The historic development of the first geobarometer for spinel lherzolite based on the distribution of calcium between existing olivine and clinopyroxene is reported here.
Kohler and Brey(1990) suggested the geobarometer as follows:
P(10⁸Pa)=(−TlnD_Ca−11982+361T)/56.2 for T≥ (1275.25+2.827P)  (K),
P(10⁸Pa)=( −TlnD_Ca−5792−1.25T)/42.5 for T≤(1275.25+2.827P)  (K).
Based on their calibrated experiments for calcium exchange between olivine and clinopyroxene from wide temperature-pressure range.