Hydrothermal growth of OH-phlogopite single crystals II. Role of Cr and Ti adsorption on crystal growth rates

OH-phlogopite mica overgrowths on natural seeds have been obtained at P_sol=1 kbar and 522–535°C growth temperatures by means of the thermal gradient method of hydrothermal growth. Solutions used were KOH (6.5 m)-H₂O with Cr or Ti introduced as additives. The growth kinetics of the lateral single faces of the mica have been measured as a function of the nominal relative supersaturation applied and then compared to similar measurements formerly made from Cr- and Ti-free hydrothermal solutions. It is shown that Cr stimulates a sharp transition of growth mechanisms from spiral growth to two-dimensional nucleation or one-dimensional roughening for all lateral faces as the supersaturation increases. On the other hand, Ti causes the latter two possible growth mechanisms to predominate over the entire investigated supersaturation range (3.4–10.2%). Cr as well as Ti additives decrease the growth kinetics of all lateral faces. This is indicative of a growth retardation effect due to adsorption. Furthermore, Cr and Ti act as growth habit modifiers, i.e. they adsorb differently onto the two groups of lateral forms: {11l}, {02l} and {13l}, {20l}. Ti and Cr stabilize both a pseudohexagonal habit, bounded by {11l} + {02l} for Ti, but by {13l} + {20l} for Cr. Chemical sector-zoning takes place for Cr and Ti among lateral forms. Both incorporate more in the {13l} and {20l} sectors than they do in the {11l} and {02l} ones, with a much stronger concentration contrast for Ti. The incorporation of both transition elements appears as poorly sensitive to the growth kinetics of the faces but more likely controlled by crystal-solution interface phenomena.