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Hematite
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Postage stamps dedicated to Hematite!
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images © 2001 by Göran Axelsson
The name 'hematite' comes from the Greek word 'haimatites',
meaning blood-like, in reference to the bright red color of powdered hematite.
The color is derived from the iron content of the mineral. Hematite occurs in
many types of igneous, metamorphic and sedimentary rocks. In igneous rocks,
hematite occurs as grains within granite. It also occurs as crystals associated
with quartz deposited by volcanic gases and by high temperature hydrothermal
solutions. Hematite often occurs in metamorphic rocks, which formed from
ferruginous sediments, and in contact metamorphic deposits. The commercially
important hematite deposits are mainly of sedimentary origin, formed from the
weathering of iron-bearing minerals. In these sedimentary deposits, hematite is
thought to have formed in lakes or seas by organic and/or chemical processes.
The hematite often occurs with intermixed layers of quartz or chert. Hematite
also occurs as inclusions in a variety of other minerals, sometimes forming
highly valued gem and lapidary materials such as sunstone, which can contain
minute hematite or goethite inclusions which reflect light creating a sparkling
sheen. It is extremely common in surface environments and is responsible for the
red staining of many soils and rocks.
Hematite is of the oxide class, with a hexagonal-rhombohedral
crystal system It commonly occurs as tabular crystals, often platy to micaceous,
sometimes showing rhombohedral faces 'beveling' the edges, typically with
triangular markings on basal planes. It occurs more rarely in rhombohedral form,
also botryoidal to reniform, massive, earthy and soft (red ocher). It also occurs as
inclusions (typically flake-like) in quartz, feldspar, and a variety of other
minerals. Hematite frequently shows polysynthetic twinning. It has a refractive index
of 2.87 to 3.15, with high (.28) birefringence. Interference colors are completely masked
by hematite's red-brown color. Pleochroism is a deep red-brown/yellow-brown.
Specific gravity is 5.26 and hardness ranges from 5.5 to 6.5. It has a metallic
to sub-metallic to dull luster, and varies from reddish-brown to gray to black in
color. It is opaque to translucent with a light to dark-red streak. It fractures
conchoidally and has no cleavage, but can show well-developed parting.
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