Role of Salts in Development of Granitic Tafoni, South Australia

Fresh Proterozoic and Paleozoic granitic rocks in South Australia possess approximately 0.06% soluble salts (chiefly Ca, Na, S, and Cl), possibly in fluid inclusions. Active tafoni developed in these rocks show small-scale flaking on their ceiling and walls. Fresh and flaking granite are similar in all respects (chemistry, mineralogy, degree of alteration, texture, and structures), except that soluble salts are 2-13x more abundant in the flakes, where they occur as gypsum and halite crystals. An external source for these salts is ruled out by their chemical character. They are most likely a concentration of soluble substances present in the fresh rock and leached to a near-surface location when available moisture is evaporated. Tafoni configuration, distribution, and mineralogy argue against either thermal changes or wetting-and-drying being major contributors to the flaking process. We believe that salts cause flaking by crystal growth. Residual strain energy in the granites could help explain how only 0.4% salts can create 10-20% fracture porosity in an otherwise fresh rock.