Description:
Brucite is a mineral that is not often used as a mineral
specimen but does have some important industrial uses.
It is a minor ore of magnesium metal and a source of
magnesia. It is also used as an additive in certain
refractories.
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". |
