Discontinuous Solid Solution in Ca-Rich Dolomites: The Evidence and Implications for the Interpretation of Dolomite Petrographic and Geochemical Data

  1. Bruce Purser,
  2. Maurice Tucker and
  3. Donald Zenger
  1. A. Searl

Published Online: 14 APR 2009

DOI: 10.1002/9781444304077.ch20

Dolomites: A Volume in Honour of Dolomieu

Dolomites: A Volume in Honour of Dolomieu

How to Cite

Searl, A. (1994) Discontinuous Solid Solution in Ca-Rich Dolomites: The Evidence and Implications for the Interpretation of Dolomite Petrographic and Geochemical Data, in Dolomites: A Volume in Honour of Dolomieu (eds B. Purser, M. Tucker and D. Zenger), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304077.ch20

Author Information

  1. Department of Earth Sciences, University of Birmingham, Edgbaston, Birmingham, B15 5TT, UK

Publication History

  1. Published Online: 14 APR 2009
  2. Published Print: 25 MAY 1994

ISBN Information

Print ISBN: 9780632037872

Online ISBN: 9781444304077

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Keywords:

  • discontinuous solid solution in Ca-rich dolomites - for interpretation of dolomite petrographic and geochemical data;
  • wavelength dispersive (WDS) electron probe data;
  • dolomite solid solution limitations;
  • model evaluation in light of published mineralogical data;
  • model predictions with data sets;
  • discontinuous solid solution in dolomites - relevance to studies of dolomitization;
  • interpretation of patterns of compositional zonation in dolomite

Summary

The collection of about 2000 electron microprobe spot analyses of mole % CaCO3 in dolomite from several different dolomite occurrences indicates that sets of compositional data are typically polymodal. Similar preferred levels of CaCO3 uptake into dolomite occur as modes in different data sets, and thus may reflect some underlying lattice constraint on dolomite compositions. The rarity of continuous compositional gradients as opposed to sharply bounded growth zones within dolomite crystals also suggests that solid solution within dolomite minerals is discontinuous. This non-ideal solid solution behaviour may be related to the size difference between Ca and Mg ions and the lattice strain associated with substitution of a large Ca ion into a smaller Mg site. It is suggested that there may be an increase in lattice stability associated with evenly spaced as opposed to random substitution of Ca ions into Mg lattice layers. The observed modes in the data sets generally correspond to the preferred levels of Ca uptake predicted by this model. The spread of analyses about predicted modes may be due partly to the inefficiency of this kind of ordering at low temperatures, such that the size of individual ordered domains is less than the width of the probe beam. An underlying bimodal distribution of mole % CaCO3 in dolomite may reflect a significant increase in dolomite stability due to intralayer cation ordering at intermediate levels of Ca uptake into Mg layers. At lower levels of Ca substitution into Mg layers, Ca ions are sufficiently dispersed for there to be little energetic advantage attached to ordering. At higher levels of Ca uptake, the overall distortion associated with the substitution of Ca ions into Mg sites vastly exceeds the energetic effects of ordering. A smaller probe data set for calcite suggests that analogous patterns of cation substitution are also favoured in calcite. Some of the microstructures observed in transmission electron microscopy of carbonates may be due to the presence of domains of differing intralayer cation ordering. Their size might reflect either crystal growth rates or degree of recrystallization during burial. More significantly, if there are gaps within the range of dolomite solid solution, the use of Ca/Mg ratios as an index of crystal growth rate or as a measure of Ca/Mg ratios in the precipitating solution, and the interpretation of compositional growth zonation within dolomite, will have to be re-evaluated.