Fibre growth in wet salt aggregates in a temperature gradient field

 

Bas den Brok, Cees Passchier, Michel Sieber
Intitut für Geowissenschaften, Becherweg 21, D-55099 Mainz, Germany (denbrok@mail.uni-mainz.de)

Abstract of manuscript published in the Mineralogical Magazine Vol. 62 (4), pp. 527-532

 

Abstract

Intense fibrosity develops in wet porous NaCl crystal aggregates (grain size 250-500 Ám) held in a temperature (T) gradient field (0.5-4 °C/mm) at temperatures between 20 and 50-60°C. In-situ microscopic observation of the process shows, that fibre growth is associated with T-gradient driven motion of tiny gas (air, water vapour) bubbles present in the saturated intercrystalline aqueous NaCl solution. Gas bubbles move trough the intercrystalline pore fluid into the cold direction. They only move if they are right next to an NaCl crystal; bubbles that are "free" do not move. Each bubble is "pushed" into the cold direction by a growing crystal fibre of the same diameter as the bubble itself. Fibres apparently grow due to oversaturation of the NaCl solution at the hot side of the gas bubble. Crystals dissolve at the cold side of the gas bubbles, apparently by undersaturation of the NaCl solution there. Thus, bubbles dissolve their way through NaCl-crystals and aggregates. Intense fibrosity develops within weeks.