Microcrystalline structure of milled dolomite and its constituent carbonates

Abstract

The prolonged mechanical treatment of a small quantity of dolomite in a stainless-steel ball mill was insufficiently severe to cause disintegration into the component carbonate phases. Instead the dolomite was structurally degraded with the development of severe apparent lattice strain of about 4%, a unit cell dilated by about 0.6%, and locally misoriented carbonate ions. Contamination of the dolomite by wear fragments was slight, being at the rate of 10 ppm of Fe per hour. Similarly ground calcite and magnesite, in a 1:1 mole ratio, underwent mechanochemical transformation to dolomite. Magnesite was gradually incorporated into microregions of the calcite at a rate that was first order with respect to the unconverted calcite. The apparent lattice strain in the mixed carbonate varied in a cyclic manner; the lattice strain normal to (104) planes was a maximum of about 8% when the magnesite content of the mixed carbonate was about 30 mole%. The d-spacing was then 2.973 Å, which is about halfway between that of unstrained calcite, 3.035 Å, and dolomite, 2.904 Å. Eventually all the calcite that had not transformed to aragonite was converted into dolomite. The microstructure then closely resembled that of dolomite which had been milled for a similar time. Ground calcite is soft enough to give nonporous aggregates of welded crystals. The crystals of the harder dolomite, and magnesite, fracture to produce less strongly welded aggregates with pores spanning the micro, meso, and macro size ranges. In doing so, the specific surfaces of the latter each pass through a maximum between 11 and 12 m 2/g after 20 hr of grinding.