Crystal morphology of monoclinic potassium feldspars

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Abstract
The influence of the crystal structure on the crystal morphology of monoclinic potassium feldspar has been investigated according to the theory of Hartman and Perdok. There are a limited number of PBCs, i.e., uninterrupted chains of strong bonds, consisting exclusively of T(=Si,Al) –O bonds. These are parallel to [001], 〈1/21/20〉, [101], [100], 〈1/21/21〉, [010] and [102]. Many other PBCs are present having both T – O and K – O bonds as strong bonds, e.g., 〈unk/21/21〉, 〈3/21/21〉, [unk01], [100], 〈3/21/210〉, 〈1/21/22〉, [201] and 〈111〉. The most important crystal faces can now be classified as either F 1 faces parallel to at least two PBCs having only T – O, or as F 2 faces parallel to at least two PBCs, one of which contains in addition K – O bonds. The F 1 faces are {110}, {001}, {010}, {[unk]01} and {[unk]11}. The F 2 faces are {130}, {021}, {[unk]21}, {[unk]12}, {100} and {[unk]01}. The remaining faces parallel to the various PBCs are S faces. Crystals grown in nature display only F faces as the most important faces. The presence of S faces is induced by internal factors other than crystal structure or by external factors such as impurities and supersaturation.

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