Abstract The polymorphic relations for Mg3(PO4)2 and Mg2PO4OH have been determined by reversed experiments in the temperature-pressure (T-P) range 5001100C, 230 kbar. The phase transition between the low-pressure phase farringtonite and Mg3(PO4)2-II, the Mg analogue of sarcopside, is very pressure dependent and was tightly bracketed between 625C, 7 kbar and 850C, 9 kbar. The high-temperature, high-pressure polymorph, Mg3(PO4)2-III, is stable above 1050C at 10 kbar and above 900C at 30 kbar. The low-pressure stability of farringtonite is in keeping with its occurrence in meteorites. The presence of iron stabilizes the sarcopside-type phase towards lower P. From the five Mg2PO4OH polymorphs only althausite, holtedahlite, -Mg2PO4OH (the hydroxyl analogue of wagnerite) and -Mg2PO4OH were encountered. Relatively speaking, holtedahlite is the low-temperature phase (600C), -Mg2PO4OH the high-temperature, low-pressure phase and -Mg2PO4OH the high-temperature, high-pressure phase, with an intervening stability field for althausite which extends from about 3 kbar at 500C to about 12kbar at 800C. Althausite and holtedahlite are to be expected in F-free natural systems under most geological conditions; however, wagnerite is the most common Mg-phosphate mineral, implying that fluorine has a major effect in stabilizing the wagnerite structure. Coexisting althausite and holtedahlite from Modum, S. Norway, show that minor fluorine is strongly partitioned into althausite (KDF/OH 4) and that holtedahlite may incorporate up to 4 wt% SiO2. Synthetic phosphoellenbergerite has a composition close to (Mg0.90.1)2Mg12P8O38H8.4. It is a high-pressure phase, which breaks down to Mg2PO4OH+Mg3(PO4)2+H2O below 8.5 kbar at 650C, 22.5 kbar at 900C and 30 kbar at 975C. The stability field of the phosphate end-member of the ellenbergerite series extends therefore to much lower P and higher T than that of the silicate end-members (stable above 27 kbar and below ca. 725C). Thus the Si/P ratio of intermediate members of the series has a great barometric potential, especially in the Si-buffering assemblage with clinochlore+talc+kyanite +rutile +H2O. Application to zoned ellenbergerite crystals included in the Dora-Maira pyrope megablasts, western Alps, reveals that growth zoning is preserved at T as high as 700725C. However, the record of attainment of the highest T and/or of decreasing P through P-rich rims (1 to 2 Si pfu) is only possible in the presence of an additional phosphate phase (OH-bearing or even OH-dominant wagnerite in these rocks), otherwise the trace amounts of P in the system remain sequestered in the core of Si-rich crystals (5 to 8 Si pfu) and can no longer react.