MAGNUSSONITE

Mn₁₀As³⁺₆O₁₈(OH,Cl)₂  
Cubic

Magnussonite was found at Sterling Hill in 1934 and analyzed by Lawson Bauer. The studied specimen, in the Harvard Mineralogical Museum, was recognized as magnussonite by Frondel (1961). The crystal structure was determined by Moore and Araki (1979) using Swedish material. New chemical data were given for both Sterling Hill and Långban specimens, and a new Sterling Hill occurrence was described by Dunn and Ramik (1984) who proposed the above formula. Magnussonite has not been reported from Franklin.

Description

Magnussonite occurs as massive material; no euhedral crystals are known. It varies in color from green (predominant) to light brown and dark brown. The luster is vitreous, and no cleavage was observed. Frondel (1961) reported the index of refraction for Bauer’s magnussonite to be n = 1.983. X-ray methods are suggested for facile verification.

Composition

Magnussonite is a manganese arsenite hydroxide mineral, with small amounts of Mg, Fe, and Ca. Appreciable Cu, and at Sterling Hill, Zn, are present. Bauer’s original analysis of specimen HU#-95036, given by Frondel (1961), missed Cu and Cl, but did find H₂O to be present. Analytical data were given by Dunn and Ramik (1984), and several analyses are given in Table 25. In a detailed discussion, Dunn and Ramik showed that magnussonite has     (OH) > Cl and 4(OH,Cl) per 12 As, instead of the two reported by Moore and Araki (1979). Additionally, it was shown that magnussonite has persistent amounts of Cl and Cu present in all specimens. Analysis of material from a subsequent 1984 discovery (#161159) shows it to have Zn > Cu. Much work remains to be done on magnussonite.

Occurrence and paragenesis

The first occurrence of magnussonite was of a very small amount of green material associated with zincite in a selvage on a slickenside on willemite/franklinite/zincite  ore (HU#-95036).

The second occurrence, from the central zincite zone near the 800 level at Sterling Hill, consisted of veins of green or brown magnussonite. Some magnussonite is present within the adjacent granular ore, but it is recognized only with difficulty. Some associated minerals are native copper, alleghanyite, and zincite.

A third occurrence was found at Sterling Hill by John Kolic in 1984 and consisted of locally abundant veins of clean magnussonite up to 2 cm thick, associated with mcgovernite and zincite. It is perhaps noteworthy that all local magnussonite is associated with zincite.

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