Etch pits on basal cleavage faces of apophyllite crystals

Abstract

Basal cleavage faces of apophyllite crystals were etched in the base etchant (dil HF) containing different concentrations of ammonium fluoride as a poisoning salt. At low concentrations, square pyramidal pits formed with their sides parallel to the < 100> direction. At higher concentrations, pits with their sides parallel to the < 1 1 0 > direction were formed. The lateral growth of a pit is dependent on pH value of the etching solution. Activation energy for dissolution at a pit (active site) was found to decrease with increasing concentration of poison in the base etchant. The change in orientation of the pit is attributed to adsorption of aqueous complexes having different chemical constitutions which have different directional adsorption sites in apophyllite.

Kinematics of apophyllite leaching—A terrace–ledge–kink process within phyllosilicate interlayers

“…However, as has been suggested by Kostov [22] and confirmed by Akizuki and Konno [23], the principal habit forms of apophyllite are {1 1 0} and {1 0 1}. Therefore, the results of Joshi and Ittyachen [19,20] and Pande and Vadrabade [21] have to be reconsidered taking into account the data reported by Kostov [22] and Akizuki and Konno [23]. By doing this the orientation of etch pits reported by Pande and Vadrabade [21] are consistent with the present results.…”

“…An octagonal shape of the pits at medium concentration has been attributed to roughly equal reactivity of both {1 0 0} and {1 1 1} faces at these conditions. Pande and Vadrabade [21] studied HF etching of the basal surface of apophyllite using different concentrations of ammonium fluoride as inhibitor and obtained similar results. They attributed the observed changes to different adsorption sites of aqueous complexes on apophyllite.…”

Ammonium-Bearing Fluorapophyllite-(K) in the Magnesian Skarns from Aleului Valley, Pietroasa, Romania

“…In the definition of crystallographic forms, Ref. [10] used the crystallographic orientation conducted by [34], describing a combination of basal pinacoids {001}, prisms {100}, and low pyramids {111}. As shown by [35,36], this orientation must be reconsidered, but the combination of faces remains the same.…”