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 <100> 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.