CIANCIULLIITE

Mn(Mg,Mn)₂Zn₂(OH)₁₀.2-4H₂O 
Monoclinic, C2/m, a = 15.47(2), b = 6.369, c = 5.576 Å,
b = 101.29^o, Z = 2

	Figure 22-51. Tabular platy crystal of cianciulliite from Franklin. Incipient cleavages are well-developed. Field of view is approximately 1 mm. Photograph courtesy of Herb Yeates.

Cianciulliite was first described from Franklin by Dunn et al. (1991). It has not been reported from Sterling Hill.

Crystal structure

The crystal structure of cianciulliite has a brucite-type layer of Mg and Mn octahedra with tetrahedrally coordinated Zn atoms capping the octahedral holes. The apical (OH) of Zn tetrahedra are hydrogen-bonded to the water layer. Some of the water is readily lost in experimentation (Grice and Dunn, 1991).

Description

Cianciulliite occurs as small (0.6 mm) euhedral crystals with highly lustrous faces, some of which are curved. The crystals are slightly tabular on {100} and the {100} pinacoid is dull to pearly; this differential luster is the most distinctive physical feature of this mineral. Incipient {100} perfect cleavages are abundant, adding to this anomalous luster (Figures 22-51 and 22-52). Cianciulliite is dark reddish brown in transmitted light, but crystals appear dark brown to black in hand specimen. The mineral is very soft, and the density, calculated for 2 water molecules per 5 divalent cations, is 2.87 g/cm³.

	Figure 22-52. Sharp, well-formed crystals of cianciulliite from Franklin. Incipient cleavages are well-developed. Field of view is approximately 1 mm.

Optically, cianciulliite is biaxial with undetermined sign; the indices of refraction are between 1.76 and 1.92 and were calculated to be 1.79 - 1.80 for sodium light. In reflected light, cianciulliite is weakly anisotropic, gray, weakly bireflectant, and not observably pleochroic. In oil, the color in reflected light is a slightly darker and bluer gray; internal reflections are orange-red and clearly evident. Reflectance measurements and color values were given by Dunn et al. (1991). There is no discernible fluorescence in ultraviolet.

Composition

Cianciulliite is a manganese magnesium zinc hydroxide hydrate mineral. The composition of the type material, recalculated as described by Dunn et al. (1991), is MnO 29.7, MgO 9.86, ZnO 33.36, H₂O 27.08, total = 100.0 wt. %, with two molecules of water per 5 divalent cations, recalculated by difference.

Occurrence and paragenesis

Cianciulliite is known on but one small museum specimen from the Franklin Mine. The matrix is a mixture of willemite and zincite which has replaced crystals of an unknown species. An irregular coating of presumed Mn-oxides is followed by a final crystallization of cahnite and cianciulliite. To date, cianciulliite is a rare mineral.

Name

Cianciulliite was named in honor of John Cianciulli of Sussex, New Jersey, in recognition of his assistance to scientists and his substantial contributions to the Franklin-Sterling Hill area mineralogical institutions, especially his nurturing of the Franklin Mineral Museum.

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