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
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).
Back to Minerals