SUSSEXITE

MnBO₂(OH)                                    Orthorhombic, P2₁2₁2₁, a = 10.617, b = 12.666, c = 3.302 Å, Z = 4.

	Figure 24-17. Sussexite (massive light gray) with altered franklinite-bearing ore (lower area) from Sterling Hill. Specimen is 21 cm in maximum dimension. Smithsonian Institution, #R15679. Photo by the author.

Sussexite was first described from Franklin by Brush (1868a, 1868b, 1901); his formula was confirmed by re-examination of type material by Penfield and Sperry (1888). Pointevin and Ellworth (1924) provided additional analytical and optical data, as did Palache (1928a). Takéuchi (1957) provided X-ray data, and Frondel and Ito (1965a) reported sussexite from Sterling Hill. Moore and Araki (1974) referred to an unpublished study yielding the structural formula Mn²⁺₂(OH)[B₂O₄(OH)].

Description

Sussexite occurs at Franklin principally as fibrous masses; the fibrosity is commonly parallel to subparallel, but may be random. Fascicles are tight or loose, long or short, flexible or brittle; it occurs in diverse habits. It also occurs in hexagonal arrays either as relicts after pyrochroite or epitactic thereon. Crystal fibers may be loose or tightly packed into dense, tough masses, which yield a sharp noise when struck by a hammer. Cherty material is also found at Franklin.

These various textures also occur at Sterling Hill, but the preponderance of material there is massive, and some is cherty; grain boundaries are commonly not visible. Much of this material is pink to violetish pink and breaks with an even to subconchoidal fracture. Chalcedonic, quartzite-like, and varying textures grade insensibly into each other, sometimes in concentric patterns (Figure 12-39). In other specimens, grain boundaries are sharp.

Sussexite has much color variation: fibrous material is commonly white to gray to light pink; cherty material varies in color from pink to pinkish violet to red; and yellow or violet is less common. False colors are induced by minute inclusions of other species; zincite colors sussexite orange, for example. The density is 3.30 g/cm³, but lower values are commonly obtained. There is a cleavage parallel to the fibrosity, as reported by Frondel and Ito (1965a). Optically, sussexite is biaxial and negative; varying values of the indices of refraction have been given by the above cited investigators. There is no discernible fluorescence in ultraviolet. Sussexite resembles some fibrous amphiboles, fluoborite, and other species.

Composition

Sussexite is a manganese boron hydroxide mineral and the Mg-analogue of szaibelyite. Local material contains much magnesium in solid solution towards szaibelyite, a species as yet unknown locally. Palache (1935) summarized the then-extant analytical data. An analysis of Sterling Hill sussexite (Frondel and Ito, 1965a) yielded: MgO 16.84, MnO 35.78, ZnO 3.71, CaO 0.15, Fe₂O₃ 0.26, B₂O₃ 33.50, H₂O 9.57, insol 0.19, total = 100.00 wt. %.

Occurrence and paragenesis

Sussexite from Franklin is associated with willemite, calcite, franklinite, and massive red zincite; rhodochrosite, pyrochroite, and leucophoenicite are also associated minerals. The association of calcite and willemite is sporadic. The common mode of occurrence is as veins in the common ore. Such veins may be wide or narrow (Figure 12-44) and not uncommonly have simple mineralogies; veins of only sussexite are common. Sussexite may comprise part of the material locally referred to and labeled as calcozincite. Porcelaneous to chalky sussexite occurs with pyrochroite. Cherty material is much less common here than at Sterling Hill, but occurs as violetish veins with light yellow willemite and as an intimate violet pink mixture with hodgkinsonite. The latter was described by Palache (1928a, 1935) and restudied by Dunn and Bostwick (1982), who found it to be a mixture of hodgkinsonite and sussexite. Palache (1935) noted sussexite occurrences in the Hamburg, Trotter, Taylor, and Parker Mines.

At Sterling Hill, the predominance of sussexite is massive, cherty to fine-grained-fibrous aggregates (Figure 24-17). Kilogram specimens are common. Sussexite accompanies sheared and recrystallized assemblages, some of which are of large scale, especially in the lower reaches of the mine. It is associated with franklinite, willemite, calcite, and, commonly, light-orange recrystallized zincite in pods. Rhodochrosite, manganoan calcite, dolomite, serpentine, pyrochroite, hematite, and, less commonly, brucite and dypingite are also associated. This suite of minerals is largely uninvestigated.

Name

Presumably, the mineral sussexite was named for Sussex County, in which both ore deposits are located and which is mentioned in the title of the description, but Brush did not say so explicitly. “Sussexite” is also a rock-name for a particular variety of nepheline syenite found in Beemerville, Sussex County, New Jersey, the type locality (Aurousseau and Washington, 1922).

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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This page created: January 11, 2001