Table 1 Compositions of garnets (Grt-I and Grt-II) and majorite.

Compositions for synthetic majorites at 12.5 and 14 GPa are also listed for comparison.

Natural samplesSynthetic majorite
Grt-I(6)*Grt-II(4)Majorite(6)P = 12.5 GPaP = 14 GPa
SiO240.31(17)42.40(46)44.20(38)43.89(24)44.03(16)
TiO20.06(4)0.10(2)0.10(2)00
Al2O322.54(21)22.13(118)7.09(41)10.75(65)10.80(93)
Cr2O30.04(4)1.09(85)1.10(44)1.69(5)1.61(9)
FeO17.05(24)7.71(9)18.47(103)15.94(75)15.58(81)
MnO0.71(11)0.30(3)0.32(5)00
MgO11.39(27)20.79(35)27.16(28)25.10(22)25.25(16)
CaO8.01(27)5.06(43)1.35(32)1.28(7)1.31(4)
Na2O0.02(1)0.02(1)0.03(1)00
Total100.14(29)99.60(20)99.83(6)98.71(33)98.63(32)
Cations per 12 oxygen atoms§
Si3.003(11)3.014(36)3.181(28)3.192(8)3.199(19)
Ti0.003(2)0.006(1)0.005(1)00
Al1.979(13)1.854(97)0.602(35)0.921(52)0.925(76)
Cr0.002(2)0.061(48)0.063(25)0.097(3)0.092(5)
Fe3+0.005(15)0.046(32)0.963(69)0.598(44)0.585(46)
Fe2+1.057(5)0.413(39)0.148(54)0.371(9)0.362(17)
Mn0.045(7)0.018(2)0.020(3)00
Mg1.265(27)2.203(38)2.914(26)2.721(14)2.735(12)
Ca0.639(23)0.385(31)0.104(24)0.100(5)0.102(4)
Na0.003(1)0.002(1)0.004(2)00
Fe3+/∑Fe0.02(2)0.11(8)0.87(5)0.62(2)0.62(2)
XcatMj0.022(10)0.193(29)0.192(21)0.199(32)
P(GPa)||6.4(0.4)12.9(1.1)12.9(0.8)13.2(1.3)

*Number of electron microprobe analyses.

†1 SD in the rightmost digit.

‡Total iron oxide.

§Fe3+ and Fe2+ are calculated using the charge balance method.

XcatMj = 0.5 × (XMj1 + XMj2), where XMj1 = (Si + Ti − 3) + Na and XMj2 = 1 − 0.5 × (Al + Cr + Fe3+) + 1.25 × Na.

||The pressures were calculated using our calibrated equation P(GPa) = 5.61 + (37.98 × XcatMj), with Fe3+/∑Fe ~ 0.55.