AKROCHORDITE


(Mn2+,Mg)5(AsO4)2(OH)4 · 4H2O,
manganese magnesium arsenate hydroxide hydrate




Akrochordite, a mineral known from few localities worldwide, was first described (in 1922) from Långban, Sweden.  The 1979 find of this mineral in the Sterling mine (Dunn, 1981) marked only the second occurrence worldwide of the species; it is not known from Franklin.  Most of the better specimens are in institutional collections, and only rarely are examples offered on the open market.  As shown below, most Sterling mine akrochordite occurs as brown, radial sprays of flattened crystals that formed within narrow fracture openings in willemite-franklinite ore; only one specimen is known where the mineral had sufficient room to form a large, complete spheroid (AKR2).  All known examples of this mineral from the Sterling mine are from the area of the 1979 find.

References

Dunn, P.J. (1981), Akrochordite, a second occurrence: Sterling Hill, New Jersey. Mineralogical Magazine, vol. 44, p. 235-236. [Announces the second world locality for the species and gives chemical analyses of three specimens. Mentions the near-absence of Mg in the Sterling mine specimens and suggests they may qualify as the Mn-analogue of akrochordite as then defined on the basis of the Långban material]

Dunn, P.J. (1995), Franklin and Sterling Hill, New Jersey:  the world’s most magnificent mineral deposits. Privately published in five volumes, 755 pp. [Description of akrochrodite on p. 655-656, with a chemical analysis on p. 654]
 
King, V.T., King, N.E., Betancourt, P.P., Bostwick, R.C., Chin, Peter, Hecht, T.J., Kuitems, S.M., Moritz, Harold, Nemetz, J.D., Nikischer, A.J., Sanford, Stephen, Van Fleet, J.A., and Verbeek, E.R. (2021), The Mineralogy of Franklin and Ogdensburg, New Jersey—A Photographic Celebration. Privately published, 1400 pp. [Two photos of akrochordite shown on p. 761]

Moore, P.B., Sen Gupta, P.K., and Schlemper, E.O. (1989), Akrochordite, (Mn,Mg)5(OH)4(H2O)4(AsO4)2:  a sheet structure with amphibole walls. American Mineralogist, vol. 74, p. 256-262. [Detailed discussion of the crystal structure of akrochordite]

Moore, P.B. (1995), Closest-packing and hydrogen bonds in minerals of the Franklin Marble:  The Picking Table, vol. 36, no. 1, p. 7-26. [Describes akrochordite as a packet structure containing amphibole-like packets based on cubic close packing and hypothesizes a link to the high pH and reducing nature of the fluids from which it and other such minerals formed]



Images




Splayed rosettes of akrochordite, granular, high-grade, brown willemite-franklinite ore, Sterling mine, Ogdensburg, NJ
Earl R. Verbeek
Splayed rosettes 0.5-2.5 cm in diameter of akrochordite thinly coating a fracture surface in granular, high-grade, brown willemite-franklinite ore. This is the best of several akrochordite specimens owned by the Franklin Mineral Museum and is notable for the large size of several of the rosettes; in most other specimens they are only 5-7 mm across (Dunn, 1995, p. 656). From the SPEX/Gerstmann collection, this specimen now bears the catalog number FMM-1916. This is from the 1979 find of akrochordite described by Pete J. Dunn in his 1981 paper; he states that only about 20 specimens were recovered.

Identifier: AKR1a
Locality: 1220 undercut pillar, 30 ft below 800 level, Sterling mine, Ogdensburg
Specimen size: 13 x 8.5 x 6.5 cm



Splayed rosettes of akrochordite, granular, high-grade, brown willemite-franklinite ore, Sterling mine, Ogdensburg, NJ
Earl R. Verbeek
Close-up view of the large akrochordite rosette at center-right of the specimen AKR1a. The rosette is 2.0 cm in diameter.

Identifier: AKR1b
Locality: 1220 undercut pillar, 30 ft below 800 level, Sterling mine, Ogdensburg
Specimen size: 13 x 8.5 x 6.5 cm





Spherical aggregate of akrochordite crystals, high-grade willemite-franklinite ore, Sterling mine, Ogdensburg, NJ
Earl R. Verbeek
This specimen, a remarkable one for the locality, shows a spherical aggregate of akrochordite crystals implanted on a minor fault surface in high-grade willemite-franklinite ore. This is the only such large (1.2 cm diameter) spherule known from Sterling Hill. In nearly all other specimens the akrochordite formed within narrow seams and had little room to develop beyond thin, patchy coatings, as in AKR1. The finder of this specimen, John Kolic, reported that the blasted ore on the day of the find, in August 1979, was exceptionally rich in fibrous to felty sussexite, and that he found the akrochordite sphere by picking through a thick mass of such material, which had probably protected it from damage during the blast. John proudly displayed this specimen, known to locals as “The Rock Wart,” during the Franklin-Sterling Gem and Mineral Show every year in Franklin. It should be noted that akrochordite was named in 1922 by Gustav Flink in allusion to ακρόχορōου (akrochordon), the Greek word for wart.

Identifier: AKR2a
Locality: 1220 undercut pillar, 30 ft below 800 level, Sterling mine, Ogdensburg
Specimen size: 9.5 x 5.5 x 5 cm