ORE MINERALS 117
Manganite
Mn3+O(OH)
Opaque, metallic dark grey or black manganite crystals are
MANGANITE occurs
prismatic and striated lengthways. They typically have flat with other manganese
or blunt terminations and are often grouped in bundles. oxides and minerals,
Manganite can also be massive or granular and is then such as rhodonite
hard to distinguish from other manganese oxides, such as (shown here) and
rhodochrosite.
pyrolusite. Usually, manganite forms in low temperature
hydrothermal veins, hot spring manganese deposits, and
by alteration of other
manganese minerals.
striations
on faces
flat
terminations
bundles
of crystals
COMPOSITION Oxide.
CRYSTAL SYSTEM Monoclinic.
CLEAVAGE/FRACTURE Perfect/Uneven.
LUSTRE/STREAK Metallic or submetallic/Dark
reddish brown.
HARDNESS/DENSITY 4 / 4.29–4.34.
KEY PROPERTIES Black mineral distingished
from pyrolusite (p.118) by streak.
Hausmannite
Mn2+Mn23+O4
Hausmannite is dark brown or black and is usually
granular or massive. Well-formed crystals, although less
common, are more distinctive. They appear octahedral in AN ORE mineral of the
shape, but often have additional faces. An ore of Kalahari manganese
manganese, hausmannite field in South Africa,
hausmannite forms
forms in hydrothermal massive aggregates
veins. It also occurs and fine crystals.
where manganese-rich
rocks have been
massive habit
metamorphosed.
submetallic
lustre
crystals
appear
octahedral COMPOSITION Oxide.
CRYSTAL SYSTEM Tetragonal.
CLEAVAGE/FRACTURE Perfect/Uneven.
LUSTRE/STREAK Submetallic/Reddish brown.
HARDNESS/DENSITY 5.5 / 4.84.
KEY PROPERTIES Black octahedral-shaped
crystals, perfect cleavage, reddish brown
streak.
118 ORE MINERALS
Pyrolusite
Mn4+O2
This common opaque black or dark grey mineral is usually
PYROLUSITE occurs in
the metamorphosed fibrous or fine-grained; prismatic crystals are rare. It is
sedimentary rocks of found as botryoidal masses, concretions, and as coatings
the Green Mountains, that may be powdery or dendritic. Pyrolusite, a secondary
Vermont, USA. mineral in manganese deposits, also forms in bogs, lakes,
and shallow seas. marly
limestone
black dendritic
pyrolusite
COMPOSITION Oxide.
CRYSTAL SYSTEM Tetragonal.
earthy habit CLEAVAGE/FRACTURE Perfect/Uneven.
LUSTRE/STREAK Metallic to dull/Bluish black.
HARDNESS/DENSITY 6–6.5 / 5.06.
black KEY PROPERTIES Black mineral,
mass distinguished from other manganese oxides
by streak, crystal shape, or chemical analysis.
Psilomelane, Wad
Mixed manganese oxides
Black manganese oxides that lack distinct crystals are
difficult to tell apart, and the problem is greater when they
MANGANESE, iron, and occur in mixtures. Soft, unidentified manganese oxides are
copper oxide minerals known in general terms as wad, and when they are hard, as
colour Castle Rocks, psilomelane. In the past, psilomelane has also
Michigan, USA.
been used as a name for
botryoidal the barium manganese
habit
oxide romanèchite
and it often contains
this mineral.
hard, black
mass
COMPOSITION Oxides and hydroxides.
CRYSTAL SYSTEM Various.
CLEAVAGE/FRACTURE None/Uneven.
LUSTRE/STREAK Earthy/Usually black.
HARDNESS/DENSITY Varies according to soft powdery
composition. mass
KEY PROPERTIES Soft or hard black masses;
no distinct crystals visible. no obvious WAD
crystals
ORE MINERALS 119
Rhodochrosite
Mg2+CO3
Rhodochrosite is usually pink, rarely brown, and forms
THESE HYDROTHERMAL
rhombohedral or dog-tooth crystals like calcite. Banded veins of rhodochrosite,
rhodochrosite comes from Argentina and Peru, and makes galena, and fluorite
an attractive decorative stone, but the finest crystals are are in the roof of the
Sweet Home mine,
from the Sweet Home mine and elsewhere in Colorado, Colorado, USA, and
USA, and are bright cherry red. other localities.
Rhodochrosite is found mainly
rhombohedral
in hydrothermal deposits crystal
with black manganese
oxides and minerals
such as fluorite,
galena, and
sphalerite.
cherry red
crystals
quartz crystal
vitreous
lustre
pale silvery-
yellow
pyrite pink
rhodochrosite
bands of
pink and
red
polished surface
COMPOSITION Carbonate.
NOTE
CRYSTAL SYSTEM Trigonal. Manganese secondary minerals tend to be pink
CLEAVAGE/FRACTURE Perfect, or black. Minerals containing other chemical
rhombohedral/Uneven or conchoidal. elements can also be a particular colour. Copper
LUSTRE/STREAK Vitreous/White. secondaries are blue or green; those of cobalt,
HARDNESS/DENSITY 3.5–4 / 3.7. lilac pink, and of nickel, apple green. Chromium
KEY PROPERTIES Pink mineral that fizzes in secondaries are green or purple-red, while iron
dilute HCl; does not fluoresce under UV light. minerals are often yellow, brown, or green.
120 ORE MINERALS
Rhodonite
CaMn4Si5O15
Rhodonite is a pink manganese mineral and an attractive
THIS RHODONITE
nodule contains black semi-precious gemstone. It is usually found as coarse,
manganese oxides and cleavable or fine-grained compact masses veined with black
brown bustamite. manganese oxides. It can also be granular, but crystals are
much less common. Rhodonite comes from
manganese deposits formed by
hydrothermal, metamorphic,
and sedimentary processes.
tabular crystals
massive
rhodonite
COMPOSITION Silicate.
CRYSTAL SYSTEM Triclinic.
CLEAVAGE/FRACTURE Perfect/Conchoidal
to uneven. polished pebble
LUSTRE/STREAK Vitreous/White.
HARDNESS/DENSITY 5.5–6.5 / 3.57–3.76. black
manganese
KEY PROPERTIES Harder than rhodochrosite
oxides
(p.119) and does not fizz in dilute HCl.
Braunite
2+ 3+
Mn Mn 6SiO12
Not the most striking of minerals, braunite is an important
ore of manganese. It is opaque, brown-black, or dark grey,
BRAUNITE is an and is typically granular or massive. Crystals are less
important manganese common and are pyramidal, nearly
ore in South octahedral in shape. Braunite is
Africa.
found with manganese oxides
and forms by metamorphism
or the weathering of
manganese deposits.
submetallic mass of small
lustre crystals
COMPOSITION Oxide or silicate.
CRYSTAL SYSTEM Tetragonal.
CLEAVAGE/FRACTURE Perfect/Uneven to
subconchoidal.
LUSTRE/STREAK Submetallic/Brownish black
to steel grey.
HARDNESS/DENSITY 6–6.5 / 4.72–4.83.
crystal appears KEY PROPERTIES Shape, streak, and association.
octahedral
