Heterogeneous microstructures in oolitic carbonates

Several bioclastic and oolitic carbonates have been studied with the transmission electron microscope. Aragonite has been distinguished from calcite on the basis of the SAD patterns and the morphology. The aragonite occurred either as microscopic crystals varying in length from 0.3 to I mm, or as larger masses twinned on {l l0}. It has been recognized in Pleistocene to Recent samples, but not in any of the Mesozoic or Paleozoic ages. The study of the calcite in the Jurassic Twin Creek Oolite (Wisconsin) and other lime-stones with the TEM has revealed the presence of a modulated fnicrostructure iir low Mg cal-cite. The microstructure has two basic morphologies: one is a fiae modulation with a wave-length of around l50A; the other is coarser, with a wavelength of approximately 500A. The modulations are not always distinctly planar, and occasionally they are superimposed upon each other. Both the coarse and fne modulations are found to be parallel to {l0l4i: r. The microstructure is similar in appearance and orientation to a structure found by Reeder (1980) in ancient calcian dolomites. No apparent chemical diflerence was found when comparing calcite crystals that have the microstructure to those that lack it. The modulated microstructure was found to be rotated by e twins formed during deforma-tion and is therefore older. It is suggested that the modulated structure is the result of an or-dering phase transformation from a calcium carbonate in which the COi groups are dis-ordered, resulting in the formation of planar faults. Alternatively, defects and partial disorder developed during crystal growth. The substitution of cations such as magnesium, calcium, iron, or manganese would increase the likelihood of the formation of faults. This may explain why a similar microstructure is so pervasive in calcian dolomites. Disorder appears to occur in secondary calcite crystals, i.e., those which formed by dissolution and reprecipitation from another carbonate