(Holohedral; Pinacoidal).
Here there is only a centre of symmetry. All the forms are pinacoids,
each consisting of only two parallel faces. The indices of the three
pinacoids parallel to the axial planes are {100}, {010} and {001};
those of pinacoids parallel to only one axis are {hko}, {hol} and
{okl}; and the general form is {hkl}.
[Illustration: FIG. 65.--Crystal of Axinite.]
Several minerals crystallize in this class; for example, the
plagioclastic felspars, microcline, axinite (fig. 65), cyanite,
amblygonite, chalcanthite (CuSO4.5H2O), sassolite (H3BO3); among
artificial substances are potassium bichromate, racemic acid
(C4H6O6.2H2O), dibrom-para-nitrophenol, &c.