Magnesium carbonate (MgCO2)
Color: Colorless, transparent,
white, grayish white, yellowish to brown
Streak: Nearly white
Hardness: 3.75 - 4.2
Specific Gravity: 3.00
Cleavage: Perfect, three directions
Habit: Crystals are not common.
Aggregates are massive, compact, coarse
to fine granular, chalky, porcelaineous,
lamellar, and fibrous.
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
in Which this Mineral is Found
San Luis Obispo
Specimen Description: Magnesite, 10 cm
wide, Stanislaus County. California
Division of Mines and Geology Library.
Photographer: Max Flanery, CGS.