Brucite

Brucite is the mineral form of magnesium hydroxide (Mg(OH)₂). It commonly forms when periclase alters in marble, appears as a low-temperature hydrothermal vein mineral in metamorphosed limestones and chlorite schists, and develops during the serpentinization of dunites. It is typically found alongside serpentine, calcite, aragonite, dolomite, magnesite, hydromagnesite, artinite, talc, and chrysotile. Its structure is layered, resembling cadmium iodide, with hydrogen bonds holding the layers together.

First described in 1824 by François Sulpice Beudant, it was named after American mineralogist Archibald Bruce (1777–1818), who discovered it. A fibrous variety is called nemalite, occurring as fibers or laths that usually elongate along the [1010] direction, though sometimes along [1120].

Notable occurrences include Wood's Chrome Mine at Cedar Hill Quarry in Lancaster County, Pennsylvania, USA. Yellow, white, and blue brucite with a botryoidal habit was found in the Qila Saifullah District of Balochistan, Pakistan, and later in the Bela Ophiolite of Wadh, Khuzdar District, also in Balochistan. Other localities include South Africa, Italy, Russia, and Canada, but the most significant discoveries are from the US, Russia, and Pakistan.

Synthetic brucite is mainly used as a precursor to magnesia (MgO), a refractory material and thermal insulator. It also serves as a flame retardant, as it thermally decomposes to release water, similar to aluminum hydroxide and mixtures of huntite and hydromagnesite. It is an important source of magnesium for industry. While generally considered safe, brucite can be contaminated with naturally occurring asbestos fibers.

In cement and concrete exposed to seawater containing both Mg²⁺ and SO₄²⁻ ions, the precipitation of poorly soluble brucite enhances gypsum formation during sulfate attack. This precipitation drives the chemical equilibrium toward gypsum and ettringite, an expansive phase that causes mechanical stress in hardened cement paste. However, brucite has a small molar volume (24.63 cm³/mol), which may clog the porous network and slow the diffusion of harmful ions, delaying decalcification of the C-S-H phase (the binder in concrete) and its transformation into M-S-H. The exact mechanism of brucite’s role in degradation is debated. If it had a high molar volume, it would be considered a swelling phase like ettringite, but that is not the case. Whether it causes expansion is unclear. If it replaces another phase locally (topochemical replacement) and has a smaller molar volume, no expansion is expected—only reduced porosity. But if many tiny crystals grow between existing ones, even with a small molar volume, they could exert crystallization pressure, leading to tensile stress, expansion, and cracking. Prolonged contact between seawater or magnesium-rich brines and concrete can cause durability issues, especially for submerged structures with steel reinforcements, due to pitting corrosion from chloride ions. Using dolomite as aggregate in concrete with high-alkali cement can also trigger brucite precipitation through the dedolomitization reaction, as seen in alkali-aggregate reactions. For this reason, dolomite is prohibited as an aggregate in concrete.