NELENITE

(Mn,Fe)₁₆Si₁₂O₃₀(OH)₁₄[As³⁺₃O₆(OH)₃]
Hexagonal, R3m, a = 13.418, c = 85.48 Å, Z = 12,
or Monoclinic, C2/m, a = 23.240, b = 13.418, c = 7.382 Å,
b = 105.21^o

	Figure 18-15. Nelenite (dark gray, center) showing perfect cleavage, associated with actinolite (black); the composite is a breccia fragment in a coarse-grained breccia cemented by white calcite from Franklin. Specimen is 7 cm in maximum visible dimension. Smithsonian Institution, #C6219 (holotype specimen). Photo by the author.

Nelenite was originally described as ferroschallerite by Bauer and Berman (1930). The name was in allusion to the chemical similarity to schallerite and the presence of much Fe in the material. This mineral was later re-examined by Frondel and Bauer (1953) who, on the basis of its X-ray powder diffraction pattern, discredited it as an As-bearing type of friedelite; Hey (1956) suggested that this discreditation of ferroschallerite as friedelite was premature. Subsequent to the re-characterization of schallerite by Dunn et al. (1981c), ferroschallerite was re-examined and found to be a polymorph of schallerite (Dunn and Peacor, 1984). Because this phase does not have the schallerite structure and has Mn > Fe in all specimens the older name was discarded as a gross misnomer, and the mineral was renamed nelenite. Nelenite is isostructural with friedelite and/or mcGillite. It was found only at Franklin.

Description

Nelenite is found as anhedral fragments up to 30 x 45 mm in size, with perfect {0001} cleavage; no euhedral crystals have been found. It is distinctly brown; turbid material is light brown. The luster is vitreous. The density is 3.46 g/cm³ (meas.), 3.45 g/cm³ (calc.). Optically, nelenite is uniaxial, negative, with indices of refraction w = 1.718 and e = 1.700; it may exhibit weakly biaxial behavior. Pleochroism is moderate with O = light brown and E = colorless. Nelenite is not discernibly fluorescent in ultraviolet. Broken fragments resemble some local manganoan diopside (schefferite) or yeatmanite. Nelenite is easily distinguished from these two minerals by its optical character and lower indices of refraction, respectively. Nelenite is distinguished from friedelite by higher indices of refraction and from schallerite by its X-ray powder pattern and perfect cleavage.

Composition

Nelenite is a manganese iron arsenic silicate hydroxide mineral of the friedelite group. In contrast to schallerite, Fe substitutes for Mn to a large extent and comprises up to 5.8 of the 16 octahedrally coordinated cations. In addition, Zn, Mg, and Ca also substitute for Mn, in small yet consistent amounts. Because Fe³⁺ may play a role in other manganese arsenic silicates, the oxidation state of Fe in nelenite was carefully checked and found to be almost wholly divalent. Several analyses of nelenite are given in Table 15; they are chosen to show the range of Fe substitution for Mn.

Occurrence and paragenesis

Nelenite was first found in 1925 and reportedly came from the 1597 pillar, approximately 50 feet south of the Trotter Shaft at Franklin. The type material is associated with light-yellow willemite which is rimmed by a stilpnomelane-group mineral. The type specimen is a breccia consisting of actinolite, calcite, nelenite, and willemite (Figure 18-15). Although Bauer and Berman (1930) referred to this amphibole as zinc-manganese-cummingtonite (later shown to be tirodite), their reference was to other nelenite samples.

The known nelenite specimens apparently came from at least four assemblages, as evidenced by textural relations and the compositions of the associated amphiboles (Dunn and Peacor, 1984). Four specimens are found with zincian, ferroan tirodite and spessartine; two of these four are also associated with franklinphilite. In addition, a complex sample has nelenite associated with willemite, rhodonite, apatite, and lennilenapeite (Dunn et al., 1984b).

Name

Nelenite is named for Joseph A. Nelen, chemist at the Smithsonian Institution, in recognition of his contributions to the chemical analysis of the complex manganese arsenic silicates from Franklin and Sterling Hill.

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