BRUCITE
Brucite – Mg(OH)2 – is a relatively rare hydroxide. It is an important industrial mineral, being a source of the metal magnesium and of magnesia, in addition to applications in refractories, plastics and many other uses.
It is classified in the Brucita Group. Very rarely it forms large crystals, but the literature reports cases of crystals up to 19 cm. The foliate, granular, fibrous and plaque masses reach 50 cm in length. Macroscopically, it closely resembles talc and chlorite and is sold in the form of a white powder.
Brucite constitutes the “brucite layers”, which are found in the clay mineral structures of the Chlorite Group and the Montmorillonite/Smectite Group.
There is a relatively rare fibrous variety (“nemalite”) and another one with Mn. May contain Zn, Mn and Fe. It is pyroelectric and can form epitaxies with pyroaurite.
1. Characteristics
Crystal system: Trigonal.
Color: White, greenish, blue, gray, honey yellow to red-brown and deep brown (when with Mn).
Habit: Tabular, fibrous, massive, foliate masses and rosettes.
Cleavage: {0001} perfect.
Tenacity: Sectile. Plates are flexible. Fibers are plastic.
Twinning: No.
Fracture: Irregular, fibrous, micaceous.
Mohs Hardness: 2.5 – 3
Parting: No.
Streak: White.
Lustre: Greasy, pearly.
Diaphaneity: Transparent.
Density (g/cm³): 2.39
2. Geology and Deposits
Brucite occurs as a product of alteration of periclase (MgO) in marbles formed by metamorphism on dolomitic limestones, in brucite-marbles (pencatite, predazzite). Also in magnesian scorns.
Furthermore, it occurs in low-temperature hydrothermal veins, in metamorphic limestones (marbles) and in chlorite-schists. Also in serpentinites, by serpentinization of dunites.
In concrete and cement with excess Mg, brucite neoformation occurs, causing problems.
3. Mineral Associations
In serpentinized ultramafic rocks it occurs with magnetite, chromite, nepheline, chlorite, serpentine, talc and chrysotile.
In dolomitic marbles and magnesian skarns, it is associated with carbonates (calcite, aragonite, dolomite, magnesite, hydromagnesite), wollastonite, forsterite, spinel, minerals from the Humite Group and periclase.
It also occurs with rare minerals such as sturmanite, ettringite, pyroaurite, jouravskite and artinite.
Also with quartz, manganite and anhydrite.
4. Transmitted Light Microscopy
Refraction indices: nω: 1.560 – 1.590 nε = 1.580 – 1.600
The indices increase with the increase of Fe and Mn contents.
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless. Never shows pleochroism.
Relief: Moderate.
Cleavage: Perfect on {0001}.
Habits: Usually forms foliate or spiral masses. It can be tabular, micaceous, massive or fibrous. Fibrous brucite occurs around periclase grains, marking the beginning of periclase change.
In marbles it forms pseudomorphs (cubic, octahedral) on periclase.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Birefringence between 0.010 and 0.020: colors up to the end of the 1st order: grey, yellow, orange, red, purple and deep blue.
When in fibers, it presents gray colors like serpentine.
It can present anomalous colors of up to 2nd order, can be red-brown or blue of 1st order.
Extinction: Parallel and mottled.
Elongation sign: ES(-). Important, because micas, talc and pyrophyllite have positive elongation sign!
Twins: No.
Zoning: No.
CONVERGENT LIGHT
Character: U(+), may be biaxial. Due to the tiny size of the grains, the figure is difficult to obtain.
2V angle: there may be, anomalous, small.
Alterations: is usually a product of alteration.
Magnesite can be partially altered to brucite.
Olivine, when it undergoes serpentinization, changes to serpentine, brucite, talc and carbonates.
Brucite, on the other hand, easily alters to hydromagnesite and sometimes serpentine, periclase, and other Mg-containing minerals.
May be confused with: other micas, so it is important to consider the paragenesis of the rock.
Talc, muscovite and pyrophyllite are very similar, but their interference colors are more intense, they are biaxial and have ES(+). This last feature is very important for differentiation.
Gypsum has lower interference colors, is biaxial, and has low relief.
Chlorite usually presents in PPL slightly greenish colors.
Serpentine is almost indistinguishable from brucite of fibrous habit and low interference colors (grays), but serpentine (=antigorite) has low relief.
5. Reflected Light Microscopy
Reflected light microscopy is not the recommended analytical method for the identification of brucite. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with brucite.
Sample preparation: the polishing of brucite is not of good quality, as with other phyllosilicates. Thus, brucite also stands out for its worse polishing than that of the accompanying minerals.
PLANE POLARIZED LIGHT – PPL
Reflection color: Dark gray.
Pleochroism: No.
Reflectivity: Very low (<<10%)
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Anisotropy was not observed.
Internal reflections: Widespread, clear, colorless, milky.
May be confused with: many other transparent minerals of light colors. It is not possible to identify brucite with reflected light. On the other hand, knowing of its existence in the rock, the mineral is easy to identify.
