Magnesite

Magnesite

Magnesite
 

Composition: Magnesium carbonate (MgCO2)

Physical Properties:
Color: Colorless, transparent, white, grayish white, yellowish to brown
Streak: Nearly white
Luster: Vitreous
Hardness: 3.75 - 4.2
Tenacity: Brittle
Specific Gravity: 3.00
Cleavage: Perfect, three directions

Crystallography:
System: Hexagonal
Twinning: Rare
Habit: Crystals are not common. Aggregates are massive, compact, coarse to fine granular, chalky, porcelaineous, lamellar, and fibrous.

General Information:
Magnesite is closely related to calcite, and the two minerals are similar in most physical properties. However, magnesite is slightly heavier and harder than calcite, is less readily soluble in acids, and is not nearly as abundant. Magnesite cannot be scratched with a copper penny but it can be scratched with a knife blade, and it will effervesce only in strong hot acids. Crystals of magnesite have perfect cleavage, but the specimen is made up of an aggregate of very tiny magnesite grains so this cleavage is not visible. Large crystals of this mineral are rare. In California, magnesite occurs principally as white veins in serpentine, a dark green rock. Such veins are characteristically an inch or so thick, but in places they are several feet thick. Magnesite has been mined from many of the larger veins, and used in the manufacture of a special type of cement (called magnesium oxychloride cement, or simply "magnesite"). The mineral also is mined for production of magnesium chemicals and magnesium metal. Name refers to its composition.

California Counties in Which this Mineral is Found
 

Alameda
Del Norte
El Dorado
Fresno
Humboldt
Inyo
Kern
Kings
Lake
Los Angeles
Madera
Mariposa
Mendocino
Monterey
Napa
Nevada
Placer
Riverside
San Benito
San Bernardino
San Francisco
San Luis Obispo
Santa Barbara
Santa Clara
Sonoma
Stanislaus
Tulare
Tuolumne
 

Information on Specimen Photo:
Specimen Description: Magnesite, 10 cm wide, Stanislaus County. California Division of Mines and Geology Library.
Photographer: Max Flanery, CGS.