Magnesite
MgCO3, Magnesium Carbonate

Chemistry: MgCO3, Magnesium Carbonate
Class: Carbonate
Group: Calcite
Uses: an ore of magnesium
Specimens: Magnesite does not ordinarily form good crystals, but can make up a substantial portion of some rock types. It forms commonly from the alteration of magnesium-rich rocks during low grade metamorphism while they are in contact with carbonate-rich solutions. Magnesite has the same crystal structure of calcite, hence its inclusion into the calcite mineral group. 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.Physical Characteristics:
Color is white or gray, also tinted yellow or brown.
Luster is vitreous.
Transparency crystals are translucent to transparent only in individual crystals.
Crystal System is trigonal; bar 3 2/m.
Crystal Habits are usually massive forms such as lamellar, fiberous and course to fine grained rocks. Crystals are extremely rare, but when found are in the form of rhombohedrons or hexagonal prisms with a pinacoid termination.
Cleavage is perfect in three directions forming rhombohedrons.
Fracture is conchoidal to uneven.
Hardness is 4 - 4.5.
Specific Gravity is approximately 3.0 (average).
Streak is white.
Associated Minerals: calcite, dolomite, aragonite, strontianite and serpentine.
Other Characteristics: effervesces easily only in hot dilute hydrochloric acid.