Brucite
Brucite is Mg(OH)2, hardness 2.5 and density 2.39. Forming it from periclase more than doubles the volume, which is why brucite-bearing marble falls apart.
Brucite is magnesium hydroxide, Mg(OH)2, hardness 2.5, density 2.39, with perfect {0001} cleavage, a waxy lustre and sectile, flexible plates. A specimen is normally massive rather than crystallised. It forms by hydration of periclase, MgO, and that reaction takes the molar volume from 11.26 cm3 to 24.40 cm3 — an expansion of 2.17 times, which destroys the rock it happens in.
In short
- The periclase-to-brucite reaction expands 2.17 times, and the arithmetic is short. MgO has a formula mass of 40.30 and a density of 3.58, so one mole occupies 11.26 cm3. Mg(OH)2 has a formula mass of 58.32 and a density of 2.39, so one mole occupies 24.40 cm3. The ratio is 2.167.
- That is why brucite-bearing marble disintegrates. The expansion happens inside a rigid crystalline rock, and the rock loses. The same reaction is the reason magnesia refractories fail if they are allowed to take up water before use.
- Brucite is sectile with flexible plates and elastic fibres. The Handbook records the distinction precisely: separable plates are flexible, fibres are elastic. Flexible means it stays bent; elastic means it springs back. Very few species are recorded as both.
- Gibbsite is the near-twin, and density does not separate them. Al(OH)3 at 2.40 against Mg(OH)2 at 2.39 — a difference of 0.4%, far inside measurement error. Hardness helps a little: gibbsite is 2.5 to 3.5 against brucite's flat 2.5.
- Crystals reach 19 cm tabular on {0001} and fibres reach 50 cm, but the Handbook notes brucite is rarely found in crystalline masses despite occurring at many localities.
| Species | Formula | Hardness | Density | Cleavage | What separates it |
|---|---|---|---|---|---|
| Brucite | Mg(OH)2 | 2.5 | 2.39 | Perfect on {0001} | — |
| Gibbsite | Al(OH)3 | 2.5 to 3.5 | 2.40 | Perfect on {001} | Densities are 0.4% apart; aluminium against magnesium |
| Portlandite | Ca(OH)2 | 2 | 2.23 | Perfect on {0001} | Softer and lighter; a rare species of burnt contact rocks |
| Talc | Mg3Si4O10(OH)2 | 1 | 2.58 to 2.83 | Perfect on {001} | Much softer and greasy; a silicate |
| Gypsum | CaSO4·2H2O | 1.5 to 2 | 2.317 | Perfect on {010} | Softer; cleaves to clear plates |
| Antigorite | (Mg,Fe)3Si2O5(OH)4 | 2.5 to 3.5 | 2.65 | Perfect on {001} | Denser, and green rather than white |
| Muscovite | KAl2(AlSi3O10)(OH)2 | 2.5 parallel, 4 across | 2.77 to 2.88 | Perfect on {001} | Plates are elastic, not flexible; denser |
The volume calculation, worked out in full
Periclase is MgO, cubic, with a density of about 3.58. Brucite is Mg(OH)2, hexagonal, with a measured density of 2.39 and a calculated density of 2.368, per the Handbook of Mineralogy. The reaction that turns one into the other is the addition of a molecule of water: MgO + H2O → Mg(OH)2.
Molar volume of periclase: formula mass 24.305 + 15.999 = 40.30 g/mol; divided by 3.58 g/cm3 gives 11.26 cm3/mol.
Molar volume of brucite: formula mass 24.305 + 2 × (15.999 + 1.008) = 58.32 g/mol; divided by 2.39 g/cm3 gives 24.40 cm3/mol.
Ratio: 24.40 / 11.26 = 2.167. One mole of magnesium occupies 2.17 times as much space as brucite as it did as periclase.
This is the same class of calculation as the dehydration arithmetic on the autunite page, run in the opposite direction. There, losing water contracted the structure by 18% by volume and the crystals flaked. Here, gaining water expands it by 117%, and the host rock fails.
