THE MINERAL BRUCITE
- Chemical Formula: Mg(OH)2, Magnesium Hydroxide
- Class: Oxides and Hydroxides
- Group:
Brucite - Uses: A minor source of metallic magnesium, a source of magnesia and as a refractory additive.
Specimens
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".
PHYSICAL CHARACTERISTICS:
- Color is white or colorless with shades of gray, blue and green.
- Luster is vitreous or waxy; cleavage surfaces have a pearly luster.
- Transparency Crystals are translucent and rarely transparent.
- Crystal System is trigonal; bar 3 2/m
- Crystal Habit is 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.
- Cleavage is perfect in one direction, basal.
- Fracture is uneven.
- Hardness is 2 - 2.5
- Specific Gravity is 2.4 (slightly below average)
- Streak is white.
- Other Characteristics: cleavage flakes and fibers are flexible but not elastic.
- Associated Minerals are calcite, wollastonite, nepheline, talc, aragonite, serpentine, chromite, dolomite, magnesite, periclase and other magnesium minerals.
- Notable Occurrences include Unst, Shetland Islands, England; Aesbestos, Wakefield and Black Lake, Quebec, Canada; Aosta, Italy; Brewster, New York, Wood's Mine, Texas, Gabbs, Nevada, Crestmore, California and Berks Co., Pennsylvania, USA.
- Best Field Indicators are crystal habit, luster (especially on cleavage surfaces), lack of soapy or greasy feel and flexible but inelastic flakes and fibers.
 Amethyst Galleries' Mineral Gallery MINERALS |
BRUCITE specimen bru-1
$ 300.00
$ 300.00
Dims: 4.9" x 4.3" x 2.0"(12.4 x 10.9 x 5.1 cm)
Wt: 1 lb., 0.2 oz.(458 g)
Asbest, Ekaterinberg, central Urals, Russia
This Brucite specimen lacks any definite crystal form, but is quite large and has an interesting green-blue color. It takes the form of a foliated mass that shows evidence of crystalline tendencies, but not clearly. It has a generally waxy luster, except for areas that show its perfect cleavage, which have a pearly luster. It is translucent, but large parts of it are opaque. It seems as if there is another mineral in the specimen, but most likely it is nearly pure Brucite. Probably the most impressive aspect of this specimen, other than what it actually is, is its Russian locality, which I have never heard of before.
bru-1 ($300.00)
Asbest, Ekaterinberg, central Urals, Russia
BRUCITE specimen bru-2
$ 50.00
$ 50.00
Dims: 1.3" x 1.0" x 0.5" (3.3 x 2.5 x 1.3 cm)
Wt: 8.8 g
Cedar Hill Quarry, Lancaster County, Pennsylvania, U.S.A.
This small thumbnail specimen consists of 3 intergrown and incomplete Brucite crystals. The largest of them has a diameter of 1" (2.5 cm) and a thickness of 0.2" (5 mm), and the other two are about half that diameter. All occur in a flattened trigonal tabular form that has a rounded but definable hexagonal outline. Damage to each is considerable, but does not directly affect their edges. All have a pearly-white color and a dull, silky luster on their intact surfaces; the surfaces that have undergone cleavage show a brighter, pearly luster that makes me think of mica. Though they are rather milky in appearance, they still show a dim translucence. The base on which these crystals rest appears to be made up almost entirely of bits of other broken Brucite crystals.
bru-2 ($ 50.00)
Cedar Hill Quarry, Lancaster County, Pennsylvania, U.S.A.
BRUCITE specimen bru-3
$ 75.00
$ 75.00
Dims: 2.7 x 2.3 x 1.1" (6.9 x 5.8 x 2.8 cm)
Wt: 2.57 oz. (72.8 g)
Boulby Potash Mine, Cleveland, Yorkshire, England
Though this specimen is composed almost entirely of Brucite, only a small percentage of the material shows any crystalline form. These formations are confined to two opposing surfaces on the specimen, which is likely part of a crust One of the surfaces shows tiny, botryoidal orbs that are heavily intergrown and dusted with microscopic crystals, giving them a slight sparkle. These orbs do not exceed 3 mm in diameter, and they show a few small areas of noticeable damage. However, these damaged areas allow one to see a few cross-sections of crystals, which contain visible concentric layering. Under magnification, one can see that each of these nodules is actually a cluster, of compact, fibrous, radial needles. The opposing surface also shows some definite botryoidal form, but these orbs are covered with larger, visible trigonal prismatic crystals that are transparent, quite clear, and possess a vitreous luster. All of the material has a uniform pale-green coloration that is slightly to intensely rust-stained in several areas.
bru-3 ($ 75.00)
Boulby Potash Mine, Cleveland, Yorkshire, England
BRUCITE specimen bru-4
$ 30.00
$ 30.00
Dims: 3.7 x 2.7 x 2.5" (9.4 x 6.9 x 6.4 cm)
Wt: 1 lb., 2.1 oz. (514 g)
Gabbs Valley Mine, Meneral County, Nevada, U.S.A.
This large hand specimen consists simply of a chunk of massive, shapeless Brucite. There is no evidence of either crystal form or crystalline tendencies present, and all surfaces are rough, dull, and uneven. The material has a dingy pale yellow-gray coloration, though there is a patch of almost black material at one end, and is basically opaque. There is no host or base rock present. We have left the original label (made of a strip of surgical tape) on the specimen; it is very difficult to read the locality off of it, though, because it is badly stained and the lettering is somewhat faded.
bru-4 ($ 30.00)
Gabbs Valley Mine, Meneral County, Nevada, U.S.A.
BRUCITE specimen bru-5
$ 35.00
$ 35.00
Dims: 1.25x1.17x0.55" (3.17x2.98x1.41cm)
Wt: 0.22oz (6.15g)
San Benito County, California, USA
A single large ball of brucite rests on a sliver of matrix. The brucite is off-white (just a hint of brown), and the individual crystals in the perfect radial cluster appear transparent, with triangular terminations. The brucite and the matrix are covered with a scattering of tiny white artinite clusters, some of which are broken open, revealing the radial pattern of fine white fibers of artinite.
bru-5 ($ 35.00)
San Benito County, California, USA