SCHALLERITE

Mn₁₆Si₁₂O₃₀(OH)₁₄[As³⁺₃O₆(OH)₃]
Hexagonal, P6₃, a = 13.40, c = 14.28 Å, Z = 2

	Figure 18-16. Pyramidal crystal of schallerite from the original material found at Franklin. Crystal is 1.1 mm in maximum dimension.

Schallerite was first described from Franklin by Gage et al. (1925). It was subsequently restudied by Bauer and Berman (1928), who showed a chemical relationship with friedelite, established the arsenic as trivalent instead of pentavalent as initially reported, and described a new occurence in spheroidal aggregates. The polytypic relationship among schallerite, manganpyrosmalite, and friedelite was suggested by Frondel and Bauer (1953) and discussed by Kashaev and Drits (1970). McConnell (1954) determined that schallerite is hexagonal and discussed the layered aspects of the structure relative to the chemical composition.

	Figure 18-17. Resorbed crystals of schallerite from Franklin. Field of view is 0.25 mm in maximum dimension.

The chemical aspects of schallerite were the focus of much speculation. Berman (1937) had suggested substitutions involving (Si-As) and (OH-Cl), and Hey (1956) suggested an additive solid-solution mechanism for As rather than substitution. Takéuchi et al. (1969) showed that the schallerite structure and other variants could be derived from the simpler manganpyrosmalite structure. Dunn et al. (1981c) provided much analytical and descriptive data and proposed the extant formula for schallerite. Schallerite is clearly related to friedelite, manganpyrosmalite, nelenite, mcGillite, and pyrosmalite, the first three of which occur at these deposits.

Crystal structure

The crystal structure of schallerite was decribed by Kato and Watanabe (1992). They found that schallerite has a two-layer structure and that arsenic in the form of As₃O₆ is present in the voids of twelve-membered rings. They also communicated a finding by other researchers that schallerite may be two-layer orthorhombic.

Description

The original schallerite occurs as massive material, some with fine pyramidal crystals (Figure  18-16). The color varies from reddish-brown for massive aggregates to light pinkish-orange for transparent crystals. The cleavage is imperfect on {0001}, and the luster is vitreous to slightly waxy. The density is 3.37 g/cm³ (meas.), in good agreement with values calculated for the theoretical formula. Optically, schallerite is uniaxial, negative, with w = 1.704 and e = 1.679 (Gage et al., 1925). There is no discernible response to ultraviolet. Schallerite is distinguished from friedelite and manganpyrosmalite only with difficulty. The higher density and refractive indices of schallerite are useful, but because mixtures are common, X-ray diffraction, optical, and microprobe techniques are best employed.

Composition

Schallerite is a manganese arsenic silicate hydroxide mineral of the friedelite group. Its composition, a matter of ambiguity for many years, was redefined by Dunn et al. (1981c). The arsenic is clearly trivalent, and the formula given in the heading is consistent with all known good data. Substitution of Mg and Zn for Mn is common, although minor in percentage of total cations. Manganese varies from 6.7 to 7.5 of the 8 octahedral cations. Silicon and arsenic are relatively constant in clean material. Ferric iron was detected, but its role is unclear; it varies from 0.1 to 0.4 atoms per 8 octahedral cations. The schallerite formula can be derived from that of friedelite by the substitution of two O for two (OH), coupled with the addition of As³⁺(OH). Schallerite which occurs in spheroidal aggregates has lower As₂O₃ values; these might be due to intimate, interlayer mixing of other minerals, such as friedelite and manganpyrosmalite, neither of which contains essential As; either one would serve to lower resultant arsenic values. Microprobe analyses of several schallerites are given in Table 15. Additional analyses have very similar values. Analyses of friedelite/schallerite mechanical mixtures showed no solid solution of As; such mixtures are composed of materials of end-member compositions.

Occurrence and paragenesis

Schallerite was first found between the 500 and 600 levels at Franklin (Foshag et al., 1927). This occurrence was of massive open-vein material, covered with a thin layer of calcite; removal of this calcite by immersion in a dilute solution of hydrochloric acid revealed superb crystals of schallerite (Figure 18-16).

	Figure 18-18. Schallerite spherules (“augens”) in fine-grained rhodonite matrix which is white in this photograph. Specimen is 6 cm in maximum dimension. Smithsonian Institution, #C5834. Photo by Vic Krantz.		Figure 18-19. Thin-section of one-half of a schallerite spherule, showing the radial arrangement of numerous pyramidal crystals. Field of view is 6 mm in maximum dimension. Photo by the author.

In addition, schallerite also occurs as spheroidal aggregates, composed for the most part of rhodonite and schallerite crystals (Figures 18-18 and 18-19) in a rhodonite groundmass containing minor barite and franklinite. A detailed description of these assemblages, which occur in hand-sized masses, was given by Dunn et al. (1981c).

	Figure 18-20. Schallerite crystals from a vein occurrence at Franklin. Field of view is 0.2 mm in maximum dimension.		Figure 18-21. Schallerite crystals from a vein occurrence at Franklin. Field of view is 0.5 mm in maximum dimension.

In addition to these principal occurrences, schallerite has been found in small amounts, associated variously with sarkinite, manganberzeliite, hedyphane, and some manganoan serpentines. It also occurs as intergrowths with friedelite; and as light brown impure platy aggregates with celestine (HU #122311). In recent years it has been found in Kodnitztal in Austria (J. Abrecht, pers. comm.). Schallerite has not been found at Sterling Hill.

Name

Schallerite was named in honor of Dr. Waldemar T. Schaller, an eminent American mineralogist.

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