Plagioclases

Placental lactogens Plagioclase Planchette counting Planck s constant Planck s law Plane tree Planiblock Planography Plantago ovata Plantains... 

Clayton, J. L. (1986). An estimate of plagioclase weathering rate in the Idaho batholith based upon geochemical transport rates. In "Rates of Chemical Weathering of Rocks and Minerals" (S. M. Coleman and D. P. Dethier, eds). Chap. 19, pp. 453-466. Academic Press, New York. 

Date et al. (1983) found the existence of an Na20-depleted dacite mass with a lateral dimension of 1.5 x 3.0 km immediately below the ore horizon (Figs. 1.23 and 1.24) and the mass is useful indicator of exploration of Kuroko ore deposits. This Na20 depletion is considered to be due to the destruction of plagioclase attacked by potassium-... 

In zone (1), quartz, K-feldspar, epidote, chlorite, prehnite and sphene are predominant alteration minerals. Epidote, prehnite and carbonate replace plagioclase phenocryst. Epidote often occurs as a veinlet with several millimeters wide, together with prehnite. K-feldspar, calcite and quartz tend to occur as a veinlet. Chlorite replaces pyroxene... 

As suggested by Grant (1982) and Giggenbach (1980), if the fluids which are initially in equilibrium with the plagioclase-clay-calcite assemblage at high temperatures... 

Fig. 2.37. Phase diagram for Ca0-Na20 Si02-(Al203)-H20 system in equilibrium with quartz at 400°C and 400 bars. Plagioclase solid solution can be represented by the albite and anorthite fields, whereas epidote is represented by clinozoisite. Note that the clinozoisite field is adjacent to the anorthite field, suggesting that fluids with high Ca/(H+) might equilibrate with excess anorthite by replacing it with epidote. The location of the albite-anorthite-epidote equilibrium point is a function of epidote and plagioclase composition and depends on the model used for calculation of the thermodynamic properties of aqueous cations (Berndt et al., 1989).

The flK-f of plagioclase from MORB is generally lower than that of back-arc basin igneous rocks. Therefore, the higher m +/mY + of midoceanic ridge fluids compared with back-arc basin fluids could be explained in terms of Na-feldspar/K-feldspar/hydro-thermal fluids equilibrium. 

Berndt, M.E., Seyfried, W.E. Jr. and Janeckey, D.R. (1989) Plagioclase and epidote buffering of cation ratios in midocean ridge hydrothermal fluids Experimental results in and near the supercritical region. Geochim. Cosmochim. Acta, 53, 2283-2300. 

Hydrothermal alteration minerals from midoceanic basalt are analcite, stilbite, heulandite, natrolite-mesolite-scolecite series, chlorite and smectite for zeolite facies, prehnite, chlorite, calcite and epidote for prehnite-pumpellyite facies, albite, actinolite, chlorite, epidote, quartz, sphene, hornblende, tremolite, talc, magnetite, and nontronite for green schist facies, hornblende, plagioclase, actinolite, leucoxene, quartz, chlorite, apatite, biotite, epidote, magnetite and sphene for amphibolite facies (Humphris and Thompson, 1978). 

Among the minerals mentioned above calcite and kaolinite may be important for controlling /CO2 of terrestrial geothermal waters. It was cited by Giggenbach (1981) that /CO2 (or Xco2> mole fraction of CO2) of terrestrial geothermal waters is controlled by plagioclase -1- CO2 = calcite -H kaolinite . 

Figure 4. Fits of lattice strain model to experimental mineral-melt partition coefficients for (a) plagioclase (run 90-6 of Blundy and Wood 1994) and (b) elinopyroxene (ran DC23 of Blundy and Dalton 2000). Different valence cations, entering the large cation site of each mineral, are denoted by different symbols. The curves are non-linear least squares fits of Equation (1) to the data for each valence. Errors bars, when larger than symbol, are 1 s.d. Ionic radii in Vlll-fold coordination are taken from Shannon (1976).

Figure 5. Variation in apparent lattice-site Young s Modulus ) with molar anorthite content of the host crystal for the plagioclase partitioning experiments of Blundy and Wood (1994)., Ej and...

A further feature of the fit parameters obtained by Blundy and Wood (1994) for plagioclase and clinopyroxene was that the partitioning parabolae become tighter E increases) and displaced to lower as charge increases (Law et al. 2000 Blundy and Dalton 2001). These observations have since been confirmed by a large number of... 

Plagioclase has a single large cation site (M) into which all U-series elements partition. This site is normally occupied by Ca and Na, with coordination number increasing with increasing Na content. For simplicity we will assume VIII coordination... 

Lattice strain parameters for 3+ cations entering plagioclase are difficult to derive because is clearly larger than La, meaning that one limb of the partitioning parabola is not... 

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