Nelenite, a Manganese Arsenosilicate of the Friedelite Group, Polymorphous with Schallerite, from Franklin, New Jersey
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MINERALOGICAL MAGAZINE, JUNE 1984, VOL. 48, PP. 271-5 Nelenite, a manganese arsenosilicate of the friedelite group, polymorphous with schallerite, from Franklin, New Jersey PETE J. DUNN Department of Mineral Sciences, Smithsonian Institution, Washington, DC 20560, USA AND DONALD R. PEACOR Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA ABSTRACT. Nelenite, (Mn,Fe)16Si1203o(OH)'4[As~+06 typically related to schallerite. We have examined (OHh], is a polymorph of schallerite and a member of the this material in light of current knowledge of the friedelite group. X-ray diffraction patterns can be indexed crystal chemistry of friedelite and schallerite and on a supercell with a = 13.418(5) and c = 85.48(8) A, have found it to be a unique and valid species. The space group R3m, but by analogy with TEM results on mcGillite and friedelite, the structure is based on a old name,jerroschallerite, is a misnomer in that this one-layer monoclinic cell with a 23.240, b 13.418, material is not the Fe-analogue of schallerite and = = does not have the schallerite structure. Hence, we c = 7.382 A, {3= 105.21°, and space group C21m. Chemi- cal analysis yields Si02 31.12, FeO 17.12, MgO 0.12, have renamed it to avoid confusion. ZnO 3.63, MnO 29.22, As203 12.46, H20 6.42, sum = We take pleasure in naming this new mineral 100.09 %. Analysis of a number of samples indicates that nelenite in honour of Joseph A. Nelen, Chemist at Fe substitutes for Mn up to 5.8 of the 16 octahedrally the Smithsonian Insitution, in recognition of his coordinated cations, but that the Si: As ratio is constant. contributions to the chemistry of minerals, in The strongest lines in the X-ray powder diffraction particular his analytical studies of the complex pattern (d, 1110) are: 2.552,/00; 2.878,70; 1.677,60; 3.55,60; arsenosilicates of manganese which are found at 1.723,50. Nelenite is brown in colour with a vitreous luster and Franklin and Sterling Hill. The name and the perfect {OOOI}cleavage, which easily distinguishes it from species were approved by the IMA Commission on schallerite. The Mohs' hardness is approximately 5. The New Minerals and Mineral Names, prior to pub- density is 3.45 g/cm3 (calc.) and 3.46 g/cm3 (obs.). Nelenite lication. The holotype material is deposited in the is uniaxial negative with E = 1.700 and w = 1.718 (both Smithsonian Institution under catalog no. C6219. 10.004). Nelenite was formerly known asferroschallerite, Co type samples are in the Smithsonian Institution which is a misnomer. It was found in the Franklin Mine, under catalog no. R7824 and at Harvard University Franklin, Sussex County, New Jersey, in the 1920s. It under catalog no. H 92791-a. Additional speci- occurs in several parageneses, associated with actinolite, mens repose in the private collections of John tirodite, albite, garnet, feldspars, and several members of Baum, John Kolic, and in the Spex-Gerstmann the stilpnomelane group in coarse-grained assemblages with pegmatitic texture and a breccia likely derived from collection. this rock. Nelenite is named in honour of Joseph A. Nelen, Designation of the holotype is non-ambiguous chemist at the Smithsonian Institution. due to Lawson Bauer's habit of writing the chemi- cal analysis directly on the unused portion of the A SCHALLERITE-LIKE mineral was described by analysed sample. We note here that the cotype Bauerand Berman (1930) and namedferroschaller- samples R7824 and H-92791-a are identical in all itein allusion to the high iron content, relative to details to the holotype, are extremely close in manganese, and the chemical similarity to schaller- composition, and all three can be fitted together, ite, Mn16Si12030(OH)14[As~+06(OHh]' It was indicating they were once a single sample. We re-examined by Frondel and Bauer (1953), who retain the terms holotype and cotype, even though considered it to be a variety of friedelite, MngSi6 they may seem to be redundant because of the 0ls(OH)10' considered at that time to be poly- reconstruction, so as to indicate precisely which @ Copyright the Mineralogical Society 272 P. J. DUNN AND D. R. PEA COR fragment of this breccia was used for crystallo- and mcGillite was shown (Donnay et al., 1980) to be graphic and physical and optical measurements. an apparent twelve-layer polytype with c = 85.66A Physical and optical properties. Nelenite is light (although it differs in composition in having 2CI to medium brown in colour and transparent, with replacing 2(OH)). Dunn et al. (1981) showed that no apparent colour-zoning. The cleavage is perfect, specimens of friedelite, which supposedly were parallel to {0001}, and is developed only with three-layer polytypes, gave mcGillite-like diffrac- moderate difficulty. Cleavage plates are flat and tion