The Topography of Crystal Faces. II. The Topography of Cleavage Faces of Mica and Selenite

摘要: The multiple beam interference procedure described in part I is applied to the examination of topography cleavage surfaces mica and selenite. precision attained extremely high, particularly case mica. Mica. All specimens examined exhibit considerable hills dales which may extend up heights depths 1/200th mm. extreme cases. Sharp 'cleavage lines' are revealed, representing discontinuities level (steps). These vary length from quite small values several millimetres. smallest step measured 41 A, only 2 'molecules'. Evidence given showing that steps integral multiples 'molecular' thickness, 20 A. Steps varying some 577 molecules recorded, majority being under 'molecules' height. error evaluation step, best cases 3 A units. Up down occur a random way. value remains constant along line', proving separated by this line 'parallel'. area surface between pair lines highly uniform at least true 30 probably molecular plane. observations reveal objectively 'invisible steps' found on growing crystals it. Selenite. selenite reveals large number long mostly roughly parallel, although inclined angles 15 degrees. characteristic shown absent, but specimen investigated exhibited cylindrical curvature, curved same direction as lines'. observed these 16 146 faces adjacent sides (in contrast with mica) usually each other, slope line. minute, $0\cdot 011$ 063$ min. arc fringes very ragged nowhere smooth High dispersion shows due existence secondary structure consisting elongated facets. dimension (at ten times width most cases) lines. facets order 1/50th wide way few