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Formula: ZnTiO3
Simple oxide of zinc and titanium,
ilmenite group, forms a series with
ilmenite
Crystal System: Trigonal
Specific gravity: 4.98 calculated
Hardness: 5
Streak: Brown-black
Colour: Dark brown, black
Environments
Pegmatites
Metamorphic environments
Localities
At the Sierras de Córdoba, Ambul District, San Alberto Department, Córdoba Province, Argentina, ecandrewsite
and other ilmenite-group minerals have been found in
amphibolites, in an area where
zinc is a relatively widespread element in the associated metasedimentary and
metaigneous sequences. Ilmenite group minerals occur as anhedral to subhedral,
tens to a hundred micrometer-sized relic inclusions in titanite, with
compositions along a discontinuous solid-solution trend ranging from manganese-
and iron- bearing ecandrewsite toward
ilmenite.
It is believed that ecandrewsite and zinc-rich
ilmenite compositions were attained by metasomatic fluid–mineral reactions
during retrograde regional metamorphism, ie after the replacement of protolithic
zinc-bearing ilmenite group species by
titanite. The original composition of the
ilmenite group species might have been
zinc-poor ilmenite; however, the
attainment of ecandrewsite compositions possibly needed an external supply of
zinc provided by the fluid. This is the first worldwide report of
ecandrewsite in amphibolites; so far it has been described in quartz-rich
metasediments, quartz-gahnite exhalites,
kyanitic schists,
nepheline syenites, metamorphosed volcanic hosted massive
sulphide mineralisations, and albitites. The presence of
ecandrewsite in amphibolite, as has been proved for
zinc-rich ilmenite and
gahnite in other metasedimentary sequences elsewhere in the world, could become
another pathfinder or indicator mineral for zinc-enriched portions of the crust
(CM 60.677-686).
At the type locality, the Melbourne Rockwell mine, Little Broken Hill, Broken Hill district, Yancowinna county, New South
Wales, Australia, ecandrewsite occurs as euhedral tabular grains, up to 50 by 150 microns. It is disseminated in
quartz-rich metasedimentary rocks of
amphibolite-granulite
facies and coexists with
almandine-spessartine,
iron-rich gahnite and
rutile. Associated
amphibolites contain
zinc-bearing ilmenite
(AM 74.501).
At the San Valentín mine, Sancti Spiritu, La Unión, Murcia, Spain, ecandrewsite grains up to 50 microns across
occur intergrown with zinc-bearing
ilmenite in
primary oxide-sulphate-carbonate ores
(AM 74.501).
At the Pilanesberg Alkaline Ring Complex, Moses Kotane Local Municipality, Bojanala Platinum District Municipality,
North West, South Africa, ilmenite-group
titanates are relatively common
accessory minerals in lujavrite. They crystallised in three generations
and form a continuous ecandrewsite–pyrophanite solidsolution. The
earliest generation of iron- and
manganese- rich ecandrewsite of widely varying composition is
replaced by zinc-rich pyrophanite. The
latest generation is pyrophanite with only traces of
zinc, showing a compositional evolution toward
iron-rich pyrophanite. The three
generations of zinc-manganese
titanates crystallised during the alteration of the
lujavrite by evolving deuteric alkaline fluids. The enrichment in
zinc, manganese and
titanium in these fluids resulted from the decomposition of earlier-formed
zinc- and manganese- rich
titanium- and zirconium- bearing
silicates. This is the first reported occurrence of a near-complete
ecandrewsite–pyrophanite solid solution series
(CM 42.1169-1178).
At Death valley, Inyo county, California, USA, in addition to kyanite-bearing
fine-grained schist, other metamorphic rocks of the footwall are
mylonitic
quartz-feldspar
gneiss, granitic
pegmatite and
marble. A new occurence of
ilmenite-ecandrewsite solid solution from the
kyanite-bearing schist is reported.
Because ilmenite typically is not a host of significant amounts of
zinc in metamorphic rocks, the paragenesis of
zinc-bearing ilmenite-ecandrewsite
solid solution is of interest
(CM 31.425-436).
Associated minerals include almandine,
kyanite, zinc-bearing
ilmenite, monazite-(Ce),
muscovite, quartz,
chlorite, zircon and
titanite
(HOM).
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