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3- Solid angles
It is the angles resulting from meeting more than two crystal faces.
The previous three parts determine the body of the crystal and its
external shape.
4- Interface Angles:
The angles between any two faces are known as the angle between their
faces. This angle is estimated by the value of the angle between the two
perpendiculars lines on faces or by the value of the angle supplementary
to the angle between the two faces. It helps distinguish different minerals
from each other because it constant.
