Schallerite



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). 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. Analyses of friedelite/schallerite mechanical mixtures showed no solid solution of As; such mixtures are composed of materials of end-member compositions.
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 occurrence 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.
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. Takeuchi 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.
The crystal structure of schallerite was described 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.
The original schallerite occurs as massive material, some with fine pyramidal crystals. The color varies from reddish-brown for massive aggregates to light pinkish-orange for transparent crystals. 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. 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.
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.
In addition, schallerite also occurs as spheroidal aggregates, composed for the most part of rhodonite and schallerite crystals 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). 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. Schallerite has not been found at Sterling Hill. (Dunn, 1995)


 Location Found: Franklin (Type Locality)
     
 
 Year Discovered: 1924
     
 Formula: (Mn,Fe)16Si12As3O36(OH)17
 Essential Elements: Arsenic, Hydrogen, Manganese, Oxygen, Silicon
 All Elements in Formula: Arsenic, Hydrogen, Iron, Manganese, Oxygen, Silicon
     
 IMA Status: Valid - first described prior to 1959 (pre-IMA) - "Grandfathered"
     
     
 To find out more about this mineral at minDat's website, follow this link   Schallerite

     
 References:
Dunn, Pete J. (1995). Franklin and Sterling Hill New Jersey: the world's most magnificent mineral deposits. Franklin, NJ.: The Franklin-Ogdensburg Mineralogical Society. p.481

Frondel, Clifford (1972). The minerals of Franklin and Sterling Hill, a checklist. NY.: John Willey & Sons. p.75


The Picking Table References
 PT Issue and PageDescription / Comment
View IssueV. 29, No. 2 - Fall 1988, pg. 3Notes from the Laboratory & Changes to the List of Species From Franklin and Sterling Hill, Pete J. Dunn, John L. Baum, Changes to the Unique List, Schallerite
View IssueV. 23, No. 1 - Spring 1982, pg. 9Mineral Notes Research Reports, Schallerite
View IssueV. 23, No. 1 - Spring 1982, pg. 11Additional Information on Schallerite and Friedelite and Caryopilite, Pete J. Dunn
View IssueV. 17, No. 2 - September 1976, pg. 4Mineral Notes - Schallerite (small article)
View IssueV. 9, No. 1 - February 1968, pg. 16The Exclusive Minerals of Franklin/Ogdensburg, N.J. (as of January 1968) by Frank Z. Edwards - Schallerite (Short Note)
     
Images

     
Schallerite (sperules), rhodonite, calcite, willemite and franklinite from Franklin, NJ
Schallerite (reddish brown sperules), rhodonite (pink), calcite (white), willemite (greenish tan) and franklinite (black) from Franklin, NJ. Photo by Robert A. Boymistruk.







All content including, but not limited to, mineral images, maps, graphics, and text on the Franklin-Ogdensburg Mineralogical Society, Inc. (FOMS) website is licensed under a
Creative Commons Attribution-NonCommercial 4.0 International License.
Creative Commons License