Calcite phase relations

Fig. 2. Logarithmic activity diagram depicting equilibrium phase relations among aluminosilicates and sea water in an idealized nine-component model of tire ocean system at the noted temperatures, one atmosphere total pressure, and unit activity of H20. The shaded area represents (lie composition range of sea water at the specified temperature, and the dot-dash lines indicate the composition of sea water saturated with quartz, amotphous silica, and sepiolite, respectively. The scale to the left of the diagram refers to calcite saturation foi different fugacities of CO2. The dashed contours designate the composition (in % illite) of a mixed-layer illitcmontmorillonitc solid solution phase in equilibrium with sea water (from Helgesun, H, C. and Mackenzie, F T.. 1970. Silicate-sea water equilibria in the ocean system Deep Sea Res.).

Figure 2.1. Generalized phase relations in the unary system CaCC>3. "A" -aragonite "I" through "V - calcite polymorphs with metastable fields indicated by (). Dash-dot line at 800°C represents transition encountered on cooling runs solid line at lower T represents transition encountered on heating runs. (After Carlson, 1980.)...

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.)...

Phase relations in the system Ca0-C02-H20 from 0 to 100°C are shown in Fig. 6.1. Given the fact that CO2 pressures in natural environments are rarely less than about 10 bar, and that equilibrium between calcite and portlandite is at a CO2 pressure of ]0 bar at 25°C, calcite is clearly the stable phase in Earth-surface environments. 

Poly(L-isocyanoalanyl-D-alanine) with a regular distribution of carboxylic acid-terminated side chains was taken as a model template to investigate the relation between the structure of a polymeric template and a developing calcite phase [105]. A series of carboxylate-containing polyamides were synthesized [106-108] for the purpose of crystallization of CaCOa. HeUcal... 

Because one mole of calcite reacts with one mole of quartz, the molar reaction rates of phase A and B are identical, which equals the molar growth rate of wollastonite (D). Therefore, the linear reaction rate and u are related as follows ... 

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. 

The mineralogical phase composition of the sample SW [86] (in wt %) is 90% 5% clinoptilolite and 10% 5% others, which include montmorillonite (2-10 wt %), quartz (1-5 wt %), calcite (1-6 wt %), feldspars (0-1 wt %), magnetite (0-1 wt %), and volcanic glass (3-6 wt %). Employing this sample and a pure clinoptilolite, whose TCEC fluctuates between 2.0-2.2 mequiv/g depending on the Si/Al relation of the clinoptilolite monocrystal, it is possible to indirectly evaluate the total cation-exchange capacity of the sample SW as follows ... 

A knowledge of past lake-levels may be used to interpret variations in precipitation/evaporation - commonly called effective precipitation - and which is clearly dependent upon air temperature. Evaporite mineralogy is a key component of such studies. For example, in Death Valley, California, the interpreted dry periods are associated with glauberite, gypsum and calcite in palaeo-mudflat deposits, whereas abundant calcite, scarce CaS04 minerals and halite with mud layers, typify wet periods, including a perennial-lake phase (Li et al., 1997 Lowenstein et al., 1999). A crude lOOka cyclicity is noted, possibly related to Milankovitch forcing. 

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