I agree with Ralph Bottrill.
What characterizes adularia is the rhombohedral form. It has actually been
shown that "sure" adularia can have very different structural state, due to
different factors: see Dong & Morrison, 1995 (Mineralium Deposita, 30, 11-19).
Coming back to the Western Italian Alps, within the Cervo body (mostly
sienite to monzonite)quartz-rich hydrothermal veins (+ minor sulphides)
contain in their outer portions well shaped adularia. The host monzonite
shows strong potassic alteration: particularly, primary K-feldspar is surely
overgrown by hydrothermal K-feldspar. All minerals are very cloudy due to
late weathering and possibly also late hydrothermal alteration, so optical
characters are difficult to see: however, due to the fact that secondary
K-feldspar definitely occurs adjacent to veins containing euhedral adularia,
I would call this K-feldspar adularia as well.
Rob: in this case, however, this "adularia" is not diffused in the plutonic
rocks, but clearly related to veins.
There are no published papers about this and other occurrences in the
Western Alps, I'm still working on it in order to understand the P-T
conditions of emplacement and fluid composition. It will take some time...
Barium content: I do not know of any Ba-rich adularia associated with
massive sulfide deposits. In the epithermal adularia-sericite deposits of
Marmato (Caldas, Colombia), adularia typically shows a variable barium
content (BaO: 1.5 - 4 wt%).
Piergiorgio
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Piergiorgio Rossetti
Dipartimento di Scienze Mineralogiche e Petrologiche
Università di Torino
via Valperga Caluso, 35
10125 Torino, Italy
Tel.: +39 011 6707107
Fax: +39 011 6707128
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