Minerals labradorite

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 They are derivatives of silica in which about one-half or one-quarter of the silicon atoms have been replaced by aluminum.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. 

FTIR Spectra. After reaction, the mineral samples were subjected to ultrasonic treatment in acetone for periods of up to 1 hour. Transmission IR spectra of the released product fines from reactions of 60-mesh labradorite and microcline with 10 2 mol dm 3 CsCl in distilled water at 150 C for 47 days are presented in Figure 3. The spectrum obtained from of the labradorite reaction product (Figure 3(a)) is identical with that of a sample of natural pollucite (Figure 3(c)). The spectrum obtained from the microcline reaction product (Figure 3(b)) contains bands in the 500 cm""1 to 800 cm"1 region, due to unreacted microcline, in addition to the major bands of pollucite. Heating the samples overnight at 105 C resulted in the disappearance of the water absorption bands at about 3500 cm 1 and 1680 cm 1. 

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. 

All of these minerals are used in ceramics and refractories, and labradorite is sometimes used as a building stone. 

Beran A. (1987) OH groups in nominally anhydrous framework structures an infrared spectroscopic investigation of danburite and labradorite. Phys. Chem. Mineral. 14, 441-445. 

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. 

J. Arndt, W. Hummel, and I. Gonzalez-Cabeza, Displectic labradorite glass from the Manicouagan impact crater I. Physical properties, crystallization, structural and genetic implications. Phys. Chem. Minerals 8, j. 230—239 (1982). 

K.L. Geisinger, A. Navrotsky, and J. Arndt, Enthalpy of diaplectic labradorite glass. Phys. Chem. Minerals 13, pp. 357-359 (1986). 

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... 

Hochella, M.F., Jr., H.B. Ponader, A.M. Turner, and D.W. Harris. 1988. The complexity of mineral dissolution as viewed by high resolution scanning Auger microscopy Labradorite under hydrothermal conditions. Geochim. Cosmochim. Acta 52 385-394. 

Initial mineral grains were ground in a jaw crusher. The plagioclase and the apatite are sieved at 0.5-1 and 1-2 mm, respectively. The 0.5-1 mm fraction was magnetically sorted in order to remove grains containing ilmenite. Apatite and labradorite grains were also treated ultrasonically and washed with distilled water in order to remove the fine particles. 

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). 

The feldspar minerals have similar physical properties and often occur as prismatic or tabular crystals in igneous rocks, or as more anhedral grains in metamorphic and sedimentary rocks. They are colourless when fresh but are more commonly white due to incipient alteration impurities or inclusions result in coloured varieties, with green-brown alkali feldspars found in some metamorphic rocks, and orthoclase commonly found as pink. The surfaces of feldspar crystals are often iridescent due to twinning on a microscopic scale, with labradorite characterised by blue surface iridescence. Feldspars readily alter under hydrothermal action or chemical weathering to form members of the clay minerals group (. v.). Sodium-rich feldspars commonly decompose to form montmorillonite, in the presence of limited water, or to kaolinite with excess water alkali feldspars typically form illite or kaolinite sub-group (qq.v.) clay minerals (Deer et al, 1992 Rutley, 1988). 

FELDSPAR. The most common mineral in crystalline rocks. Hardness 6.0-6.5 Mohs. Usually occurs as small grains intimately associated with other minerals, but commercial deposits are obtained from pegmatites. Feldspars form a group of which the principal types are potash spar (ortho-clase, microcline), soda spar (alhite), lime spar (anorthite), and lime-soda spar (ohgo-clase, andesine, labradorite and bytownite). They are aluminum sdicates of potassium, sodium and calcium. 

Casey WH (1991) On the relative dissolution rates of some oxide and orthosilicate minerals. J Colloid Interface Sci 146 586-589 Casey WH, Westrich HR (1992) Control of dissolution rates of orthosilicate minerals by local metal-oxygen bonds. Nature 355 157-159 Casey WH, Carr MJ, Graham RA (1988a) Crystal defects and the dissolution kinetics of rutile. Geochim Cosmochim Acta, 52 1545-1556 Casey WH, Westrich HR, Arnold GW (1988b) The surface chemistry of labradorite feldspar reacted with aqueous solutions at pH = 2, 3 and 12. Geochim Cosmochim Acta 52 2795-2807... 

---