Conditions of Formation of Calcite Crystals of Tabular and Acute Rhombohedral Habits

Calcite crystals are grown under pure conditions by the method of counterdiffusion of calcium and carbonate ions in water. An extension of the diffusion method is used making possible the crystallization of calcite in a medium containing calcium or carbonate ions in excess of their stoichiometric amounts in calcite. Calcite crystals grown in the presence of an excess of Ca2+ ions exhibit an elongated habit determined by the prevailing development of some of the acute negative rhombohedra φ. {0221}, θ. {0441} or π. {0881}, whereas with an excess of CO ions the habit varies from thick ‐to fine‐tabular due to the considerable development of the pinacoid (c {0001}). The habit of crystals grown in a medium in which both ions are approximately in stoichiometric amounts or in which the excess of one of them frequently alternates with an excess of the other, is determined by the supersaturation and varies from basic rhombohedron to acute rhombohedron m. {4041}. The results are discussed in the light of some of the existing theoretical ideas on the influence of admixtures on habit. It is shown that the order of morphological importance which can be derived on the basis of various extensions of Bravais' law agrees neither with the experimental results nor with the observations in nature. The particular habits observed in experiments, tabular and acute rhombohedral, are exactly those which Kalb had included in his morphogenetic order of calcite crystals as first and fifth habit types. Since they are the result of specific conditions in the medium they ought to deviate from the general order. The remaining three habit types, therefore, will determine the habit variations of calcite due to the decrease of supersaturation.