Monoclinic pinacoid set with biaxial indicatrix3D Model
This model of a monoclinic pinacoid set (in purple) demonstrates the relationship between the optical indicatrix (in yellow) and vibration directions (α, β & γ) to the crystallographic axes (x, y & z - all in purple). In monoclinic the β vibration direction (in green) coincides with the y crystallographic axis, while the other two vibration directions do not coincide with crystal axes (since the x & z axes are not orthogonal). In this instance the x-axis is at ~115° to z. The γ vibration direction (in red) is at an angle (γ-z angle) of 22° to the z-axis (similar to monoclinic amphiboles), while the α vibration direction (in blue) is close to the the x-axis. The two optic axes (in yellow) in this case form an acute (2V) angle (60° in this case) about the α vibration direction, consistent with the optically negative character of most monoclinic amphiboles.
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