Dolomites phase relations

Figure 8.2. Phase relations in the system Ca0-Mg0-HCl-H20-(CC>2). At A. 25°C, 0.001 kb B. 150°C, 1 kb C. 300°C, 3 kb. Mag, magnesite Dol = dolomite Calc = calcite Hydro-Mag = hydromagnesite. Saturation lines for brucite (a) and lime (b) are also shown. Average seawater composition, river water composition, A some subsurface water compositions. (After Bowers et al 1984.)...

Goldsmith J.R., Graf D.L., Witters J., Northrop D. (1962) Studies in the system CaC03-MgC03-FeC03 1. Phase relations 2. A method for major element spectrochemical analysis 3. Compositions of some foreign dolomites.. /. Geol. 70, 659-688. 

It is tempting to place significance on the relative magnitudes of the saturation indices calculated for various minerals and then to relate these values to the amounts of minerals likely to precipitate from solution. The data in Table 6.6, however, suggest no such relationship. Thirteen minerals are supersaturated in the initial fluid, but the phase rule limits to ten the number of minerals that can form only two (dolomite and quartz) appear in the final phase assemblage. 

The listed chemical formulae are ideal and most of these minerals contain trace and minor elements which undoubtedly affect the CL. Several of these minerals have polymorphic or compositional varieties which also may, or do, show CL (e.g. the silica polymorphs quartz, cristobalite, tridymite phosphate compositional varieties apatite, whitlockite, farringtonite, buchwaldite carbonate compositional varieties calcite, dolomite, magnesite). Glass and maskelynite (shock modified feldspar), although not strictly minerals, are relatively common. Below are described the CL observations for the most common phases including enstatite, feldspar and forsterite and they are related to their use for interpreting the mineralogy of meteorites. The observations for the other minerals are sporadic and many details have yet to be studied. 

In such cases, the alkali enters the vapor phase by vaporization from coal minerals, dolomite (10) or limestone, as in sulfurscrubbing processes, or from air-ingested salt particles (e.g., in marine environments). Hardesty and Pohl ( ) have recently reviewed the major problem areas and data limitations relating to the properties of coal mineral matter and ash. 

Clnysotile belongs to the serpentine group of minerals, v arieties of which are found in most of the important mountain ranges and precambrian shields (8). Only a small part of these serpentine occurrences are in the asbestiform clnysotile variety. Chrysotile fibers are found as veins in serpentines or related minerals in serpentinized ultramafic rocks and in serpentinized dolomitic marbles (9). It has been suggested that the ultrabasic rocks (forsterite, Mg-rich pyroxenes, and ampliiboles) are first attacked in an hydrothermal process and transformed in serpentines in a later hydrothermal event, the serpentines are partially redissolved and crystallized as chrysotile fibers (9). (Heady, the genesis of each chrysotile deposit must have involved specific features related to the composition of the precursor minerals, the stress and defomiations in the host matrix, the water content, the temperature cycles, etc. Nonetheless, it is generally observed that the chemical composition of the fibrous phase is closely related to that of the surrounding rock matrix (9). 

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