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The Gemstone Rhodochrosite

PHYSICAL PROPERTIES:
Chemistry: MnCO3
Composition: Manganese Carbonate
Color: red to pink, sometimes
almost white, yellow and brown.
Class: Carbonates
Group: Calcite
Crystal system: trigonal; bar 3 2/m
Crystal habit: rhombohedrons and
more rarely scalahedrons.
Fracture: uneven
Hardness: 3.5 - 4
Specific gravity: 3.5
Refractive Index:
Pleochroism:
Luster: vitreous to resinous
Streak: white
Cleavage: perfect in three directions
Transparency: Crystals are
transparent to translucent
Associated Minerals: pyrite, fluorite, quartz,
and ore sulfides calcite, ankerite, alabandite, rhodonite, bementite,
spessartine, manganite
COMPOSITION:
Manganese Carbonate (61.7% MnO, 38.3% CO2
with any or all of the following Present: iron, calcium,magnesium, zinc
and cobalt)Rhodochrosite is usually a mineral of copper and lead ore veins,
but sometimes occurs, like siderite, in pegmatite's. At Butte, Montana,
it is an ore of manganese. It commonly alters to black manganese oxides
on weathering and the black stains are very apparent on the containing
rocks. Good specimens are not common.
ENVIRONMENT:
Rhodochrosite forms in veins of copper, lead and silver
which have been altered by hot water (hydro thermals), and as a secondary
deposit in sedimentary layers of manganese oxide.
CRYSTAL DESCRIPTION:
Crystal Habits include the rhombohedrons and scalahedrons with
rounded or curved faces that can obscure the crystal shape. Some crystals
can be flattened to a bladed habit and these are sometimes aggregated
into rosettes or minute crystals into spherules. Also botryoidal, globular,
stalactitic, layered, nodular, vein-filling and granular. Twinning is
somewhat common forming penetration twins and contact twins similar to
calcite's twins.
TESTS:
Identification of rhodochrosite is fairly easy despite a
few similarly colored minerals
such as rhodonite. Rhodonite is harder and has different cleavage; but
perhaps the
best distinguishing factor is its lack of reaction to acids. Rhodochrosite
will dissolve
slowly in cold, and rapidly in warm, hydrochloric acid with effervescence.
Demonstrating
it's carbonate chemistry. Basically, any rose-pink carbonate is considered
rhodochrosite. However some calcites with a small amount of manganese
impurities can be pink in color.
The manganese replaces some of the calcium ions in calcite but a complete
series between
calcite and rhodochrosite is not established. Differentiating pink calcite
from rhodochrosite may require a fluorescence test as rhodochrosite is
distinctly non-fluorescent and manganese is a fluorescent activator in
calcite.
DISTINGUISHING CHARACTERISTICS:
Pink and white banding in massive forms, non-fluorescence
and specimens effervesce easily with dilute acids. The cleavage
and hardness (and acid test) show it to be of the carbonate group. The
borax bead test shows it to be a manganese mineral and eliminates about
everything else. The pink color is the best guide.
USES:
As a minor ore of manganese, an ornamental and semiprecious gemstone and
as mineral specimens. Specimens. In a massive form its pink and white
bands are extremely attractive and are often used in semiprecious jewelry.
Rhodochrosite is often carved into figurines
and tubular stalactitic forms are sliced into circles with concentric
bands that are truly unique in the mineral kingdom. Fine crystals are
sometimes cut into gemstones, but rhodochrosite's softness and brittleness
limit it as a gemstone for everyday use.
LOCALITIES:
The famous Sweet Home Mine and American Tunnel in Colorado. The Humboldt
Mine in Cochise County, Arizona. Butte, Montana, Franklin, New Jersey
and many California sites. Rhodocrosite is also found in The Huaron Mine
Catamarca, Argentina. Peru; Kara oba, Kazakhatan; Sacrimb, Transylvania,
Romania; Cornwall, England; Harz Mountains, Germany; Tsumeb, Otavi, Namibia;
Santa Eulalia and Magdalena, Mexico; N'Chwanging Mine, Hotazel, South
Africa; Mont Saint-Hilaire, Quebec, Canada. The best specimens have come
from the Sweet Home Mine in Colorado.
FACTS & HISTORY:
Rhodochrosite means rose-colored The name rhodochrosite derives from the
Greek words rhodos, meaning rose and khros meaning color. This is the
distinctive color of most specimens although some may appear brown or
light gray.
John Ghist’s Earth Science class at Platte Canyon High School near
Bailey, Colorado was studying rocks and minerals when they became aware
that Colorado did not have a State Mineral. After some debate, the students
decided that rhodochrosite, because of its red color (similar to Colorado
= reddish in Spanish) should be the state mineral. They wrote a letter
to State Representative Carl Miller suggesting that rhodochrosite be designated
the official State Mineral. Representative Miller introduced the legislation
and within three months rhodochrosite was designated the Colorado State
Mineral.
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