Feldspar alkali

Mineral-Melt Partitioning ofU-Series Nuclides Alkali feldspar... 

Figure 19. Variation in dimension of the X-fold cation site in alkali-feldspar for 2+ cations (), obtained by fitting the experimental data of Icenhower and London (1996) for Dca, >sr and Dsa at 0.2 GPa and 650-750°C. In performing the fits was set at 91 GPa for all runs. Error bars are 1 s.d. The positive slope is consistent with measured changes in metal-oxygen bond length from albite to orthoclase (cf Fig. 6). The solid line shows the best-fit linear regression given in Equation (35).

The above 2 fit parameters can also be used to derive Dpb from Dsr, assuming that the proxy relationship is valid for alkali feldspar. For the Icenhower and London (1996) experiments we obtain a further linear correlation with Xor... 

As Sr is highly compatible in alkali feldspar under all conditions, it is likely that so too is Pb, whatever the validity of the Sr-Pb proxy relationship. Leeman (1979) determines Z)pb of 0.84-1.37 for separated sanidine phenocrysts (Or- so) from various acid volcanic rocks, in broad agreement with Equation (37). 

Figure 20. Variation in calculated for three experimental alkali-feldspar-melt partitioning...

In most minerals the U-series elements are highly incompatible. There are, however, several exceptions. We calculate that radium will be compatible in phlogopite and alkali-feldspar and that these two minerals will dominate the magmatic radium budget in cases... 

Guo J, Green TH (1989) Barium partitioning between alkali feldspar and silicate melts at high temperature and pressure. Contrib Mineral Petrol 102 328-335... 

Icenhower J, London D (1996) Experimental partitioning of Rb, Cs, Sr, and Ba between alkali feldspar and peralnminons melt. Am Mineral 81 719-734... 

Figure 12 continued, (d)-(g) Mineral isochrons for rhyolites from the Olkaria volcanic center (Kenya), (d) and (f) alpha spectrometry resnlts from Black et al. (1997). (e) and (g) TIMS resnlts from Henmaim and Davies (2002). All the rhyolites have eraption ages between 3.3 and 9.2 ka. Note that the same sample (570) analyzed in both stndies gives rather different ages (f and g). Same abbreviations of mineral names as in Fignre 10 + Qz qnartz KF alkali feldspar Amph amphibole Bt biotite. 

Alkali feldspars (K.NajAlSijOg) form a continuous solid solution series at high temperatures, but tend to exsolve upon cooling into a potassium-rich phase and a sodium-rich phase, resulting in perthitic intergrowths (2). Sodic plagioclases and potassium-rich feldspars tend to be studied together, in part because of similarities in structure, composition, occurrence, and weatherability (e.g., 1[). 

Intracrystalline ordering in alkali feldspars (Al ceeds according to different kinetics ... 

Hemingway B. S., Krupka K. M., and Robie R. A. (1981). Heat capacities of the alkali feldspars between 350 and 1000 K from differential scanning calorimetry, the thermodynamic functions of alkali feldspars from 298.15 to 1400 K, and the reaction quartz + jadeite = albite. Amer. Mineral, 66 1202-1215. 

Kroll H. and Ribbe P. H. (1983). Lattice parameters, composition and Al, Si order in alkali feldspars. In Reviews in Mineralogy, vol. 2 (2d ed.), P. H. Ribbe (series ed.), Mineralogi-cal Society of America. 

Lin T. H. and Yund R. A. (1972). Potassium and sodium self-diffusion in alkali feldspar. Contrib. Mineral Petrol, 34 177-184. 

Long P. E. (1978). Experimental determination of partition coefficients for Rb, Sr and Ba between alkali feldspar and silicate liquid. Geochim. Cosmochim. Acta, 42 833-846. 

Martin R. F. (1974). Controls of ordering and subsolidus phase relations in the alkali feldspars. 

Mason R. A. (1982). Trace element distributions between the perthite phases of alkali feldspars from pegmatites. Min. Mag., 45 101-106. 

Parson I. (1978). Alkali feldspars Which solvus Phys. Chem. Minerals, 2 199-213. 

Powell M. and Powell R. (1977a). Plagioclase-alkali feldspar thermometry revisited. Min. Mag., 41 253-256. 

Schairer J. F. (1950). The alkali feldspar join in the system NaAlSi04-KAlSi04-Si02. J. Geol, 58 512-517. 

Seek H. A. (1972). The influence of pressure on the alkali-feldspar solvus from peraluminous and persilicic materials. Fortschr. Mineral, 49 31—49. 

Stewart D. B. and Ribbe P. H. (1969). Structural explanation for variations in cell parameters of alkali feldspars with Al/Si ordering. Amer Jour Sci., 267A 444-462. 

Thompson J. B. Jr., Waldbaum D. R. and Hovis G. L. (1974). Thermodynamic properties related to ordering in end member alkali feldspars. In The Feldspars, W. S. MacKenzie and J. Zussman, eds. Manchester Manchester University Press. 

Willaime C. and Gaudais M. (1977). Electron microscope study of plastic defects in experimentally deformed alkali feldspar. Bull Soc. Fr. Mineral Cristallogr., 100 263-271. 

Yund R. A. (1974). Coherent exsolution in the alkali feldspars. In Geochemical Transport and Kinetics, Hofmann, Giletti, Yoder, and Yund, eds. New York Carnegie Inst. Washington and Academic Press. 

Based on mineralogical and textural similarities (supported by geochemistry), the rocks of the syenitic dyke swarm can be subdivided into three groups (1) nepheline syenite - nephelinolite group (NS-N), (2) syenite - monzodiorite group (SM), and (3) an alkali feldspar granite dyke. 

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