1. which one of the following defines a hexagonal dipyramid? (a) a vertical six fold axis of symmetry (b) a horizontal mirror plane (c) six vertical mirror plan

1. Which one of the following defines a hexagonal dipyramid? (A) A vertical six fold axis of symmetry (B) A horizontal mirror plane (C) Six vertical mirror planes at an angle of 30° with each other (D) A mirror plane that is perpendicular to the vertical six fold axis of symmetry 2. If (326) is the Miller Index of a crystal face. then the value ofx in the corresponding Weiss Parameter of the same face, xa: yb: zc is 3. The retardation of a uniaxial negative mineral of thickness 0.03 mm is 5160 nm in its principal section of indicatrix. If the refractive index corresponding to the E-ray is 1.486. the value of the refractive index (correct to three decimal places) of the O-ray is _-- 4. If a crystal contains 5 faces and 8 edges., the number of vertices in the crystal is_