Robert
It may be adularia. This can have variable optics, up to sanidine-type
disorder.
I have seen this replacing plagioclase in high-level syenite porphyries at
Cygnet, Tasmania, as part of a potassic alteration zone about some gold
mineralisation.
Regards
Ralph Bottrill
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Tasmanian Geological Survey
Mineral Resources Tasmania (Dept of Mines - Tasmania)
P.O. Box 56, Rosny Park, Tasmania 7018, Australia
Aus Tel: 03-6233-8359 Intl Tel: +61-3-6233-8359
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"Geology is a capital science to begin with as it requires nothing but a
little reading, thinking and hammering."
Charles Darwin, 1835.
-----Original Message-----
From: Robert Willan [SMTP:[log in to unmask]]
Sent: Wednesday, 16 February 2000 1:32
To: [log in to unmask]
Subject: Sanidine in plutonic rocks??
Best wishes for 2000 everyone!
Has anyone come across sanidine in plutonic rocks, and if so what is
its significance?
It looks like orthoclase (brownish colour, dusty appearance), is
anhedral and interstitial, replacing plag and forming graphic intergrowths
with quartz.
But it has a small 2V (10-30°) is almost uniaxial (characteristic of
sanidine) and is optically negative .
The intrusion is a small (1-2 km) med grained granodiorite stock
with marginal propylitic alteration. It contains granophyric intergrowths,
miarolitic cavities and mm-scale veinlets of quartz, pyrite, pyrrhotite,
magnetite, chalcopyrite and bornite. There is an extensive propylitised
aureole and about 1 km away, areas of argillic, sericitic and advanced
argillic alteration.
Any examples of sanidine in plutons?
Thanks - Rob
----------------------------------------------
Dr Robert CR Willan,
Magmatic-Hydrothermal Processes
Geological Sciences Division,
British Antarctic Survey,
Cambridge CB3 0ET, UK
Tel: 01223 221420
FAX: 01223 362616
Email: [log in to unmask]
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