Clinohedral Class
(Hemihedral; Domatic).
Crystals of this class are symmetrical only with respect to a single plane. The only form which is here geometrically the same as in the holosymmetric class is the clino-pinacoid {010}. The forms perpendicular to the plane of symmetry are all pedions, consisting of single planes with the indices {100}, {100}, {001}, {001}, {hol}, &c. The remaining forms, {hko}, {okl} and {hkl}, are domes or “gonioids” (γωνία, an angle, and εἶδος, form), consisting of two planes equally inclined to the plane of symmetry.
Examples are potassium tetrathionate (K2S4O6), hydrogen trisodium hypophosphate (HNa3P2O6·9H2O); and amongst minerals, clinohedrite (H2ZnCaSiO4) and scolectite.
5. ANORTHIC SYSTEM
(Triclinic).
In the anorthic (from ἀν, privative, and ὀρθός, right) or triclinic system none of the three crystallographic axes are at right angles, and they are all of unequal lengths. In addition to the parameters a : b : c, it is necessary to know the angles, α, β, and γ, between the axes. In anorthite, for example, these elements are a : b : c = 0.6347 : 1 : 0.5501; α = 93° 13′, β = 115° 55′, γ = 91° 12′.
Holosymmetric Class
(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}.