Ti-andradite (melanite) has been found in a metapyroxenite layer in the
upper part of the Malenco ultramafics (Italy), coexisting with
clinochlore, diopside and magnetite. Field observations, as well as
major and trace element bulk-rock composition, strongly suggest a
cumulate origin for the layer. Textural relationships indicate that
Ti-andradite formed during two different metamorphic stages. Under peak
metamorphic conditions (400-450-degrees-C, 5+/-2 kbar) Ti-andradite
grew in an assemblage of diopside, clinochlore, magnetite and rare
ilmenite and perovskite. Later, retrograde brittle deformation induced
formation of veins containing the paragenesis Ti-andradite,
vesuvianite, diopside, chlinochlore, magnetite and accessory
perovskite. The Ti-andradite varies considerably in TiO2
(0.11-9.62wt%), Fe2O3 (14.3-30.5 wt%), Al2O3 (0.65 3.90 wt%),
Cr2O3 (< 0.18-0.98 wt%) and SiO2 (32.1-36.1 wt%); this is
mostly, but not entirely, due to distinct zoning. Ti-andradite contains
0.32 to 0.66 wt% H2O as determined by infrared spectroscopy and 0.83
to 1.76 wt% FeO. The CaO shows almost no variation (34.1+/-0.7 wt%)
and Ca completely fills the dodecahedral site. Single crystal site
refinements indicate that no tetrahedral Ti or Fe replaces Si. Titanium
incorporation is attributed to similar degrees of substitution along
the exchange vectors Ti3+ Fe--1(3+), Ti4+ Al(IV)Al-1(VI)Si-1 and (Fe2+,
Mn2+, Mg2+)Ti4+ 2Fe--1(3+). The presence of mixed valence states of
both Fe and Ti suggests a low oxygen fugacity during crystallization of
Ti-andradite. Mass balance calculations indicate an isochemical origin
of the first generation of Ti-andradite in the clinopyroxenite layer.
Its occurrence is restricted to antigorite-free mineral assemblages
containing clinochlore of 0.95 < X(Al) < 1.1. The hydrothermal
crystallization of Ti-rich andradite in veins demonstrates Ti mobility
in aqueous fluids under moderate P-T conditions. The zonation patterns
indicate disequilibrium conditions during vein crystallization. As no
fluorine-, carbonate- and phosphate-bearing minerals were found, OH- is
most probably the ligand complexing Ti.