patterns, and thus these phases differed only in highly lustrous, and it is this quality and appear- composition in the anion sites, but not in the nature ance of the cleavage that immediately distinguishes of their polytypic repeats, although diffuseness in nelenite from the other members of the friedelite the diffraction pattern of friedelite indicated dis- group, which have cleavage fragments with slightly order in the stacking sequence. or grossly warped and curved surfaces. The streak Iijima (1982a, b) has shown, using high-resolution is light brown. The hardness (Mohs) is approxi- TEM lattice imaging techniques in which the mately 5. The lustre of cleavage surfaces is vitreous; sequences of layers are directly imaged, that the that of fracture surfaces is slightly resinous and mcGillite crystal structure is actually a complex decidedly duller than cleavage surface lustre. The stacking of several polytypes. The basic repeat unit density, determined using heavy liquid techniques, consists of one of two kinds of layer, both of which is 3.46 g/cm3, in excellent agreement with the have monoclinic geometry and which are desig- calculated value of 3.45 g/cm3 and 3.44 g/cm3 value nated IMl and 1M2' A third layer type, which is obtained by Bauer and Berman (1930). theoretically possible, was not observed. Iijima Optically, nelenite is uniaxial (-) with indices notes that the 1M 1 layer (which has space group of refraction e = 1.700 and w = 1.718 C:1::0.004), C2/m) is dominant and that it is repeated with measured in sodium light. It occasionally exhibits regular occurrences of 120° rotation twinning on biaxial behaviour with 2V near 0° and is pleochroic (001), giving rise to a variety of poly types. This layer with e = colourless and w = light brown. Nelenite has dimensions a = 23.279, b = 13.498, c = is not discernibly fluorescent in ultraviolet radia- 7.390 A, and fJ = 105.3°. A critical point is that tion, nor is it phosphorescent. Iijima implies that the two-layer polytype has a X -ray crystallography. Single crystal photographs structure that is not identical to that of schallerite; of nelenite were obtained using precession camera he notes that the relation between the three-layer techniques. These photographs are nearly identical polytype (which is trigonal) and friedelite (which is to those described by Dunn et al. (1981) for trigonal) remains to be investigated. The significant friedelite which, in turn, are nearly identical to conclusion is that because nelenite is shown to be those of mcGillite (Donnay et al., 1980). The isostructural with friedelite, its structure must be principal differences are that photographs of based largely on the IM1 layers. Such units are friedelite and nelenite exhibit diffuseness in reflec- different from those of schallerite. The relation tions, which are spread out parallel to c*, while between schallerite and friedeIite is therefore not a mcGillite gives sharp reflections. The relations in polytypic one, sensu strictu. diffraction patterns were noted by direct compari- Ozawa et al. (1983) have observed similar rela- son of equivalent photographs obtained for the tions for friedelite and mcGillite, obtaining dimen- different species. The structures of nelenite and sions for a one-layer monoclinic unit cell close to friedelite thus appear to be identical and, in turn, those of Iijima. They further note that friedelite is identical to that of mcGilIite except that they are the disordered equivalent of mcGillite. The status relatively disordered in stacking sequence. As the of mcGillite, friedelite and, by analogy, nelenite true symmetry of mcGilIite and friedelite (and thus appears to be well established. therefore nelenite) is a complex matter that is not Because the single-crystal X-ray diffraction unambiguously determinable by X-ray diffraction patterns of nelenite, mcGillite, and friedelite are alone, we give here a discussion of the relations nearly identical, nelenite must also have a structure among these closely related phases. based on a one-layer unit. Donnay et al. (1980) Manganpyrosmalite, schallerite, and friedelite interpreted the X-ray diffraction data for mcGillite were until recently thought to be simple polytypes as being due to a twelve-layer hexagonal polytype of MnsSi60 ls(OH)lO (with schallerite having having a very pronounced substructure with A = some essential As) (Frondel and Bauer, 1953). All a/4 and C = c/4. Because of that pseudosymmetry, were thought to be hexagonal with a ~ 13.4 A and indexing of powder diffraction data on the basis of with values of c that are multiples of 7.15 A, the the IMl unit cell parameters gives rise to multiple thickness of a single layer. Manganpyrosmalite, indices for most reflections, such that the para- schallerite, and friedelite were interpreted to have meters of the monoclinic cell cannot be refined by one-, two-, and three-layer structures, respectively, least-squares.