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Wild Horse Magnesite
PHYSICAL PROPERTIES:
Chemistry: MgCO3,
Composition: Magnesium Carbonate
Color: Color is white or gray, also
tinted
yellow or brown.
Class: Carbonate
Subclass:
Group: Calcite
Crystal system: trigonal; bar 3 2/m
Crystal habit: rhombohedrons or hexagonal
prisms with a pinacoid termination.
Fracture: conchoidal
Hardness: 4 - 4.5
Specific gravity: 3.0
Refractive Index:
Pleochroism:
Luster: vitreous
Streak: white
Clevage: perfect in three directions
Transparency: crystals are
translucent to transparent
Associated Minerals: calcite, dolomite,
aragonite, strontianite and serpentine
COMPOSITION:
Magnesium carbonate (47.6% MgO, 5 2.4% C02,
often with some iron and calcium) Many
of the properties of magnesite are either identical or similar to those
of calcite. However, the magnesium ion does not allow the carbonate ion
(CO3) to interact as easily with cold acids, as
the calcium ion does in calcite. This provides the best means of distinguishing
magnesite from calcite. However, dolomite(MgCa(CO3)2)
can be almost indistinguishable from magnesite.
ENVIRONMENT:
Usually in sedimentary beds from a hot-water (hydrothermal)
al- teration of serpentine which creates solid white veins in the parent
rock. Small free-growing crystals were found in ser- pentine fissures
on Staten Island, New York. Huge quantities of the dull white material
have been mined as sources of mag- nesia and magnesium in Washington and
California.
CRYSTAL DESCRIPTION:
Usually in dull white microcrystalline masses. Crystals are extremely
rare, but when found,
they are in the form of rhombohedrons. Also found in small prismatic needles.
A marble-like
variety with very coarse grain is found in Brazil. This deposit is the
source of large Ice-land-spar like crystals and cleavages. Has been found
in cavities with quartz and other minerals. This stratified deposit probably
represents the completion of the magnesia-enrichment
process which takes place in limestones to transform them to dolomites.
Magnesite has the same crystal structure of calcite, hence its
inclusion into the calcite mineral group.
DISTINGUISHING CHARACTERISTICS:
The white, dull, fine-grained porcelaneous masses
can be identified by their behavior in acid. Both the marble-grained and
transparent rhombs can be confused with calcite or dolomite, but are heavier
and make no response to cold hydrochloric acid.
LOCALITIES:
Austria; Bahia, Brazil; Korea; China; California,
USA and many European localities.
Wild Horse is the marketing
name for a new find of magnesite/hematite from the Gila Wilderness Area
in Southern Arizona
USES:
an ore of magnesium; mineral specimens; for gemstones and carving
material.
TESTS:
The tongue adheres to the porcelaneous material. Dis- solves with
bubbles in hot
hydrochloric acid.
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