Brucite

Brucite is a specific mineral form of magnesium hydroxide. Its chemical formula is Mg(OH)2. This mineral is important because it serves as a major source of magnesium for many industrial uses. It also plays a complex role in how building materials like cement and concrete react to their environment. Brucite can form through several different geological processes. It is a common alteration product of periclase found in marble. It also appears as a low-temperature hydrothermal vein mineral in metamorphosed limestones and chlorite schists. Additionally, it forms during the serpentinization of dunites.

At a microscopic level, brucite has a very organized structure. It adopts a layered CdI2-like structure. These distinct layers are held together by hydrogen-bonds. The mineral can grow in different shapes depending on its environment. It often appears in fibers or laths. These crystals are usually elongated along the [1010] direction. Sometimes they grow along the [1120] crystalline directions. A specific fibrous variety of brucite is known as nemalite.

Scientists have been documenting brucite for nearly two centuries. The mineral was first described in 1824 by François Sulpice Beudant. He named the mineral after Archibald Bruce. Bruce was an American mineralogist who lived from 1777 to 1818. This historical naming connects the mineral to the early era of mineralogical study. Today, we understand its chemical properties much more deeply than early researchers could.

Brucite can be found in many notable locations around the world. In the United States, a significant site is Wood's Chrome Mine. This is located at the Cedar Hill Quarry in Lancaster County, Pennsylvania. In Pakistan, researchers have found colorful varieties of the mineral. In the Qila Saifullah District of Baluchistan Province, brucite can appear in yellow, white, and blue colors. It also has a botryoidal habit there. Another discovery occurred in the Bela Ophiolite of Wadh, in the Khuzdar District of Pakistan. Other important examples come from Russia, Italy, Canada, and South Africa.

Industry relies on brucite for several essential products. Synthetic brucite is mainly consumed as a precursor to magnesia (MgO). Magnesia is a useful refractory material and a thermal insulator. Brucite is also a significant source of magnesium for various industrial processes. It can even function as a flame retardant. This happens because it thermally decomposes to release water. This process is similar to how aluminium hydroxide and mixtures of huntite and hydromagnesite work. However, users must be careful because brucite can be contaminated with naturally occurring asbestos fibers.

One of the most complex roles of brucite is its effect on cement and concrete. When concrete is exposed to seawater, magnesium ions are present. This can lead to the precipitation of brucite, which is poorly soluble. This precipitation can drive the chemical equilibrium of a reaction to the right. This process enhances the formation of gypsum during a sulfate attack. This can lead to the formation of ettringite, which is an expansive phase. The expansion of ettringite creates mechanical stress in the hardened cement paste.

There is an ongoing scientific debate regarding how brucite affects the durability of concrete. Brucite has a small molar volume. Because of this, it might actually help clog the porous network in the hardened cement paste. This clogging could hinder the diffusion of harmful reactive species. This might delay the decalcification of the C-S-H phase, which is the "glue" phase responsible for cohesion. However, if tiny crystals grow between existing ones, they could exert crystallization pressure. This pressure could result in tensile stress, expansion, and cracking. Because of these risks, using dolomite as an aggregate in certain concrete is prohibited to prevent brucite precipitation.