Question

in which crystal system angle is 90?

Answer 1

Answer:

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Explanation:

Note: Rhombohedral is a sub-category of hexagonal.

Note: 'Group' refers to type of optical properites. First observe:

Is the crystal always dark under crossed polars ?

If yes - isotropic; If no - anisotropic (GROUP 1)

If light passes through the crystal under cross polars, can one (uniaxial) or two (biaxial) orientations be found where the crystal appears dark? (GROUP 2)

If the crystal is uniaxial it belongs to the tetragonal or hexagonal crystal systems, if it is biaxial it belongs to the orthorhombic, monoclinic, or triclinic crystal systems.

Shapes of unit cells are determined by the crystal system, with the following results:

CRYSTAL SYSTEM AXES LENGTHS AXIAL ANGLES

CUBIC All equal All 90

TETRAGONAL a=b not = c all 90

HEXAGONAL two are equal 2 are 90, one 120

ORTHORHOMBIC all unequal all are 90

MONOCLINIC all unequal two are 90

TRICLINIC all unequal none are 90

These are the detailed shapes associated with these crystal systems!

Example showing how optical properties that provide information about the crystal system can help distinguish similar looking gems!:

Blue sapphire - hexagonal - thus uniaxial.

Blue colored cubic zirconia (looks like sapphire) - cubic thus isotropic

Tanzanite (blue like sapphire) - orthorhombic thus biaxial

Answer 2

Answer:

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