DJW Home

mineral photo

   P-T Home

Main Sections

1. Microprobe analyses

2. Mineral groups

3. Solid solutions

4. Thermobarometers

5. Uncertainties

6. P-T calculations

7. Phase diagrams



Docs and downloads

Activity coding



Bugs and quirks

Bulk compositions

Modal proportions

Spreadsheet tools

Practical Aspects of Mineral Thermobarometry

Mode calculations in THERMOCALC

THERMOCALC's modal amounts

All THERMOCALC's calculations are done in moles and molar percentages of the oxide system components. So, the essential pieces of knowledge for decoding their meaning are

THERMOCALC’s "modes" might be more appropriately called oxide-molar proportions, since the "mode" of a particular phase is the proportion of the total number of moles of the oxides under consideration which are combined in that phase. The relationship with the volume proportions is not all that simple:

Converting "modes" to volume proportions

  1. Divide by the number of moles of oxides per formula unit, to get the number of moles of each phase. In difficult cases, such as where not all end members contain the same number of moles of oxides, you will need to consider the calculated composition of the phase, and the divisor will not be a whole number.
  2. Estimate the molar volume of the phase, using the calculated phase composition and interpolating between the molar volumes of the end members if necessary.
  3. Multiply the molar volume by the number of moles of the phase, to get relative volume proportions, and normalise them to 100% for volume percentages.

Note that the volume percentages will not necessarily include all the phases in the rock. For calculations in KFMASH projected from muscovite and quartz, muscovite is included in the modes, but quartz is not. Before comparing with real rocks, the volume percentages will need to be re-normalised to take account of the quartz content of the natural samples, plus the volume of any other phases which are not included in the model system.

^ Top

This page last modified 12 October 2004