On computer-drawn projections of the structure of diopside (CaMgSi?O?) the PBCs are sought. These are <001>, <100>, <010>, <110>, <10[-2]>, <102>, <11[-2]>, <112>, <012> and <011>. Using a broken bond model the attachment energies can be calculated and so the F faces (containing two or more PBCs) can be classified in the order of decreasing importance. In the case of free atoms in the crystallizing phase one obtains : {110}, {010}, {100}, {111}, {[-2]21}, {111}, {021}, {001}, {221} and {011}. The results are identical for the other monoclinic pyroxenes : hedenbergite, augite, jadeite and acmite. Drawings of the theoretical growth form of the crystals are given assuming the attachment energy to be proportional to the growth rate of a face. When the eventual presence of Si?O? groups in the crystallizing phase is considered as an external factor, the growth form, as far as it depends on the crystal structure is the one calculated on the basis of free atoms. Diopside, hedenbergite and augite are bound by {110}, {010}, {[-1]11}, {111} and jadeite and acmite by {110} and {[-1]11}. It is questionable whether the various forms appearing in nature are always due to external factors or whether the hypothesis that EaH is directly proportional to the growth rate is justified.