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The Mineral johannsenite

Olive-Green Johansennite

Johannsenite is a rare member of the pyroxene group that is isomorphous with Diopside and Hedenbergite. It often occurs with black manganese oxide dendrites on its surface. Johannsenite alters to become the pink mineral Rhodonite. Johannsenite is named in honor of Albert Johannsen (1871-1962), a petrologist and professor at the University of Chicago.

Chemical Formula

CaMnSi₂O₆

Color

Gray, brownish-gray, greenish-gray, olive-green, light blue, cream yellow. May also be banded.

Crystal System

Monoclinic

Properties

Streak
White

Hardness
5 - 6

Transparency
Translucent to nearly opaque

Specific Gravity
3.4 - 3.6

Luster
Vitreous to greasy

Cleavage
1,2 - prismatic at cleavage angles of 87º and 93º (Characteristic of minerals in the pyroxene group). May also exhibit parting in one direction.

Fracture
Uneven to splintery

Tenacity
Brittle

Crystal Habits

As elongated and stubby prismatic crystals, in thick tabular crystals, in radiating and fibrous aggregates, massive, and in rounded masses.

Additional Information

Composition
Calcium manganese silicate, sometimes with iron

In Group
Silicates; Inosilicates; Pyroxene Group

Striking Features
Crystal habits, localities, color, and cleavage habits

Environment
Manganese-rich deposits of contact metamorphic rocks.

Rock Type
Metamorphic

Noteworthy Localities

Johannsenite is found in slender acicular crystals and in banded masses at Monte Civillina, Recoaro, Italy; and in olive green crystals in the North Mine, Broken Hill, New South Wales, Australia. Brown crystals associated with Rhodochrosite come from the Uchucchaqua Mine, Oyon Province, Peru. In the U.S., the premier locality is the Iron Cap mine, Landsman Camp, Graham Co., Arizona. Other occurrences are Hanover, Grant County, New Mexico; and Franklin, Sussex Co., New Jersey.