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unit faceunit plane

ssss1 Unit face (outlined in solid blue) in an orthorhombic crystal with three unequal unit cell edges and crystallographic axes that intersect at right angles. All parallel faces (e.g., outlined in dotted red) will have the same general relationship to the crystallographic axes and the same atomic content and properties.

Weiss parameters123

general face

Mineral planes may be parallel to one or two crystallographic axes. How do we determine the Weiss parameters for such faces and planes? The Weiss parameters of any face or plane that is parallel to a crystallographic axis are infinity (∞) because the plane never intersects the axis in question. If a set of planes is parallel to two crystallographic axes and intersects the third, it is assumed to intersect that axis at unity. Planes that are parallel to the a‐ and b‐axes and intersect the c‐axis have the Weiss parameters (∞ : ∞ : 1). Planes parallel to the b‐ and c‐axes that intersect the a‐axis have the Weiss parameters (1 : ∞ : ∞). Planes that cut the b‐axis and are parallel to the a‐ and c‐axes (ssss1a) have the Weiss parameters (∞ : 1 : ∞). Each set of planes, with its unique relationship to the crystallographic axes possesses its own unique Weiss parameters. If a set of planes intersects two axes and is parallel to the third, only one of the Weiss parameters will be infinity. The other two will be one if, and only if, the two axes are intersected at lengths corresponding to their axial ratios. Therefore, the Weiss parameters (1 : ∞ : 1) represent planes that parallel the b‐axis and intersect the a‐and c‐axes at unit lengths. Similarly the Weiss parameters (1 : 1 : ∞) are those of planes that cut the a‐ and b‐axes at unit lengths and are parallel to the c‐axis (ssss1b).