During the growth process crystals take a variety of habits. The question why and how do crystals form such a wide variety of habits was posted in the seventeenth century [1] and is still of vital importance. The question is important not only for modern technology but also to understand the formation of minerals and crystals in living organisms. For crystals whose growth cannot be observed in situ, e.g. rock-crystals, crystals formed in living bodies or crystals synthesized by high temperature solution or hydrothermal solution methods the crystal morphology is a very important source of information [2]. The morphologies that crystals take result from the internal and external factors. To external factors we may include, for example, supersaturation or temperature, while to internal, for example specific symmetry of crystal.

In this paper we focus on the correlation between the crystal geometry, the evolution of individual surfaces, their morphological importance and stability. The presented analytical view pays attention to the big role, not appreciated enough so far, of crystal geometry in the formation of the final crystal morphology. In particular we show that for specific geometry characterized by interfacial angles the given face can increase in size for a very wide range of relative growth rates and need not be the slow-growing face [3]. Even growing faster than the neighbouring faces, such a face increases its size. On the other hand, there are other faces with other specific geometry which can decrease the size growing more slowly than one of the neighbouring faces. If the growth time is long enough, such a face may disappear and not be represented in the final crystal morphology. We show also that the seed-faces may influence crystal morphology modifying the evolution of directly neighbouring faces. The scale of such an influence depends, among other factors, on the geometry of a given crystal. This influence of seed-faces is the most significant for short growth time, however, seed-faces may also modify crystal morphology after a very long growth time. All these theoretical predictions are verified by computer experiments performed for various faces of different crystals or by real experiments performed for potassium dichromate (KBC) crystals grown by the method of decreasing temperature.

The presented geometric point of view in connection with the standard crystal growth models, as Burton-Cabrera-Frank (BCF) model [4], may give more comprehensive understanding of the crystal morphology formation and of interconnection between the external and internal factors, which determined the morphology of crystals.

1. N. Steno, De solido intra solidum naturaliter contento dissertationis prodromus, Florence (1669); English translation by J. G. Winter: The Prodromus of Nicolaus Steno’s Dissertation Concerning a Solid Body Enclosed by Process of Nature Within a Solid, Hofner, New York, (1968).
2. I. Sunagawa, Crystals, growth, morphology and perfection, Cambridge University Press, Cambridge 2005
3. J. Prywer, Prog. Cryst. Growth Charact. 50 (2005) 1.
4. W.K. Burton, N. Cabrera, F. Frank, C. Philos. Trans. Roy. Soc. Lond. Ser. A 244 (1951) 299.

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