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The Mineral magnesite

Transparent Colorless Magnesite Rhombs

The most common form of Magnesite is white, microcrystalline, porous masses that are dull in luster, and have the appearance of unglazed porcelain. Because they are porous, they adhere to the tongue when licked.

An interesting Magnesite occurrence is in Brumado, Brazil, where a deposit of clear, well formed, rhombohedrons similar to Iceland Spar Calcite found. Until this find, such crystals were, although identical in appearance to Iceland Spar Calcite, they are much rarer and sought after. Common Iceland Spar Calcite has been wrongly labeled by some unscrupulous dealers as Magnesite to stimulate sales.

Magnesite belongs to the calcite group of minerals, a group of related carbonates that are isomorphous with one another. They are similar in many physical properties, and may partially or fully replace one another, forming a solid solution series. All members of the calcite group crystallize in the trigonal system, have perfect rhombohedral cleavage, and exhibit strong double refraction in transparent rhombohedrons.

Chemical Formula

MgCO3

Color

Colorless, white, gray, yellow, brown, orange, light pink, light green

Crystal System

Hexagonal

Properties

Streak
White
Hardness
3.5 - 4.5
Transparency
Transparent to nearly opaque
Specific Gravity
3.0 - 3.3
Luster
Vitreous to dull
Cleavage
1,3 - rhombohedral
Fracture
Conchoidal to even
Tenacity
Brittle
Other ID Marks
1) Massive, porous specimens will stick to the tongue if licked.
2) Occasionally fluorescent blue or green.

Crystal Habits

Magnesite is not common in well formed crystals. Crystals are rhombohedral, sometimes single but more commonly in groupings and as cleavage fragments. Crystals may also be groupings of thin hexagonal plates, or as prismatic crystals. The most common form of Magnesite is in massive, compact, porous masses that are dull in luster. Also occurs grainy, coxcomb, botryoidal, in groups of thin, prismatic needles, and as small rounded balls.

3D Crystal Atlas

Additional Information

Composition
Magnesium carbonate, commonly with some iron and calcium
In Group
Carbonates; Calcite Group
Environment
In metamorphic serpentine rock and hornfels, in hydrothermal deposits, and in sedimentary beds.
Rock Type
Sedimentary, Metamorphic

Varieties

 -   Iron rich variety of Magnesite. Synonym of Ferro-magnesite.
 -   Iron rich variety of Magnesite, containing a greater amount of magnesium over iron. Its chemical formula is (Mg,Fe)CO3. If the amount of iron exceeds the magnesium, the mineral is called Sideroplesite which is a magnesium rich variety of Siderite.
 -   Clear, transparent, Magnesite rhombohedrons that resemble the Iceland Spar variety of Calcite.

Uses

Magnesite is an important ore of magnesium, and used as insulating material. White Magnesite is also used as a minor collectors gemstone,  polished into beads and spheres as well as carved into figures. These forms of Magnesite often have black or brown rock veins running through the mineral.

Noteworthy Localities

Excellent Magnesite specimens came from Styria, Austria, especially at Oberdorf an der Laming. Other European occurrences are Laach lake, Eifel Mts, Germany, where it occurs as rounded balls; Monterufoli, Pomarance, Tuscany, Italy; the Azcarate Quarry, Eugui, Navarre, Spain; and Albatera, Alicante, Spain.

In Australia good Magnesite comes from Mt. Bischoff, Waratah, Tasmania; and in China good specimens were found at Shangbao, Hunan Province. Clear crystals, identical in appearance to the Iceland Spar variety of Calcite, occur at Brumado, Bahia, Brazil, which is perhaps the most famous occurrence of this mineral.

U.S. occurrences are the Cedar Hill Quarry, Fulton, Lancaster Co., Pennsylvania; Wood's Chrome Mine, Texas, Lancaster Co., Pennsylvania; Bisbee, Cochise Co., Arizona; and Gabbs, Nye Co., Nevada; and in Serpentine at Staten Island, (Richmond Co.), New York.

Distingushing Similar Minerals

Calcite - The rare transparent Magnesite rhombohedrons appear identical to Calcite%, but can be distinguished by the slight hardness difference, and more efficiently by the fact that Calcite effervesces even in cold and diluted hydrochloric acid, whereas Magnesite only in hot, undiluted hydrochloric acid.
Dolomite - Cannot be distinguished from Magnesite by ordinary methods. However, Dolomite usually forms as curved crystals, whereas Magnesite does not.


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