Olivine Monticellite

Reaction 6. This reaction was investigated in detail by Finnerty and Boyd (1978) and was more recently recahbrated by Kohler and Brey (1990). Because the Ca content in olivine is R-dependent, due essentially to a large solvus between monticellite and forsterite that expands with pressure (cf. section 5.2.5), this reaction should act as a sensitive barometric function. However, the enthalpy of reaction is quite high and the effect of T on the equilibrium is also marked. The calibrated P-T slope has an inflection, the origin of which is not clear at first glance. 

Magnesium is a bright, silver-white alkaline earth metallic element (symbol Mg atomic no. 12). It is widely found as different salts in minerals such as boracite (borate), ankerite and dolomite (carbonates), carnallite (chloride), brucite (hydroxide), periclase (oxide), lazulite (phosphate), amphibole, cummingtonite, enstatite, gamierite, hornblende, humite, hypersthenite, iolite, jade, meUlite, monticellite, olivine, Pennine, saponite, sapphire, serpentine, talc, and tremolite (silicates), and epsomite, kainite, kieserite, and polyhalite (sulfates). 

At 600°-800°C if reactions are allowed to be controlled only by thermodynamics, the following should be true. There should be no free quartz and the complete elimination of quartz has been observed in spent shale mineral reactions (10, 11, 20). Orthosilicates such as olivine and monticellite will be present and this also has been observed in spent shale studies (9,10, 11, 20). If Na and K concentrations are at typical levels, the feld-spathoids leucite and nepheline should be present. Although there may be reasons for nepheline not appearing, K is a large ion and leucite is the only reasonable host for it in oil shales with low Si02 abundance. Leucite has been observed in one study of spent shale mineralogy (10). 

The structure is illustrated in Fig. 17. This compound is isostructural with olivine-like y-Ca2(Si04). The two crystallographically independent Ca atoms are here replaced by the pair nearly equal in size, Na and Y. The strip pattern of Na/Y octahedra along [007] is comparable to the analogues in CaMg(Si04) (monticellite). Here, however, the open Na... 

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