Earth Materials. John O'Brien. Читать онлайн. Newlib. NEWLIB.NET

Автор: John O'Brien
Издательство: John Wiley & Sons Limited
Серия:
Жанр произведения: География
Год издания: 0
isbn: 9781119512219
Скачать книгу
the crystal structure of the mineral. That is, they depend on the basic motif and the symmetry operations that produce the three‐dimensional crystal lattice. The nature of the crystal forms and cleavage surfaces depends on the crystal system and crystal class in which the mineral crystallized.

      4.5.1 Crystallographic axes

Schematic illustration of conventional labeling of crystallographic axes, illustrating the positive and negative ends of the three crystallographic axes and the angles between the axes for crystals in the orthorhombic system.
Crystal system Verbal description Symbolic description
Isometric (cubic) Three mutually perpendicular axes (a1, a2, a3) of equal length that intersect at right angles (a1 = a2 = a3) (α = β = γ = 90°)
Tetragonal Three mutually perpendicular axes; axes (a1, a2) are of equal length; the c axis may be longer or shorter (a1 = a2 ≠ c) (α = β = γ = 90°)
Orthorhombic Three mutually perpendicular axes of different lengths (a, b, c); two axial length ratios have been used to identify the axes: c > b > a (older) or b > a > c (newer) (a ≠ b ≠ c) (α = β = γ = 90°)
Monoclinic Three unequal axes lengths (a, b, c) only two of which are perpendicular. The angle (β) between a and c is not 90°. The a‐axis is inclined toward the observer. The b‐axis is horizontal and the c‐axis is vertical (a ≠ b ≠ c) (α = γ = 90o; β ≠ 90°)
Triclinic Three unequal axes, none of which are generally perpendicular. The c axis is vertical and parallel to the prominent zone of crystal faces (a ≠ b ≠ c) (α ≠ β ≠ γ ≠ 90°)
Hexagonal Four crystallographic axes; three equal horizontal axes (a1, a2, a3) intersecting at 120°. One longer or shorter axis (c) perpendicular to the other three. a1 oriented to front left of observer; a2 to right; a3 to back left; c vertical. Sixfold axis of rotation or rotoinversion (a1 = a2 = a3 ≠ c) (α = β = 90°; γ = 120°)
Trigonal (or rhombohedral) Axes and angles are similar to the hexagonal system; crystal symmetry is different with the c‐axis a threefold axis of rotation or rotoinversion (a1 = a2 = a3 ≠ c) (α = 120°; β = γ = 90°)

      4.5.2 Crystal forms

      Each of the crystal systems has an associated set of common (and rarer) crystal forms. Crystal forms consist of a three‐dimensional set of one or more crystal faces that possess similar spatial relationships to the crystallographic axes. Some natural crystals possess only one crystal form; others possess multiple or combined crystal forms. Crystal forms can be subdivided into two major groups: closed forms and open forms.

Schematic illustration of a pyritohedron, a closed form in which all faces have the same general relationship to the crystallographic axes.