Nanoscopic characteristics of anhydrite (100) surface growth under mild hydrothermal conditions

Anhydrite (CaSO4) is a major component of evaporitic rocks and is currently forming in submarine hydrothermal fields, where its dissolution and precipitation plays a significant role in controlling porosity. Anhydrite also is a component of undesirable scale on heat transfer surfaces during desalinization and waste water treatments. Knowledge of the molecular processes that control anhydrite growth and dissolution is crucial to understand the geochemical cycling of elements in some natural systems and to develop strategies to limit scale formation on hot surfaces in numerous industrial processes. Several works investigated the nanoscale characteristics of anhydrite growth and dissolution at 25oC [1,2]. However, primary anhydrite mainly forms at temperatures above 60oC. In this work we study the growth of anhydrite (100) surface in contact with supersaturated aqueous solutions (βanhydrite = 1-3.6) under mild hydrothermal conditions (T = 50-115oC, P = 1 atm). The experiments were performed using a hydrothermal atomic force microscope (HAFM) [3]. Our observations showed that: (i) anhydrite (100) surface grows by lateral spreading of monomolecular layers (3.5 Å in height), which originate at cleavage steps, (ii) the growth is highly anisotropic, with very significant differences in advance speed along different directions (S[001] (or S[00-1]) >> S[011] > S[010]), (iii) growth anisotropy is especially evident for <001>, with [001] and [00-1] alternating as fast and slow directions in successive monolayers, (iv) these growth characteristics determine that fast-moving steps combine with slow-moving steps to form bilayer steps around screw dislocations, (v) a reduced number of two-dimensional nuclei (≤ 1nuclei/μm2) forms at T ≥ 80oC and βanhydrite ≥ 2, (vi) two-dimensional nuclei also are monomolecular and their pseudo-triangular shape also reflects the growth anisotropy. Most of anhydrite (100) surface growth characteristics at mild hydrothermal conditions coincide with those reported for its growth at 25oC [2], which evidences that the strong structural control observed at room temperature extends into higher temperature conditions. The marked growth anisotropy results from the non-equivalent geometry of [010] and [0-10] steps in the monolayer. The existence of two-fold screw axes along [100] explains the alternation of [001] and [00-1] as fast-moving and slow-moving directions in successive monolayers. Highly anisotropic growth at the nanoscale seems to be a feature common to crystal surfaces perpendicular to two-fold screw axes. Acknowledgements: We thank Guntram Jordan for kindly providing access to the Hydrothermal Atomic Force Microscope and training J.M. in using it. We thank Spanish Ministerio de Ciencia e Innovación (CGL2007-65523-C02-01 and CGL2010-20134-C02-01) for financial support.