ALLEGHANYITE Chapter15. Nesosilicates

ALLEGHANYITE

(Mn,Mg)₅(SiO₄)₂(OH)₂
Monoclinic, P2₁/b, a = 4.815, b = 10.574, c = 8.083 Å,
a = 108.74^o, Z = 2.




	Figure 15-15. Crystal drawing (from Petersen et al., 1984) of the common habit of Sterling Hill alleghanyite, in the conventional setting.

Alleghanyite was first noted from Franklin and Sterling Hill by Cook (1969) who cleared up part of the confusion arising from earlier morphological studies. White and Hyde (1982b) examined several Franklin samples using TEM techniques; although they found several phases associated with alleghanyite (chiefly leucophoenicite and sonolite), these minerals were present as fragments and not as intergrowths. A magnesian alleghanyite from Sterling Hill was described by Petersen et al. (1984), and a crystal-structure refinement of material of similar composition was given by Francis (1985a). The data of Francis were used by Abbott et al. (1989) for a structure-energy calculation. Detailed compositional and assemblage data were given by Dunn (1985a).

Crystal structure

Sterling Hill alleghanyite, subjected to crystal-structure analysis, was found by Francis (1985a) to have ordered cations, with Zn and Mg in the smallest M(3) octahedron and Mn and Mg distributed such that the M(1) and M(2)₅ octahedra are predominantly occupied by Mn with minor Mg.

Description

Alleghanyite occurs as dark-brown massive material and as euhedral light-brown 1-3 mm crystals in veins. Morphologically, the crystals were found to be untwinned, with the principal forms {010}, {100}, {110}, {120}, {101}, {111}, and {112}. Crystal drawings of some of these Sterling Hill crystals are presented in figures 15-15 and 15-16. For such crystals the luster is vitreous; cleavage is absent; and the density is 3.75 g/cm³.




	Figure 15-16. Crystal drawing (from Petersen et al., 1984) of Sterling Hill alleghanyite which is tabular on [100]. Drawing is in the conventional setting.

Optically, this alleghanyite is biaxial, negative, 2V = 65^o, with Y= b, Z= a, and a L c = 20^o (Petersen et al., 1984). There is no discernible fluorescence in ultraviolet. Alleghanyite, like all the Mn-humites, is best distinguished from related species using X-ray methods.

Composition

Alleghanyite is a manganese silicate hydroxide mineral of the manganese-humite group; most local samples have magnesium and zinc substituting for manganese. Several microprobe analyses of alleghanyites are presented in Table 1. Samples from Sterling Hill are markedly higher in Mg than those from Franklin, reflecting the generally higher concentration of Mg in Sterling Hill silicates. Dunn (1985a) pointed out the possibility of a number of cation-ordering schemes for Mn and Mg in the secondary crystals from Sterling Hill.

The analysis of specimen #134648 is of the euhedral secondary crystals from Sterling Hill. Material with similar composition was described by Petersen et al. (1984) and Francis (1985a). Of particular interest, in all analyses of Franklin alleghanyite, is the invariance of Zn. Zinc is a common substituent in silicate phases at Franklin and Sterling Hill, and its presence is expected. However, in no studied local material is there an alleghanyite specimen without Zn, nor is there one with Zn in excess of relatively invariant amounts. Some of these alleghanyites coexist with zincite (ZnO), but the majority have only willemite as an associated Zn-phase. Fluorine is common in most alleghanyite, averaging 40% of the (OH) site in magnesian material and generally decreasing with increasing Mn content.

Occurrence and paragenesis

Alleghanyite occurs at both Franklin and Sterling Hill. At Franklin, there are few known assemblages. The best-preserved of these was identified by Palache (1928) as leucophoenicite. This material consists of veins up to 3 cm thick, associated with calcite, franklinite and sussexite. The bulk of this material consists of massive brown alleghanyite which forms euhedral crystals on exposed surfaces. Leucophoenicite is also present, both as epitaxial overgrowths on the terminations of alleghanyite crystals and as apparently randomly oriented euhedra, perhaps of a subsequent growth period. Although alleghanyite has been found on a few Franklin specimens from other assemblages, it is uncommon.

At Sterling Hill, alleghanyite occurs rarely as euhedral 1-2 mm crystals (Figures 15-15 and 15-16) and thin seams which crosscut the ore. In hydrothermal veinlets, these crystals may accompany arsenate species, such as kolicite, holdenite, magnussonite, adelite, kraisslite, chlorophoenicite, and others. Commonly associated with alleghanyite are franklinite, willemite, barite, and carbonates, all of secondary recrystallization. The observed species all occur on willemite/franklinite ore which contains abundant calcite. Some of the best samples were found in the late 1970’s, and numerous well-studied specimens are in the Harvard collection.

FOOTER LBI




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