- Chemistry: Mg3B7O13Cl,
Magnesium Borate Chloride.
- Class: Carbonates
- Subclass: Borates
- Uses: An ore of boron for boric acid and borax (a cleaning agent
and useful industrial chemical) and as mineral specimens.
Boracite is an interesting borate mineral. It has good color, clarity
and hardness to be a gemstone and yet it is rarely cut for this purpose.
Probably because it is slightly soluble in water and ordinary wear dulls
its surface. It is an attractive mineral for mineral specimens though.
Boracite is similar to quartz,
in that it has high temperature and low temperature phases. The high temperature
phase is cubic and forms nice well shaped cubes and octahedrons that are
often modified by other isometric forms. The low temperature orthorhombic
phase is the only phase that is stable at normal surface temperatures.
This means that when the high temperature phase cools down, it converts
to the low temperature phase. The conversion is easily accomplished and
does not alter the outward appearance of the crystal, preserving the isometric
forms. Therefore the specimens of boracite in a collection are actually
the orthorhombic phase in the isometric crystal forms. Mineralogists refer
to this as a pseudomorph, or "false shape".
Boracite is an evaporite mineral and is not surprisingly found with
other evaporite minerals like anhydrite,
gypsum and halite.
Its crystals are often embedded in these other evaporite minerals suggesting
that they formed later than the others, especially since the crystals are
in the high temperature phase.
A variety of boracite is called "strassfurtite" and is a fibrous
form found at Strassfurt, Germany.
Color is white to colorless and with pale tints of yellow, green
Luster is vitreous.
Transparency: Crystals are transparent to translucent.
Crystal System is orthorhombic; 2/m 2/m 2/m , isometric; 4/m
bar 3 2/m at high temperatures.
Crystal Habits include highly modified cubes and octahedrons
which are actually pseudomorphs of the high temperature isometric phase.
Also massive, fibrous, nodular and as embedded grains.
Cleavage is absent.
Fracture is uneven or conchoidal.
Hardness is 7 - 7.5
Specific Gravity is approximately 2.9 - 3.0 (average for translucent
Streak is white.
Other Characteristics: Slightly soluble in water.
Associated Minerals are anhydrite,
and other evaporite minerals.
Notable Occurrences include Yorkshire, England; Strassfurt,
Germany; Bolivia; Chactaw Salt Dome, Louisiana and Otis, California, USA
Best Field Indicators are crystal habit, color, associations,
locality, lack of cleavage and the high hardness.