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The Aluminium-in-Hornblende barometer of Johnson & Rutherford (1989)


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.


Johnson, M.C. & Rutherford, M.J., 1989: Experimental calibration of the aluminum-in-hornblende geobarometer with application to Long Valley caldera (California) volcanic rocks. – Geology (1989) 17 (9): 837–841.
See this publication on GeoScienceWorld (GSW), a nonprofit collaborative and comprehensive resource for research and communications in the Earth Sciences.

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The barometer

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Data set 1 – designations of the measurements (light blue) and Al content (light yellow):

Data set 2 – designations of the measurements (light blue) and Al content (light yellow):

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Data set 1

Designation Al-content Pressure [kbar]

Data set 2

Designation Al-content Pressure [kbar]

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Data set 1: Dots and circles
Data set 2: Filled and open squares
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Data set 1

Mean DS1 Mean value for data set 1

Data set 2

◼︎ ◼︎ ◼︎ ◼︎ ◼︎ ◼︎ ◼︎ ◼︎ ◼︎

Mean DS2 Mean value for data set 2

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