The new phosphate bearthite, Ca
2Al(PO
4)
2HO, found in high-pressure metamorphic rocks, has been synthesized from a stoichiometric mixture of γ-Al
2O
3 and CaHPO
4·2H
2O between 4 kbar water pressure, 485°C and 24.5 kbar, 800°C. Its upper temperature stability limit, bearthite ⇆ hydroxyapatite
+ corundum + berlinite + H
2O, has been tightly reversed between 485°C, 1 kbar, and 850°C, 10 kbar. We extracted the following thermodynamic parameters
for bearthite:
H
f
298/o
=−4327.25 kJ/mol and
S
298
o
=214.5 J/mol.K, using a new Δ
rCp approximation for dehydration reactions. Additional experiments on the reaction trolleite ⇆ berlinite + corundum + H
2O yield new thermodynamic data for trolleite, Al
4(PO
4)
3(OH)
3 (
H
f
298/o
=−6530 kJ/mol,
S
298
o
=252.7 J/mol.K). We derived the phase relations in the CaO−P
2O
5−Al
2O
3−H
2O system considering the phosphates bearthite, trolleite, berlinite, augelite, and hydroxyapatite. Extension to the SiO
2-bearing system barely affects the stability field of bearthite but reveals a chemical control on bearthite occurrence. The
stability of the assemblages hydroxyapatite + aluminium silicate + (quartz or corundum), with respect to bearthite + Ca−Al-silicate,
restricts bearthite to very Capoor (Dora-Maira massif) or P-rich rocks (Monte-Rosa massif; Hohe Tauern; Västan», Sweden).
The coexistence of bearthite with a Ca−Al-silicate (lawsonite) may be possible only for pressures exceeding ca. 15 kbar under
unusually low gradients of 5° C/km. Bearthite is otherwise not a high-pressure index mineral but provides thermal constraints
in relatively high-grade phosphate-bearing deposits or metasediments. Its stability under mantle
P-T conditions makes bearthite a sink for Sr and REE in subducted Ca-poor rocks.
Editorial responsiblity: W. Schreyer