Johannsenite is a calcium manganese silicate mineral of the pyroxene group and the Mn-analogue of diopside. Solid solution between johannsenite and diopside, hedenbergite, and petedunnite is limited; although Deer et al. (1978) have reported Fe/Mn as the usual solid solution at other localities, this might be precluded at Franklin by the low Fe content of silicates in general. An exception to this observation of limited solid solution is from the same assemblage as the type material. In this anomalous assemblage, solid solution is extensive and the sodium content is significant.
Johannsenite was first reported from Franklin and from numerous other localities by Schaller (1933b, 1938). Additional data were provided by Frondel (1965), but local material has been little studied since. It has not been reported from Sterling Hill.
Franklin johannsenite varies in color and habit. The type material, occurring as exsolved lath-like crystals, is dark red; that occurring in radial sprays of tightly bound crystals associated with nasonite is blue to green to light brown; and that occurring as fibrous, epitactic overgrowths on rhodonite is medium brown. The luster is vitreous; the cleavage is good; and the density is 3.52 g/cm3. There is no discernible fluorescence in ultraviolet.
Johannsenite occurs in at least three distinct assemblages at Franklin. Given its quite varied physical appearance, others may exist unrecognized inasmuch as johannsenite may mimic other species.
The type assemblage is of dark red johannsenite exsolved in pinkish orange, massive bustamite, together with exsolved willemite (Schaller, 1938). The assemblage is worthy of additional study. The material was locally abundant, and much has been preserved.
The assemblage described by Frondel (1965) from the 910 pillar on the 900 level is likely the same as that which provided the lead silicates (Dunn, 1985b). This assemblage consists of radial sprays of johannsenite with abundant manganaxinite, nasonite, willemite, unanalyzed mica, and lesser quantities of clinohedrite. The material may have been locally abundant.
One of the less abundant assemblages for johannsenite is the occurrence of stiff, fibrous, light-brown, epitactic overgrowths on rhodonite from Franklin. This is a vein assemblage, formed on franklinite/willemite ore, with manganaxinite and epidote occurring in superb 1-2 mm crystals, partly overgrown by rhodonite, the principal species in the vein, and barite. Euhedral recrystallized willemite occurs within some of the rhodonite. Johannsenite occurs as a surface alteration of rhodonite. (Dunn, 1995)

 Location Found: Franklin (Type Locality)
 Year Discovered: 1932
 Formula: CaMn2+Si2O6
 Essential Elements: Calcium, Manganese, Oxygen, Silicon
 All Elements in Formula: Calcium, Manganese, Oxygen, Silicon
 IMA Status: Approved
Fluorescent Mineral Properties

 Longwave UV light: Muted dull orange
 Additional Information: Moderately bright orange under blue light, peaking at 445 nm
 To find out more about this mineral at minDat's website, follow this link   Johannsenite

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.434

Frondel, Clifford (1972). The minerals of Franklin and Sterling Hill, a checklist. NY.: John Willey & Sons. p.63

The Picking Table References
 PT Issue and PageDescription / Comment
View IssueV. 58, No. 1 - Spring 2017, pg. 11Fluorescent Minerals of Franklin and Sterling Hill, N.J., Part 2, Richard C. Bostwick - Johannsenite
View IssueV. 17, No. 2 - September 1976, pg. 8The Post Palache Minerals - Johannsenite
View IssueV. 9, No. 1 - February 1968, pg. 7Mineralogical Data - Johannsenite/Bustamite
View IssueV. 7, No. 1 - February 1966, pg. 8Johannsenite
View IssueV. 6, No. 1 - February 1965, pg. 9Johannsenite
View IssueV. 6, No. 1 - February 1965, pg. 10Bustamite and Johannsenite
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