MAGNESIUM-CHLOROPHOENICITE

(Mg,Mn,Zn)₃Zn₂(AsO₄)(OH,O)₆ 
Monoclinic, C2/m, a = 22.99, b = 3.236, c = 7.299 Å,
b = 106.5^o, Z = 2

Magnesium-chlorophoenicite was first reported from Franklin by Palache (1935); it has not been verified from Sterling Hill, but may occur there, inasmuch as more Mg-bearing arsenates are found there than at Franklin. Magnesium-chlorophoenicite was redefined by Dunn (1981c) who provided much compositional data. Unit-cell data and X-ray powder diffraction data were given by Bayliss and St. J. Warne (1987). Magnesium-chlorophoenicite is isostructural with chlorophoenicite.

Description

Magnesium-chlorophoenicite occurs as radial aggregates of fibrous white or colorless crystals; such sprays are up to 12 mm in diameter and are commonly splayed. The species is easily stained, and crystals may appear brown on the surface. Cleavage is perfect, presumably on {100}. The density is 3.45 g/cm³. Optically, magnesium-chlorophoenicite was reported (Berman, in Palache, 1935) to be biaxial, positive, with small 2V, a = 1.669, b = 1.672, g = 1.677, and strong dispersion, r < v. There is no discernible fluorescence in ultraviolet. Three specimens were examined by the writer; on all three magnesium- chlorophoenicite occurs as white, radial sprays of acicular crystals. It is best verified using both quantitative chemical analysis for Mg and Mn and X-ray methods.

Composition

Magnesium-chlorophoenicite is a magnesium manganese zinc arsenate hydroxide mineral. The original analysis (Palache, 1935) was of admittedly impure material. Dunn (1981c) re-analysed three purported type specimens; only one had Mg > Mn, and the analysis is given in Table 25 (#H-92803), together with previously unpublished analyses of three additional specimens subsequently studied. It is noteworthy that in all four extant analyses of magnesium-chlorophoenicite there is a surfeit of zinc; Zn varies from 2.4 to 2.8 Zn per five divalent cations. X-ray powder diffraction indicates that magnesium- chlorophoenicite is isostructural with chlorophoenicite; thus, some Zn must be in octahedral coordination substituting for (Mg,Mn). Magnesium-chlorophoenicite may not be stable as an end-member and may require octahedrally coordinated Mn or excess Zn for structural stability. The maximum Mg content known to date is 1.73 Mg per 5 divalent cations.

Occurrence and paragenesis

The type magnesium-chlorophoenicite occurs as radial, 1 cm aggregates associated with zincite and carbonate minerals and was found on the 750 level at Franklin. Additional specimens seen by the writer appear to be from this assemblage. The most Mg-rich magnesium-chlorophoenicite known (Table 25, Bostwick) consists of willemite/franklinite  ore with a partially open vein filled with pink, massive, opaque hodgkinsonite, white barite, and zincite crystals. Interstitial spaces among these minerals are filled with snow-white magnesium-chlorophoenicite in radial sprays of acicular crystals. This assemblage is in turn covered by a second generation of druse barite, calcite, and hodgkinsonite crystals. The extant analyses (Dunn, 1981c), together with numerous X-ray investigations, suggest that magnesium-chlorophoenicite is very rare.

Name

Magnesium-chlorophoenicite was named for the relation to chlorophoenicite, but no formal naming statement was given by Palache (1935).

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