The matrix

The matrix of carbonate rocks consists of either fine grained carbonate mud, called micrite. Or coarser grained calcite crystals formed during diagenesis, called sparite.

Micrite is "lime mud", the dense, dull-looking sediment made of clay sized crystals of CaCO3. Much micrite today forms from the breakdown of calcareous algae skeletons. It is not clear if all ancient micrites formed in the same way. The micrite results from recrystallization of carbonate mud during diagenesis or from direct precipitation of calcite, and causes lithification of the sediment. The micrite gives the dull opaque appearance of most limestones as seen in hand specimen. If the rock consists entirely of fine-grained mud matrix, it implies deposition in a low energy environment just like in siliclastic mudstones. Some of the mud may start out as aragonite needles 5 to 10 mm in length produced by calcareous algae. But, again this becomes recrystallized to a microspar 5 to 15 mm in diameter during diagenesis.


Different microorganisms, which produce mud


• Deer, W.A., Howie, R.A., Zussman, J. (1998) Rock-forming Minerals.
• Optical Mineralogy : The Nonopaque Minerals by Phillips / Griffen
• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc
• M. J. Hibbard (1994): Petrography to Petrogenesis. Prentice Hall editore