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.