (white to violet, upper right and lower left), magnetite (large black grains at left), and white calcite (upper left). This specimen for years was in the collection of Lee Areson (LA 275) and then passed to the collection of Earl R. Verbeek (ERV-1895) upon the sale in 1988-1989 of the Areson collection by Richard Hauck. Small fragments were removed from the irregular rear face of this slab, mounted in plastic, and polished for reflected-light microscopy. The photomicrographs shown below were obtained from these polished sections.

Identifier: BTH1f
Locality: Trotter mine, Franklin
Specimen size: 9.5 x 6.5 x 0.6 cm



Dendritic mass of nickeline and associated arsenide minerals, tiny, pale violet blebs breithauptite
Hoyt B. Sutphin
The breithauptite in this photomicrograph appears as tiny, pale violet blebs, the largest barely 0.1 mm long, but most much smaller, all of them embedded in or contacting white pararammelsbergite near its contact with copper-colored nickeline. The thin straight lines in the images are polishing scratches, and the black pits mark places where mineral fragments have been plucked from the surface by abrasion.

Field of view: The horizontal width is 0.2 mm.

Identifier: BTH1a
Locality: Trotter mine, Franklin
Specimen size: 9.5 x 6.5 x 0.6 cm



Dendritic mass of nickeline and associated arsenide minerals, tiny, pale violet blebs breithauptite
Hoyt B. Sutphin
Two large and numerous much smaller grains of violet breithauptite, all of them embedded in pararammelsbergite and several in contact with coppery nickeline. Note in particular the small blebs of breithauptite in pararammelsbergite close to its contact with nickeline in the lower right quadrant of the image. The strong reflection pleochroism of breithauptite is also visible in this image: the largest grain, for example, is of lavender color, but the small grains to the left of center are pale pinkish-lavender.

Field of view: The horizontal width is approximately 0.2 mm.

Identifier: BTH1c
Locality: Trotter mine, Franklin
Specimen size: 9.5 x 6.5 x 0.6 cm