Experimental Study of the System Forsterite-Anorthite-Nepheline in Presence of Excess water under 20 kb and Variable Temperatures, a Preliminary Report: Priyanka Mukherjee, Soumitradas Gupta and Alok K. Gupta, University of Allahabad.
Abstract:Nepheline, forsterite and anorthite are essential minerals in alkali basalts. Although diopsidic pyroxene has not been included in this system, it should appear as an important phase by following reaction:
2Mg2SiO4+2CaAl2Si2O8à CaMgSi2O6.CaAl2SiO6 + Mg3Al2Si3O12 (1)
(Forsterite) (Anthotrthite) Diopside.Tschermark molecule Pyrope
It would be also interesting to see if diopside, thus produced by reaction (1), should react with nepheline to produce melilite. Note the reaction:
3CaMgSi2O6+2NaAlSiO4àCa2MgSi2O7-CaNaAlSi2O7+Mg2SiO4+NaAlSi3O8 (2) (Melilite) (Forsterite) + Albite molecule in the liquid
One of the important accessory minerals in basalt is Mg-spinel (MgAl2O4). It should be interesting to see if this mineral appears in the forsterite-nepheline as well as forestrite-anorthite joins under pressure. It should be pointed out that at higher pressures, foresterite and anorthite should react to produce garnet. Note the reaction:
Mg2 SiO4+CaAl2 Si2 O8ßàMg2CaAl2Si3O12 (3)
The system forsterite (Fo), nepheline (Ne) anorthite (An) has been studied at 20 kb under a condition of P(H2O)=P(Total). The starting materials shown by small circles are crystallized glasses. Phase relations for the two binary joins are shown in figures 1 and 2 and that of Fo-Ne-An are shown in figure 3. Appearances of spinel (Sp), diopside (Di), corundum (Co) and garnet (Ga) in (Fo)- (An)-Ne join and appearance of spinel in the join forsterite-nepheline is thus, confirmed. Corundum and Sp however disappears in the solidus region. All phases are solid solutions Table-1.
It is a six component system and the Fo-Ne-An join cuts through the phase volumes of forsterite, spinel, nepheline, diodside and corundum. Composition of various phases are shown in Table-1. The compositions of two-four phase points are shown in the diagram as follows: A (Fo5Ne92Q3) and 695±10°C, and B (Fo2Ne95Q3) and 675±10°C.