and bright tetrahedral faces _s'_ at the antilogous poles.

[Illustration: Fig. 1 Fig. 2 Fig. 3

Crystals of Boracite]

The characters so far enumerated are strictly in accordance with cubic
symmetry, but when a crystal is examined in polarized light, it will be
seen to be doubly refracting, as was first observed by Sir David
Brewster in 1821. Thin sections show twin-lamellae, and a division into
definite areas which are optically biaxial. By cutting sections in
suitable directions, it may be proved that a rhombic dodecahedral
crystal is really built up of twelve orthorhombic pyramids, the apices
of which meet in the centre and the bases coincide with the dodecahedral
faces of the compound (pseudo-cubic) crystal. Crystals of other forms
show other types of internal structure. When the crystals are heated
these optical characters change, and at a temperature of 265 deg. the
crystals suddenly become optically isotropic; on cooling, however, the
complexity of internal structure reappears. Various explanations have
been offered to account for these "optical anomalies" of boracite. Some
observers have attributed them to alteration, others to internal strains
in the crystals, which originally grew as truly cubic at a temperature
above 265 deg. It would, however, appear that there are really two
crystalline modifications of the boracite substance, a cubic
modification stable above 265 deg. and an orthorhombic (or monoclinic)
one stable at a lower temperature. This is strictly analogous to the
case of silver iodide, of which cubic and rhombohedral modifications
exist at different temperatures; but whereas rhombohedral as well as
pseudo-cubic crystals of silver iodide (iodyrite) are known in nature,
only pseudo-cubic crystals of boracite have as yet been met with.

Chemically, boracite is a magnesium borate and chloride with the formula
Mg7Cl2B16O30--A small amount of iron is sometimes present, and an
iron-boracite with half the magnesium replaced by ferrous iron has been
called huyssenite. The mineral is insoluble in water, but soluble in
hydrochloric acid. On exposure it is liable to slow alteration, owing to
the absorption of water by the magnesium chloride: an altered form is
known as parasite.

In addition to embedded crystals, a massive variety, known as
stassfurtite, occurs as nodules in the salt deposits at Stassfurt in
Prussia: that from the carnallite layer is compact, resembling
fine-grained marble, and white or