Ontario Mineral - Brucite

Rocks Minerals Canada Brucite

Mg(OH)2, Magnesium Hydroxide
Oxides and Hydroxides
A minor source of metallic magnesium, a source of magnesia and as a refractory additive.
white or colorless with shades of gray, blue and green.
vitreous or waxy; cleavage surfaces have a pearly luster
Crystals are translucent and rarely transparent
Crystal System:
trigonal; bar 3 2/m
Crystal Habits:
typically in flattened tabular crystals with rare rhombohedral terminations. Also found in lamellar and fibrous aggregates and as foliated masses. Brucite has been known to pseudomorph crystals of periclase .
perfect in one direction, basal
2 - 2.5
Specific Gravity:
2.4 (slightly below average)
Other Characteristics:
cleavage flakes and fibers are flexible but not elastic.
Associated Minerals:
calcite, wollastonite, nepheline , talc , aragonite , serpentine , chromite , dolomite , magnesite , periclase and other magnesium minerals.
Local Occurance:
York River [Tactile] Skarn
Best Field Indicators:
crystal habit, luster (especially on cleavage surfaces), lack of soapy or greasy feel and flexible but inelastic flakes and fibers.


Brucite is a mineral that is not often used as a mineral specimen but does have some important industrial uses. It is a minor ore of magnesium metal and a source of magnesia. It is also used as an additive in certain refractories.
It is brucite's structure that is interesting. The basic structure forms stacked sheets of octahedrons of magnesium hydroxide. The octahedrons are composed of magnesium ions with a +2 charge bonded to six octahedrally coordinated hydroxides with a -1 charge. Each hydroxide is bonded to three magnesiums. The result is a neutral sheet since +2/6 = +1/3 (+2 charge on the magnesiums divided among six hydroxide bonds) and -1/3 = -1/3 (-1 charge on the hydroxides divided among three magnesiums); thus the charges cancel.
The lack of a charge on the brucite sheets means that there is no charge to retain ions between the sheets and act as a "glue" to keep the sheets together. The sheets are only held together by weak residual bonds and this results in a very soft easily cleaved mineral. Brucite is closely related to gibbsite , Al(OH)3. However the extra charge in gibbsite's aluminum (+3) as opposed to brucite's magnesium (+2) requires that one third of the octahedrons to be vacant of a central ion in order to maintain a neutral sheet.
Brucite is interesting for another reason because it is often found as a part of the structure of other minerals. How can this be? Well, the neutral magnesium hydroxide sheets are found sandwiched between silicate sheets in two important clay groups: the Chlorite and Montmorillonite/smectite groups. The individual magnesium hydroxide layers are identical to the individual layers of brucite and are referred to as the "brucite layers".