Zinc is not uncommon in chromites and other spinels, especially in
hydrothermally altered rocks. There is actually a Zn endmember:
zincochromite (Nesterov & Rumjantzeva, American Mineralogist, volume 73,
pp. 931-932 (1988)
In systems with high Zn/S, large amounts of Zn will go into many other
ferromagnesian minerals, including micas, ilmenite, rhodonite, clays and
pyroxenes.
See also :
Wylie, A.G., Candela, P.A. and Burke, T.M. (1987) Compositional zoning in
unusual zinc-rich chromite from the Sykesville District, Carroll Co.,
Maryland. American Mineralogist, v. 72, p. 413-422.
Ralph Bottrill
-----Original Message-----
From: Kevin Leahy [SMTP:[log in to unmask]]
Sent: Friday, May 11, 2001 12:07 AM
To: [log in to unmask]
Subject: Zn metasomatism and chromite
Dear All,
I am currently working on a diamond exploration project where the
only
indicator minerals (of kimberlite and lamproite) turning up in the
tropically-weathered soils are chromites - this is not so unusual
because
the silicates often get toasted by the laterisation.
However, the chromites are extremely Zn rich (for chromite anyway)
around
10% ZnO, with MgO <2%, and Cr2O3 51-55%. Has anyone else ever come
across
this type of chromite? Is it likely to be mantle metasomatism? can
there be
a crustal paragenesis?
More generally, has anyone any information on the replacement of Mg
with Zn
in any other type of mineral, in a lithospheric mantle (or any
other)
setting?
Thanks
Dr Kevin Leahy
Exploration Consultants Ltd
Henley-on-Thames
UK
|