BEMENTITE

Mn₇Si₆O₁₅(OH)₈
Monoclinic, P2₁/c, a = 14.838, b = 17.584, c = 14.700 Å,
b = 95.54^o, Z = 8

	Figure 18-25. Plumose radiating bementite (pearly gray) in calcite (white) with franklinite (black) from Franklin. Specimen is 9 cm in maximum dimension. Smithsonian Institution, #139597. Photo by the author.

Bementite was first described, incompletely, from the Trotter Shaft by Koenig (1887b). A second occurrence was studied by Palache (1910), who described bementite specimens with good apparent “cleavage” from the Parker Mine at Franklin. Pardee et al. (1921) compared Franklin bementite to similar material from Washington and proposed a relation to caryopilite. Larsen, in 1925, pointed out a possible relation to serpentine. The extant information on bementite was summarized by Palache (1935) together with his own observations.

Strunz (1957), in Mineralogische Tabellen, reported that Schaller in 1954 thought bementite to be in the pyrosmalite group and that Frondel in 1956 considered it to be in the kaolinite group. Kato (1963) published a detailed study of “bementite” from many localities, but examined only the blocky material with apparent cleavage from the Parker Mine and not, as stated, the type material, which has an entirely different habit. Kato concluded that the blocky “bementite” was a mixture of two phases intergrown epitactically; the less abundant of which was similar to chamosite. The dominant of these phases he found to be monoclinic, with a = 14.5, b = 17.5, c = 7.28 Å, and b = 90^o.

Frondel (1972) reported bementite as stiff fibers on rhodonite, but this material has been examined by the writer and found to be epitactic johannsenite (Figure 17-5). Peacor and Essene (1980) investigated Franklin “bementite” as part of their study of caryopilite, but did not study type material from the original Franklin occurrence. They found that the platy, blocky Franklin “bementite” yields variable powder patterns and has non-stoichiometric composition, and they noted that the material they studied may well be different from minerals of either the friedelite group or the serpentine group. A recent review was given by Guggenheim and Eggleton (1988).

Crystal structure

A study by Kato and Takéuchi (1980) proposed a structural model for bementite as Mn₇Si₆O₁₅(OH)₈, orthorhombic, P222₁, with a = 14.5, b = 17.5, c = 7.28 x 4 (= ~29) Å, with Z = 16, but were unable to prove it, due to poor crystals.

The crystal structure of Franklin bementite was described by Eggleton and Guggenheim (1988) and by Heinrich et al. (1994), who provided the unit-cell given above. They proposed that bementite is a modulated 1:1 layer silicate with “two hexagonal sheets of octahedra, accommodating the Mn, which are alternately rotated by 22^o in the ab plane. These are interlayered by a continuous tetrahedral sheet containing pairs of six-membered rings interconnected with five- and seven-membered rings.” They also discussed the polytypism of bementite.

Description

	Figure 18-26. Platy, rectangular masses of bementite associated with calcite (white) from Franklin. The lath-like habit may result from bementite replacing barite. Specimen is 9 cm in maximum dimension. Smithsonian Institution, #R4897-1. Photo by the author.

Type bementite from the first Franklin occurrence has plumose, radiating, and stellate habits (Figure 18-25). Apart from this, bementite occurs in a wide variety of textures and habits including platy, fibrous and, rarely, very fine-grained material resembling hardened clay (Figure 18-27). Visual identification is very difficult. Material from the Parker Mine occurrence is platy for the most part, with substantial textural variance from specimen to specimen. The original bementite and much bementite in general is brown to light yellow, but that from some assemblages may be much darker brown; surface oxidation to a darker color is common. The luster is pearly except in fine-grained masses which have a dull luster and broad platy masses which can have a pseudo-vitreous luster.

	Figure 18-27. Fine-grained crackled bementite (gray in center) with impure serpentine (on the right) and calcite (white on left) from Franklin. Specimen is 10 cm in maximum dimension. Smithsonian Institution, #R6719. Photo by the author.

Cleavage was noted on {001} and {010} by Heinrich et al. (1994). The coarsely textured specimens of lath-like, blocky bementite have a very distinct, pseudocubic, apparent “cleavage” (Palache, 1935), but this bementite has formed by the replacement of barite, and the pseudocubic “cleavage” is perhaps in part an inherited relict cleavage of barite (Figure 18-26); this matter remains unstudied. The reported density varies from 2.98 to 3.20 g/gm³; the calculated value is 3.23 g/cm³. Optically, bementite was reported by Palache (1935) to be biaxial, negative, with a = 1.624, b = 1.650, and g = 1.650, presumably for the Parker Mine material described by him in 1910. Additional optical data are given by Larsen (1921).

Composition

	Table 16. Chemical analyses of bementite.

Bementite is a manganese silicate hydroxide mineral. The composition is variable, and there is much solid solution among the octahedral cations, as shown by the analyses in Table 16.

Occurrence and paragenesis  

Bementite is not a rare mineral at Franklin; it is known from a number of assemblages, but has only been preserved in volume from two of these.

The original assemblage described by Koenig (1887b) from the Trotter Mine consists of radiating, stellate, and plumose aggregates of platy to fibrous crystals, associated with calcite (Figure 18-25). The texture, as best it can be deduced from the available hand specimens, is that of a breccia wherein abundant bementite hosts large angular shards of sparser calcite.

The second major assemblage is that of platy and blocky “crystals” found in 1903 in the Parker Mine and reported by Palache (1910, 1935). Such lath-like “crystals” are up to 5 cm in size (Figure 18-26). Barite is both present and extensively replaced in this assemblage, and growth patterns of the bementite commonly intersect at 90^o. Calcite is sparse.

Aside from these two assemblages, bementite also occurs sparingly in other late-stage seams and fracture fillings as rosettes, hemispherules, 2-3 cm laths, and dendrites, commonly associated with serpentine and/or rhodonite. Several specimens suggest bementite forms by alteration of pre-existing friedelite. The worm-like “bementite” mentioned by Palache (1935) is caryopilite.

At least one specimen, which compares well in color, texture, and X-ray powder pattern with Franklin bementite, was found at Sterling Hill.

Name

Bementite was named for Clarence S. Bement, a Philadelphia mineral collector.

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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