Читать книгу Earth Materials онлайн

130 страница из 155

crystallographic axesabc

standard set of orientation rules12322

1

ssss1 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.

ssss1 The relationships of crystallographic axes and the rules for orienting crystals in each of the crystal systems. Note that the trigonal system (division) is listed independently of the hexagonal system (division) in this table.

Crystal system Verbal description Symbolic description Isometric (cubic) 123123 Tetragonal 1212 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 = 90^o; 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 123123123 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 123