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, given by Frondel (1961), missed Cu and Cl, but did find H2O to be present. Analytical data were given by Dunn and Ramik (1984). 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 shows it to have Zn > Cu. Much work remains to be done on magnussonite.
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 Langban 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.
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. X-ray methods are suggested for facile verification.
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.
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. (Dunn, 1995)

 Location Found: Ogdensburg
 Year Discovered: 1956
 Formula: Mn10As6O18(OH,Cl)2
 Essential Elements: Arsenic, Hydrogen, Manganese, Oxygen
 All Elements in Formula: Arsenic, Chlorine, Hydrogen, Manganese, Oxygen
 IMA Status: Approved
 To find out more about this mineral at minDat's website, follow this link   Magnussonite

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.672

Frondel, Clifford (1972). The minerals of Franklin and Sterling Hill, a checklist. NY.: John Willey & Sons. p.66

The Picking Table References
 PT Issue and PageDescription / Comment
View IssueV. 34, No. 2 - Fall 1993, pg. 13Recent Mineral Finds From The Sterling Mine Ogdensburg, New Jersey - Magnussonite
View IssueV. 25 No. 2 - Fall 1984, pg. 8Mineral Notes Research Reports, Magnussonite
View IssueV. 24, No. 1 - Spring 1983, pg. 6Recent Mineral Occurrences at Sterling Hill, Stephen B. Sanford, Magnussonite
View IssueV. 18, No. 1 - March 1977, pg. 11The Post Palache Minerals - Magnussonite
View IssueV. 12, No. 2 - August 1971, pg. 6Geological Notes - Magnussonite
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