Labradorite feldspar

The zeolite, scolecite, the feldspar, labradorite, and the ultra-marine, hackmanite, gave high-strength cements but all were much affected by water - the strength of the labradorite cement disappeared almost entirely - possibly because of the presence of free acid. 

The most abundant mineral in the rocks of the main body of the Basement sill studied by Hamilton (1965) is calcic plagioclase feldspar (labradorite-bytownite). 

Feldspars are the most abundant minerals in the earth s crust, accounting for about 60% of all igneous rocks. They are derivatives of silica in which about one-half or one-quarter of the silicon atoms have been replaced by aluminum. Feldspar is used in the manufacture of certain types of glass and pottery. Some feldspar crystals, such as moonstone (white perthilte), Amazon stone (green microcline), and multicolored labradorite, are used as gem stones and in architectural decorations. Some are used as a coating and filler in the production of paper. 

Cesium-ion concentrations in distilled water and synthetic ground-waters were measured after contact with the feldspars for various periods of time, over the temperature range 150°C to 200°C. It was found that for short reaction times (< 5 days), there was little reduction in the concentration of cesium ion, i.e. little sorption of Cs+ by the minerals. Removal of Cs+ from solution was enhanced by increased mineral surface area, reaction temperature and time. It was observed that in the extreme case for powdered labradorite, 98% of an initial 10 2 mol dm 3 solution of Cs+ was sorbed after 14 days at 200°C in distilled water. The morphology, composition and chemical structure of the mineral surfaces were investigated by several analytical methods, as described below. 

Plagioclase feldspars Albite, oligoclase, andesine, labradorite, bytownite, anorthite... 

Figure 5.22 Polarized absorption spectra of iron in plagioclase feldspar (from Hofmeister Rossman, 1983). (a) Visible region spectra with peaks originating from Fe3+ ions in tetrahedral coordination (b) near-infrared spectra attributed to Fe2+ ions in the Ca sites. The labradorite, from Rabbit Hills, Oregon, contains about 0.1 wt per cent Fe203 and 0.3 wt per cent FeO. (Crystal thickness 1.0 cm.)...

Play of color is the term used to describe the internal spectral colors that appear inside some specimens of materials like opal and some synthetics. The colors seem to move when the specimen is turned or the light source is moved. Labradorescence is a broad play of colors common in labradorite and other minerals (mostly feldspars) having polysynthetic twinning. 

Release of trace elements such as strontium from feldspar is also observed to be nonstoichio-metric (Brantley et al, 1998). At pH 3, bytownite, microcline, and albite aU release strontium at an initially fast rate that slows to near stoichiometric values at steady state. In addition, aqueous strontium is enriched in Sr compared to the bulk mineral early in dissolution. All feldspars smdied evenmally released strontium in isotopic abundance roughly equal to that of the bulk mineral. Nonstoichiometric release of strontium was explained by the presence of defects or accessory phases in the minerals. Taylor et al. (2000) also reported that the initial dissolution of labradorite was nonstoichiometric during dissolution in column reactors with inlet solution pH 3, but that the mineral dissolved and released strontium stoichiometrically at steady state. In contrast to the earlier work, however, Sr/ Sr in solution did not differ from that of the bulk labradorite during dissolution in the column experiments. 

Dove (1995) further summarizes evidence suggesting that adsorption of both Al " " and Fe3+ onto quartz surfaces inhibits reactivity of that phase. Inhibition of feldspar dissolution also occurs when Al is present in solution (Chou and Wollast, 1985 Nesbitt et al, 1991 Chen and Brantley, 1997). For example, Nesbitt et al. (1991) argued that adsorption of Al " " retarded the rate of dissolution of labradorite more than other cations. Furthermore, the effect of aqueous Al " " on dissolution of albite may increase with increasing temperature due to the enhanced adsorption of cations with temperature (Machesky, 1989 Chen and Brantley, 1997 note however that Oelkers (2001b) disputes this trend). In contrast, the addition of aqueous aluminum was not observed to affect the rate of forsterite dissolution at pH 3 and 65 °C (Chen and Brantley, 2000). It may be that aqueous aluminum becomes incorporated into surfaces and affects dissolution wherever the connectedness of surface silicon atoms is >0. Brantley and Stillings (1996, 1997) and Chen and Brantley (1997) suggest that Equation (51) can be used to model aluminum inhibition on feldspars. Sverdrup (1990) has reviewed the effects of aqueous Al on many minerals and incorporated these effects into rate equations. 

Casey W. H., Westrich H. R., and Arnold G. W. (1988b) Surface chemistry of labradorite feldspar reacted with aqueous solutions at pH = 2, 3 and 12. Geochim. Cosmochim. Acta 52, 2795-2807. 

Hellmann R., Penisson J.-M., Hervig R. L., Thomassin J.-H., and Abrioux M.-F. (2003) An EFTEM/HRTEM high-resolution study of the near surface of labradorite feldspar altered at acid pH evidence for interfacial dissolution-reprecipitation. Phys. Chem. Min. 30, 192-197. 

Kroll H, Schmiemann I, von Colin G (1986) Feldspar solid solutions. Am Mineral 71 1-16 Kumao A, Hashimoto H, Nissen H-LF, Endoh H (1981) Ca and Na positions in labradorite feldspar as derived from high-resolution electron microscopy and optical diffraction. Acta Crystallogr A37 229-238... 

The surface area of labradorite increased by a factor of 30 after 318 h of reaction in pH 2.0 HCl solution (Casey et al, 1989b). The increased surface area observed under these low pH conditions resulted from the formation of a highly porous, short-range ordered polymeric Si-O precipitate several tens of nanometers thick on the surface of the feldspar. Calculations made using the adsorption/desorption isotherms indicated that most of the pores had radii between 2.0 and 8.0 nm. Because thick Si-O layers are not observed on samples that have been weathered at pH >4.0, the results of this study probably have little applicability to natural soil systems. 

Casey, W.H., H.R. Westrich, T. I,. Massis, J.F. Banfield, and G.W. Arnold. 1989b. The surface of labradorite feldspar afterijser acid hydrolysis. Chem. Geol. 78 205-218. 

The prediction by our speciation model that in multiple oxides (as in quartz) Si sites are not susceptible to adsorption by H+ is confirmed by the results of surface titrations of albite, labradorite, and anorthite (Fig. 6). One can see that the net adsorption of H + at the feldspar solution interface increases markedly with the relative number of Al sites to Si sites, which is reflected by the stoichiometry of Al and Si in the different feldspars. These results help clarify the typical features of the dependence on pH of silicate dissolution (Table 2) ... 

The arrangement of defects (as in labradorite feldspar) or of grains (as in opal) can cause diffraction of the light. 

The resulting temperature estimates in column (1) range from 613°C to 1126°C. Omitting the lowest and the highest values yields a mean of 872 112°C (let) The oxygen-isotope fractionation factors reported by Chiba et al. (1989) for feldspar-pyroxene are Albite-Diopside = 1.81, Anorthite-Diopside = 0.76. For labradorite (An = 60%, Ab = 40%) the numerical value of A is ... 

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