The prograde, high pressure, transition from antigorite serpentinite to
enstatite-olivine rock occurs along a tectonically undisturbed profile
at Cerro del Almirez, SE Spain. The reactant assemblage is antigorite +
olivine with tremolite rimming precursor diopside. The product
assemblage of tremolite + chlorite + enstatite + olivine has a
spinifex-like texture with arborescent or radiating olivine elongated
parallel to [001] and with radially grown enstatite. Product
enstatite is very poor in Al2O3. Due to numerous oriented
submicroscopic inclusions of chromian magnetite, product olivine has a
brownish pleochroism and a bulk chromium content similar to precursor
antigorite. Titanian clinohumite with a fluorine content of 0.45-0.50
wt% persisted beyond the breakdown of antigorite. The partitioning of
iron and magnesium amongst the silicate phases is almost identical to
that at lower pressures. Average K-d values Mn/Mg and Ni/Mg are 0.17
and 0.70 for antigorite-olivine pairs and 1.83 and 0.22 for
orthopyroxene-olivine pairs, respectively. These data are useful in
discriminating generations of olivine grown on each other. From the
field data a phase diagram topology for a portion of the system
CaO-MgO-SiO2-H2O is derived. This topology forms the basis for
extrapolations into inaccessible P-T regions.