Ramsdellite

ramsdellite

groutite

pyrolusite

manganite

Images

Formula: MnO2 (Mn4+O2)
Simple oxide, ramsdellite group, manganese-bearing mineral
Crystal System: Orthorhombic
Specific gravity: 4.37 measured, 4.79 calculated
Hardness: 3
Streak: Black
Colour: Black, steel-grey
Environments

Hydrothermal environments

Localities

At Whale Cove, Walton, Hants county, Nova Scotia, Canada, the paragenesis of the minerals is very complex. The principal hypogene minerals are baryte, siderite, pyrite, marcasite, galena, sphalerite, tennantite, proustite, chalcopyrite and acanthite. Less common are quartz, bravoite, stromeyerite, pearceite, gersdorffite, clausthalite and penroseite.
The main supergene minerals include limonite, goethite, pyrolusite, malachite, azurite, erythrite, lavendulan, moorhouseite, aplowite, chalcanthite, szomolnokite, metavoltine and chalcocite. Of less certain origin are hematite, kaolinite, calcite and covellite.
Ramsdellite and groutite have both been found at this locality (CM 9.718-720).

At the Bridgeville iron mines, Pictou county, Nova Scotia, Canada, a specimen has been found that consists of fine platelets of ramsdellite, entirely surrounded by coarse blades of pyrolusite (AM 47.47-58).

At Blythe, San Bernardino county, California, USA, an irregular nodule has been found, approximately 3 by 2 inches, with an outer rim 1/16 to 1/4 inch wide of fine-grained psilomelane, the interior consisting of a mixture of finely crystalline ramsdellite and pyrolusite (AM 47.47-58).

At the Monroe-Tener Mine, Mesabi Range, St. Louis county, Minnesota, USA, a small hand specimen has been found with clusters of radiating shiny black crystals of ramsdellite about 6 mm long, with a black streak, embedded in a dull grey matrix (EG 52.574-577).

At the type locality, the Lake Valley Mining District, Sierra County, New Mexico, USA, ramsdellite occurs in fissures and faults in limestone. Most commonly it occurs as groups of crystals on highly ferruginous chert; less commonly as so-called "nail-head ore". Much of the ramsdellite has altered to pyrolusite (AM 47.47-58).
It occurs as thick tabular crystals and as massive material that has a platy appearance, owing to two cleavages at right angles (AM 28.615).

Origin of Ramsdellite

Ramsdellite and pyrolusite occur as weathering products of groutite and manganite respectively. Pyrolusite is recognised as the stable form of MnO2and presumably can also be formed as a primary mineral, but it is possible that ramsdellite is actually metastable and can be formed only by the oxidation of groutite. The apparently universal presence of pyrolusite disseminated in ramsdellite in an oriented position suggests that ramsdellite undergoes further, slower (reconstructive) alteration to pyrolusite as the more stable phase. Ramsdellite has not so far been observed in a mineral association that would suggest it to be of primary occurrence, that is, deposited directly from mineralising solutions (AM 47.47-58).

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