Chemistry: PbCrO4 , Lead Chromate
Class: Sulfates
Subclass: Chromates
Crystal System: monoclinic
Hardness: 2.5 - 3.
Streak: is orange-yellow
Color: orange-red-redish yellow
Transparency: Crystals are translucent
to transparent
Specific Gravity: approximately 6.0+
Refractive Index: 2.36
Fracture: conchoidal and uneven.
Luster: adamantine to greasy
Cleavage: distinct in two directions lengthwise,
prismatic.
PHYSICAL CHARACTERISTICS:
Crocoite is an unusual and beautiful redish-orange mineral specimen,
that fascinates collectors. Crocoite is well known for its distinctive
orange-red color and it's high density
and luster are the result of its lead content. It's refractive index
is also very high.
Some Crocoite aggregates are composed of bars and splinters, of many
different sizes, going
in a dozen different directions. Producing a character and quality that
is, a distinctive trait of Crocoite.
Crystal habits include long thin prisms, which are striated vertically.
Crystals are rarely large and single. Crystals form most commonly as
reticulated aggregates, acicular groupings, elongated needles, grainy,
encrusting, and as disorganized groupings of small prismatic crystals.
Crystals are sometimes partially hollow. Terminations are usually poorly
developed and appear uneven or pitted. Also, granular and columnar.
Associated minerals are wulfenite, pyromorphite, cerussite, limonite
and vanadanite.
USES:
Crocoite was at one time a main ore of chromium. But it is now too rare
to be used. Crystals are highly sought after by mineral collectors because
of its attractive color and it's wild crystal arrays.
LOCALITIES:
Crocoite is a rather unusual lead mineral, that was first found in Russia
and later in other European localities. But these localities seldomly
produced interesting mineral specimens. The main source of quality specimens
comes from the Dundas District of Tasmania, Australia. The Adelaide'
Mine, is where 95% of the world's specimen Crocoite is obtained.
Large, brilliant, well-formed crystals were obtained here in the beginning's.
But large crystals from this locality have
been scarce for some time now. Now, mostly fragile aggregates of small
crystals are found here. They are still very beautiful
specimens that show off crocoite's interesting color and character.
Crocoite specimens are fragile and care must be taken when handling
them.
The other localities listed here yielded only minor amounts. Crocoite
was found in Sverdlovsk, Ural Mountains, Russia; Callenberg, Saxony,
Germany; Nontron, France; Labo, Philippines; and Congonhas de Campo,
Brazil. In the US, only small amounts were found, only capable of being
micromounts. Occurrences include the Mammoth Mine, Tiger, Pinal Co.,
Arizona; the El Dorado Mine, Indio, Riverside Co., California; and Darwin,
Inyo Co., California.
HISTORY & FACTS:
In the mid 18th century analysis of Siberian "red lead" (PbCrO4,
Crocoite) from Siberia showed that it contained quite a lot of lead,
but also a further material. This was eventually identified as chromium
oxide. Identical in composition to chrome yellow, the artificial product
used in paint.
Chromium oxide was discovered in 1797 by Louis-Nicholas Vauquelin, who
prepared the metal itself in the following year. Starting from Crocoite
the procedure was to powder the mineral and to precipitate the lead
out through its reaction with hydrochloric acid (HCl in water). The
residue was chromium oxide, CrO3. Heating this oxide in an oven in the
presence of charcoal as a reducing agent gave the metal itself.
Vauquelin also analyzed an emerald from Peru and discovered that its
green color is because of the presence of the new element, chromium.
In fact, the name chromium is from the Greek word "chroma" meaning "color",
so named because of the many different colored compounds displayed by
chromium. A year or two after Vauquelin's discovery, a German chemist
named Tassaert working in Paris found chromium in an ore now called
chromite. This ore, Fe(CrO2)2, is now an important source of chromium.
Chromium is also used to make stainless steel.
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