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GEOREF RECORD

Silica-undersaturated portion of the system nepheline-kalsilite-SiO (sub 2) at 2 GPa [P(H (sub 2) O)=P(total)]

Alok K. Gupta, Mrigank Mauli Dwivedi, Hirak Bhattachariya and Soumitra Dasgupta
Silica-undersaturated portion of the system nepheline-kalsilite-SiO (sub 2) at 2 GPa [P(H (sub 2) O)=P(total)]
The Canadian Mineralogist (October 2010) 48 (5): 1297-1313

Abstract

The nepheline (Ne)-kalsilite (Kls)-sanidine (Sa)-albite (Ab) quadrilateral representing petrogeny's residua system for silica-undersaturated alkalic magmas has been studied at 2 GPa [P(H (sub 2) O)=P(Total)] and various temperatures. The composition of the eutectics, their assemblages and the equilibrated temperatures of the various binary joins studied at 2 GPa in presence of excess H (sub 2) O are as follows. Along the Kls-Sa join, the eutectic occurs at Kls (sub 13) Sa (sub 87) (all compositions are expressed in wt% unless specified) and 700 degrees C, where Kls, Sa (sub ss) , liquid and vapor are in equilibrium. Along the Kls-Ne binary join, the eutectic is located at Kls (sub 44) Ne (sub 56) and 650 degrees C, where Kls and Ne (sub ss) coexist with liquid and vapor. The eutectic point of the binary join Ne-Ab occurs at Ne (sub 87) Ab (sub 13) and 670 degrees C, where Ne (sub ss) , Jd (sub ss) , liquid and vapor are in equilibrium. In the case of the Ab-Kfs binary join, the eutectic lies at Ab (sub 73) Kfs (sub 27) and 630 degrees C, where Jd (sub ss) , Sa (sub ss) , quartz, liquid and vapor are the equilibrium assemblage. There are two eutectics in the silica-undersaturated portion of the Ne-Kls-SiO (sub 2) system, studied at 2 GPa in presence of excess H (sub 2) O: 1) E (sub 1) occurs at Kls (sub 18) Ne (sub 52) Qtz (sub 30) and 620 degrees C, where Jd (sub ss) , Sa (sub ss) , Ne (sub ss) , liquid and vapor are in equilibrium, and 2) E (sub 2) is located at Kls (sub 49) Ne (sub 25) Qtz (sub 26) and 615 degrees C, where Ne (sub ss) , Kls, Sa (sub ss) coexist with the liquid. In this system, the maximum (M) produced by the intersection of the join nepheline-sanidine with the nepheline-feldspar cotectic line occurs at Kls (sub 35) Ne (sub 38) Qtz (sub 27) and 635 degrees C, where Ne (sub ss) +Sa (sub ss) +Gl+V are in equilibrium. In contrast to the phase-equilibria studies made at 1 atm, 0.1, 0.2 and 0.5 GPa in presence of excess H (sub 2) O, there is no field of leucite (sub ss) at 2 GPa. The join Ab-Sa no longer acts as a thermal barrier as at low pressure. Instead, Jd (jadeite)-Sa becomes the stable thermal barrier; because of this, all compositions within the triangle Ab-Sa-Jd yield Sa (sub ss) , Jd (sub ss) and quartz at 2 GPa isobaric polythermal conditions in presence of excess H (sub 2) O. The same compositions at low pressure produce feldspathoid- feldspar-bearing assemblages. The occurrence of jadeite along with feldspathoid and feldspar in nepheline syenites arises because the bulk composition of the magma lies in the silica-deficient portion of the system Jd-Sa-Ne, whereas the occurrence of Jd-Sa- SiO (sub 2) observed in association with granitic rocks is related to crystallization of the magma at a high pressure.


ISSN: 0008-4476
EISSN: 1499-1276
Coden: CAMIA6
Serial Title: The Canadian Mineralogist
Serial Volume: 48
Serial Issue: 5
Title: Silica-undersaturated portion of the system nepheline-kalsilite-SiO (sub 2) at 2 GPa [P(H (sub 2) O)=P(total)]
Affiliation: University of Allahabad, National Centre of Experimental Mineralogy and Petrology, Allahabad, India
Pages: 1297-1313
Published: 201010
Text Language: English
Publisher: Mineralogical Association of Canada, Ottawa, ON, Canada
References: 65
Accession Number: 2011-002375
Categories: General mineralogy
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 8 tables
Country of Publication: Canada
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Association of Canada. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201102
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