THE COMPOSITION OF LIZARDITE 1T AND THE FORMATION OF MAGNETITE IN SERPENTINITES

Mossbauer data were obtained from 23 samples [20 lizardite (1T) sample s, two chrysotile samples, and one antigorite sample] taken from five geologically well-characterized serpentinites. These serpentinites rep resent both hydration and serpentine recrystallization in lizardite-ch rysotile serpentinites. Mossbauer parameters obtained from the samples used in this study were integrated with electron microprobe and H2O e xtraction yields to generate a comprehensive set of compositional data for lizardite, which was compared with modal magnetite values in the same samples. Lizardite contains up to 70% of its Fe as Fe3+ in both t etrahedral and octahedral coordination. The Fe-[6](3+) content shows a positive correlation with Fe-[4](3+) and a negative correlation with Fe-[6](2+) .Substitutions of trivalent cations in both sheets are corr elated and do not require cation vacancies (including H+) to maintain charge balance. The (Fe/Fe)-Fe-[4]-Fe-[6] and Si/Mg ratios of lizardit e, chrysotile, and antigorite are different, with the (Fe/Fe)-Fe-[4]-F e[6] ratio decreasing, and the Si/Mg ratio increasing from lizardite t o chrysotile to antigorite. The Fe(tot) content and the (Fe/(Fe + Mg)) -Fe-[6]-Fe-[6] and Fe-tot(3+)/Fe(tot) ratios of lizardite in the Jeffr ey and Woodsreef serpentinites are inversely correlated with modal mag netite, indicating that f(O2) was internally controlled in these serpe ntinites. In contrast, the Fe(tot) and the (Fe/Fe)-Fe[6]-Fe-[6] + Mg) and Fe-tot(3+)/Fe(tot) ratios of lizardite in the Cassiar serpentinite exhibit either direct or inverse correlations with modal magnetite, i ndicating internal or external control of with f(O2) externally contro lled early in recrystallization and internally controlled later in rec rystallization. The availability of Si affects internal vs. external c ontrol of f(O2) because Si exchanges with Fe-[4](3+) either to form or to consume magnetite during serpentine recrystallization.