Marcello and all,
Is this in a contact aureole? I think it is unlikely that
pectolite formed in equilibrium with the other calc-silicate
minerals, which probably formed at 600°C or so. Pectolite usually
forms veins and replacements of higher T minerals, often in
association with prehnite, and it is easily confused with xonotlite.
What are the textures of the pectolite in relation especially to
wollastonite?
eric
>At 10:47 AM 14/06/2002 +0200, Marcello Franceschelli wrote:
>>Dear all,
>> The calc-silicate rocks from the Hercynian chain of Sardinia (Italy)
>>consist of wollastonite, calcite, diopside, grossular (quartz ,
>>plagioclase) and pectolite. Can anyone suggest me the pectolite forming
>>reaction and the P-T stability conditions?
>
> Pectolite composition Ca2NaHSi3O9 is peralkaline; hence the
>pectolite-forming reaction probably involves metasomatic addition of the
>component Na2O.
> Depending on your mineral assemblages, you may be able to estimate the
>P-T conditions of pectolite formation from the equilibrium
>
>grossular + quartz = 2 wollastonite + anorthite
>
>and X(CO2) at the same P,T from
>
>calcite + quartz = wollastonite +CO2 ,
>
>as described by Skippen and Carmichael p.115-122 in Greenwood HJ editor
>1977 Short Course in Application of Thermodynamics to Petrology and Ore
>Deposits. Nowadays such P-T-X(CO2) calculations are made easy by the
>PTAX/TWQ software package developed at Univ British Columbia.
>
>Ciao, Dugald
>
>
>Dugald M Carmichael Phone/V-mail: 613-533-6182
>Dept of Geological Sciences and Geological Engineering
>Queen's University FAX: 613-533-6592
>Kingston ON K7L3N6 E-mail: [log in to unmask]
|