This is a (very) simple Al-in-Hornblende barometer. According to Johnson & Rutherford (1989), metamorphic pressures can be estimated by means of the aluminum content in hornblendes. For this purpose, the total content of aluminum (AlIV and AlVI) in hornblende, based on a structural formula with 23 oxygen atoms, is used. The formula proposed by Johnson & Rutherford is:
P (±0.5 kbar) = -3.46 (±0.24) + 4.23 (±0.13) ∙ Altot
The calibration was performed experimentally by equilibrating the required phase assemblage over the pressure range 2-8 kbar at 740-780 °C.
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