Fayalite/magnetite/quartz

There is a point of invariant equilibrium of siderite + fayalite + magnetite + quartz + graphite + fluid which fixes T — 600°C and <= 6-8 kbar. 

Figure 7 (right). Abundcuice of gas species in a fluid at 1000 bars in equilibrium with graphite + fayalite + magnetite + quartz (solid) and graphite + magnetite + wiistite (dashed). 

FeSi04 + H2O = Fc304 + 3/2Si02 + H2(g). fayalite magnetite quartz... 

The SSI (solid-state imaging) camera on board the Galileo spacecraft transmitted impressive high-resolution pictures of Io s volcanic activity. Active lava lakes, lava curtains , calderas, mountains and plateaus can be seen (McEwen et al., 2000). The Hubble telescope detected both S2 gas and SO2 in a SO2 to S2 ratio of 1 4 in the smoke trail of the volcano Pele. This value suggests an equilibrium between silicate magmas in the neighbourhood of the quartz-fayalite-magnetite buffer (see Sect. 7.2.2). 

Figure 5.51 fo2 stability field of biotite. (A) Pseudobinary phlogopite-annite mixture. Numbers at experimental points indicate observed Fe/(Fe + Mg) atom ratio. (B) Annite. HM = hematite-magnetite buffer NNO = Ni-NiO buffer MW = magnetite-wuestite buffer QFM = quartz-fayalite-magnetite buffer. From Wones and Eugster (1965). Reprinted with permission of The Mineralogical Society of America. 

In order to model the flow behavior of molten silicate suspensions such as magmas and slags, the rheological behavior must be known as a function of the concentration of suspended crystals, melt composition, and external conditions. We have determined the viscosity and crystallization sequence for a Kilauea Iki basalt between 1250°C and 1149°C at 100 kPa total pressure and f02 corresponding to the quartz-fayalite-magnetite buffer in an iron-saturated Pt30Rh rotating cup. viscometer of the Couette type. The apparent viscosity varies from 9 to 879 Pa.s. The concentration of suspended crystals varies from 18 volume percent at 1250°C to 59 volume percent at 1149 C. The molten silicate suspension shows power-law behavior ... 

Figure 27. Self-diffusion of O and in Fe304 exchanged with variable partial pressures of oxygen at 1150°C (dry). The dashed line represents the theoretical curve with a consideration of oxygen vacancy defects and interstitials (To + <9i). The sohd curve represents the theoretical curve with consideration of both oxygen vacancies and stable cluster defects involving oxygen and iron [To + (FoTpe)]. Modified from Millot and Niu (1997). At 1150°C, Ni-NiO and the QFM (Quartz-Fayalite-Magnetite) buffers lie at P02 values of roughly 10 and 10, respectively.

Fig. 1. Vitrinite reflectance measured on lignite B as a function of experimental run duration at 200°C, 2.0 kbar aqueous fluid pressure, with t02 defined by the hematite-magnetite and magnetite + quartz-fayalite solid buffers (data from Table 1 and Appendix A). The laboratory rate curve is visually estimated the approximate quasi-threshold value for %R is 0.54. Also illustrated in the computed EASY%Ro model for vitrinite maturation at 200°C. EASY%Rq rate model curves agree with experimentally determined rates for the 250 and 300°C isotherms, but are low for the 200°C and slightly high for the 400 0 runs (J. J. Sweeney, personal communication, 01/10/03).

At greater depths, the redox characteristics are mainly determined by another mineral system fayalite (Fe2Si04), magnetite and quartz (Si02) (theFMQ system). The buffer properties of the two systems can be expressed in terms of the following equations ... 

Figure 1. Values of the oxygen fugacity in various Fe-O-Si buffer systems. Key F, fayalite H, hematite I, iron M, magnetite Q, quartz and W, wustite.

H2 is produced by the interaction of water and rocks through a variety of mechanisms, and the frequent presence of H2 in geothermal emanations (e.g., [5,7,8]) suggests that such processes occur widely in the crust. Perhaps the best known mechanism involves the reduction of water to H2 by the ferrous iron component in rocks, as shown, for example, by the oxidation of fayalite (F) to quartz (Q) and magnetite (M) ... 

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