Both are worth holding as the same idea: a hydration state is a volume, and changing it is a mechanical event. A specimen that changes hydration on a shelf is not merely altering chemically; it is being pulled apart from the inside.
What the expansion does to rocks, and to specimens
Periclase forms when a dolomitic or magnesian limestone is baked at high temperature and low pressure — a contact aureole against an intrusion, most often. It is stable only while dry. Given access to water it hydrates to brucite, and the 2.17-fold expansion takes place inside a rigid marble.
The result is a rock that crumbles. Blocks that were sound when quarried spall and disintegrate over years. The same reaction is the reason magnesia refractory brick is kept dry before firing: hydration in storage destroys the brick before it ever sees a furnace.
For a collector, the practical implication is narrow but real. A periclase specimen is a specimen of a mineral that wants to become a different mineral. Brucite itself is stable — the reaction has already happened — so a brucite specimen needs no special care beyond what its softness demands. It is periclase, and periclase-bearing marble, that should be kept dry.
Brucite is on the list of associates the Handbook gives for its own occurrences: calcite, aragonite, dolomite, magnesite, hydromagnesite, artinite, talc and chrysotile. That last association puts brucite in serpentinised ultramafic rocks, which is the same setting discussed on the talc page; anything to do with fibrous material from those rocks is a matter for a conservator and the health and safety regulator, not for a mineral page.
Flexible, elastic, sectile: three words that are not synonyms
The Handbook's tenacity entry for brucite reads: Sectile; separable plates are flexible, fibers are elastic. Three distinct properties in nine words, and collectors routinely blur them.
Sectile means it can be cut with a knife into shavings without powdering. Flexible means a thin plate can be bent and stays bent. Elastic means a bent piece springs back to its original shape when released.
The flexible-versus-elastic distinction is the one that does diagnostic work, and the standard example is mica. Muscovite plates are elastic: bend one and it returns. Talc, graphite, molybdenite and brucite's plates are flexible: bend one and it stays bent. That single test separates a mica from the soft sheet species it resembles and costs nothing but a thin cleavage flake.
Brucite is unusual in being recorded as both — flexible in plates and elastic in fibres — which is a statement about two different habits of the same species rather than a contradiction. The fibrous habit is sometimes called nemalite in older literature; it is brucite, and an old label using that name is a determination to a habit, not an error. The general point about superseded names is made on why mineral names end in -ite.
Localities, and a type material oddity
Brucite occurs as a common alteration of periclase in marble, as a low-temperature hydrothermal vein mineral in metamorphic limestones and chlorite schists, and during the serpentinisation of dunites.
Two Scottish occurrences are on the Handbook's list: Unst in the Shetland Islands and Camas Mòr on the Isle of Muck. Both are serpentinite occurrences, and both are remote enough that material is scarce. Elsewhere: Wood's Chrome mine near Texas, Lancaster County, Pennsylvania, for large crystals; the Tilly Foster mine at Brewster, New York; Hoboken, New Jersey; Gabbs, Nevada; the Crestmore quarry in Riverside County, California; Asbestos and Wakefield in Quebec; Mt Vesuvius and Teulada in Sardinia; Långban and Nordmark in Värmland, Sweden; the Ethyl mine at Mutorashanga, Zimbabwe; and Phalaborwa in the Transvaal.
The species is named for Archibald Bruce (1777–1818), a physician and early American mineralogist at the College of Physicians and Surgeons in New York, who first described it. The oddity is where the type material sits: the Handbook of Mineralogy records it at the Natural History Museum, London, 1911,730. An American species, described by an American, with its type material in London — a reminder that where a specimen ends up has nothing to do with where it came from, which is the argument behind what a type locality is.
The IMA-CNMNC Master List gives brucite as grandfathered, 1818, USA. Nothing on this site is offered for sale and there is no catalogue; the route for an Unst or Wood's Chrome specimen is the wanted list, and the rest of the section is indexed under mineral species.