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Nelenite, a manganese arsenosilicate of the friedelite group, polymorphous with schallerite, from Franklin, New Jersey

Published online by Cambridge University Press:  05 July 2018

Pete J. Dunn  and
Donald R. Peacor
Pete J. Dunn
Affiliation:
Department of Mineral Sciences, Smithsonian Institution, Washington, DC 20560, USA
Donald R. Peacor
Affiliation:
Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA
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Abstract

Nelenite, (Mn,Fe)16si12O30(OH)14[O6 (OH)3], is a polymorph of schallerite and a member of the friedelite group. X-ray diffraction patterns can be indexed on a supercell with a = 13.418(5) and c = 85.48(8)A, space group Rm, but by analogy with TEM results on mcGillite and friedelite, the structure is based on a one-layer monoclinic cell with a = 23.240, b = 13.418, c = 7.382 Å, β = 105.21°, and space group C2/m. Chemical analysis yields SiO2 31.12, FeO 17.12, MgO 0.12, ZnO 3.63, MnO 29.22, As2O3 12.46, H2O 6.42, sum = 100.09%. Analysis of a number of samples indicates that Fe substitutes for Mn up to 5.8 of the 16 octahedrally coordinated cations, but that the Si: As ratio is constant. The strongest lines in the X-ray powder diffraction pattern (d, I/Io) are: 2.552,100; 2.878,70; 1.677,60; 3.55,60; 1.723,50.

Nelenite is brown in colour with a vitreous luster and perfect {0001} cleavage, which easily distinguishes it from schallerite. The Mohs' hardness is approximately 5. The density is 3.45 g/cm3 (calc.) and 3.46 g/cm3 (obs.). Nelenite is uniaxial negative with ɛ = 1.700 and ω = 1.718 (both ± 0.004). Nelenite was formerly known as ferroschallerite, which is a misnomer. It was found in the Franklin Mine, Franklin, Sussex County, New Jersey, in the 1920s. It occurs in several parageneses, associated with actinolite, tirodite, albite, garnet, feldspars, and several members of the stilpnomelane group in coarse-grained assemblages with pegmatitic texture and a breccia likely derived from this rock. Nelenite is named in honour of Joseph A. Nelen, chemist at the Smithsonian Institution.

Type
Research Article
Information
Mineralogical Magazine , Volume 48 , Issue 347 , June 1984 , pp. 271 - 275
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1984

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