ROEBLINGITE

Pb₂Ca₆(SO₄)₂(OH)₂(H₂O)₄[Mn(Si₃O₉)₂] Monoclinic, C2/m,
a = 13.27, b = 8.38, c = 13.09 Å, b = 103.86^o, Z = 2

	Figure 18-43. Two segregations of white, translucent roeblingite, the larger one within a fine-grained impure aggregate of ganophyllite and other minerals (medium gray), associated with mica (black), andradite (dark blackish-gray, top right), and willemite (light gray at top). Specimen is 10 cm in maximum dimension. Smithsonian Institution, #C6329. Photo by the author.

Roeblingite was first described as a sulfite-bearing mineral by Penfield and Foote (1897). The sulfite was determined to be sulfate by Blix (1931), and Foit (1966) provided unit-cell and powder data. New analyses were provided and the formula was revised by Dunn et al. (1982a). Dunn (1985b) provided additional data and information in a review of the lead silicates at Franklin.

Crystal structure

Moore and Shen (1984) determined the crystal structure of roeblingite using Långban material. They found the large cations (Pb and Ca) to be between sheets of composition [Mn(Si₃O₉)₂] and that the silicate radicals have relations to those of margarosanite.

Description

Roeblingite occurs as nodules of interlocking prismatic crystals. These nodules, of roughly rounded shape, are up to 15 cm in diameter, but most are small. No euhedral crystals have been noted. Roeblingite is white, opaque to semi-opaque, sometimes semi-transparent, with dull luster; fractured surfaces resemble broken surfaces of porcelain (Figure 18-43). The density is 3.50 g/cm³. Optically, roeblingite is biaxial, positive, with small 2V, and the indices of refraction (from Larsen, 1921) are a = 1.64, b = 1.64, and b = 1.66. Roeblingite is fluorescent in shortwave ultraviolet with a medium pinkish-red color. Some massive white aggregates of mixtures of finely-divided sussexite and rhodochrosite resemble roeblingite. Although Franklin roeblingite resembles bakerite, it does not resemble the extremely sparse bakerite reported from Franklin.

Composition

Roeblingite is a calcium lead sulfate silicate hydroxide hydrate mineral. Franklin samples have small amounts of Sr, presumably in substitution for Ca. The known analyses (Dunn et al., 1982a) are relatively invariant; a typical analysis is presented in Table 8. The matter of Penfield and Foote’s (1897) report of sulfite in roeblingite was addressed by all subsequent investigators; it is certain that the sulfur is present as sulfate. The nature of the H₂O content was settled by Braithwaite (1985) and Moore and Shen (1984).

Occurrence and paragenesis

Roeblingite was reported by Penfield and Foote (1897) as nodular masses from the 1000 foot level of the Parker Mine in Franklin. Palache (1935) said that Frank Nason noted it near the contacts of marble and pegmatite. Most material has been found on the dumps, and the bulk of it occurs in nodular form. The majority of these nodules occur within brownish, fine-grained aggregates containing varying amounts of ganophyllite, manganaxinite, andradite, xonotlite, hancockite, prehnite, willemite, hardystonite, clinohedrite, and barite. The nodules’ occurrence as irregular aggregates in manganaxinite is noteworthy. A subsequent report (Foote, 1898) was of roeblingite with lead from the 800 foot level.

Roeblingite is also known to occur in veins, where is it occurs with bright green willemite and coarsely-crystallized black hendricksite. At least one occurrence is in altered franklinite/willemite  ore. One coarsely-crystallized assemblage yielded a number of spectacular specimens, one of which permitted detailed observation of the sequence of formation of the well- crystallized minerals associated with roeblingite. This description was given by Dunn (1983a) in the description of charlesite. The only published description of an in-situ occurrence of roeblingite was given by Hurlbut and Baum (1960). There has been no detailed study of the possible alteration products of roeblingite. A great many altered specimens are in the collections at Harvard University. The white powdery alteration is charlesite in part.

Name

Roeblingite was named in honor of Washington A. Roebling, a noted engineer and collector of Franklin Minerals.

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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This page created: January 14, 2001