CHALCOPHANITE

(Zn,Mn,Mg,Fe)Mn⁴⁺₃O₇.3H₂O
Hexagonal, R3, a = 7.541, c = 20.824 Å, Z = 3

	Figure 22-48. Crystal drawings of chalcophanite from Sterling Hill. Drawings are from Palache (1935) who provided crystallographic data.

Chalcophanite was first described from Sterling Hill in a superb, descriptive paper by Gideon Moore (1875); the attribution to Koenig by Frondel (1972) is in error. Roepper (1882) reported a description of a mineral he called hydrofranklinite, but this was later shown to be cospecific with chalcophanite by Penfield and Kreider (1894). X-ray data were given by Berry and Thomson (1962). Chalcophanite was verified from Franklin by the writer.

Crystal structure

The crystal chemistry and the crystal structure were initially described by Wadsley (1953, 1955a). The structure was redetermined and the symmetry corrected by Post and Appleman (1988), who described it as consisting of “sheets of edge-sharing Mn⁴⁺O₆ octahedra alternating with layers of Zn cations and water molecules in the stacking sequence Mn-O-Zn-H₂O-Zn- O-Mn.”

Description

Chalcophanite occurs as small (<4 mm) platy crystals which are hexagonal in outline for the most part, as described by Palache (1935) (Figure 22-48). Crystals also may be pseudo-octahedral or rhombic in habit. Such crystals occur as druses and also occur as foliated masses, stalactites, and plumose aggregates.

	Figure 22-49. Chalcophanite crystals, partially dehydrated and showing the perfect cleavage, from Sterling Hill. Field of view is 0.6 mm in maximum dimension.

Chalcophanite also commonly occurs as crusts of radial crystals and as free-standing crystals lining vugs. Chalcophanite is dark red to black, opaque, and may have a bluish tarnish. The cleavage is perfect, appears basal on crystals of hexagonal habit, is easily developed, and is evident on even slightly altered material (Figure 22-49). The luster of freshly broken material is brilliant, splendent, and metallic; some material is dull on exposed surfaces. The density is 3.98 g/cm³.     

Optically, chalcophanite is uniaxial, strongly pleochroic as noted by Palache (1935), and has extremely strong anisotropism. Optical descriptions in reflected light are given by Ramdohr (1980) and Picot and Johan (1982). Full optical data are given by Criddle and Stanley (1986, 1993).

Composition

Chalcophanite is a zinc manganese oxide hydrate mineral. Some analyses were given by Palache (1935) and suggest limited Mn for Zn substitution. Criddle and Stanley (1986) reported ZnO 19.8, FeO 0.1, MnO₂ 65.3, H₂O (by difference) 14.8, total = 100.0 wt. % for the material they studied optically. The eight analyses by Ostwald (1985) and some unpublished analyses by this writer suggest that much Sterling Hill chalcophanite has near end-member composition, with very little, if any, substitution of Mg and Fe for Zn. However, banded, dull, fine-grained material in stalactitic aggregates may have much solid solution towards aurorite. Chalcophanite has been studied thermogravimetrically by Dasgupta (1974), and he also reported oriented transformations observed during heating. Although local material is Zn-dominant, much additional study and discussion of worldwide occurrences by Ostwald (1985, 1987) has led to a general acceptance of chalcophanite as a mineral name for material with widely varying composition.  

Occurrence and paragenesis 

	Figure 22-50. Black, lustrous chalcophanite from the mud-zone at Sterling Hill. Specimen is 12 cm in maximum dimension. Smithsonian Institution, #R6474. Photo by the author.

The principal occurrence of chalcophanite is at Sterling Hill, where it was found in association with goethite and manganese oxides with the hemimorphite deposits in the Passaic and Noble pits, and in the broader weathered part of the mine locally referred to as the “mud zone”. Here it occurs as vuggy masses (Figure 22-50), many of which are layered, and also less commonly as druses on seams in franklinite/ willemite ore. The original Sterling Hill occurrence consisted of water-worn quartz pebbles, rock fragments, and decomposed willemite, zincite, and franklinite, all held together by manganese oxides, chalcophanite among them. In general, chalcophanite is commonly associated with apparently relict franklinite, secondary goethite, hetaerolite, and hydrohetaerolite, in open-seam and porous aggregates. Birnessite, todorokite, and woodruffite are less common associated minerals.

The genesis of this occurrence has not been studied in modern times. Moore (1875) postulated that franklinite was first altered to hydrohetaerolite and limonite and that chalcophanite was formed as the result of further oxidation and hydration. This explanation was adopted by Palache who gave a description of the in situ occurrence provided by Mr. O.J. Conley (Palache, 1935, pp 22-23) and given herein in the section entitled “The geology and structure of the zinc deposits.”

Chalcophanite occurs rarely at Franklin. It was observed associated with todorokite and hemimorphite in a vein assemblage. Some hematite associated with hemimorphite from the northern end of the Franklin Mine has been mislabeled as chalcophanite.

Name

Chalcophanite is named using Greek terms in allusion to the color change it undergoes upon heating, varying from yellow-bronze to copper-red.

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