Gahnite

Rock samples collected from archived core, mine workings and outcrop were pulverized, homogenized, then analyzed using four-acid dissolution (SGS Lab, Toronto) to determine the near-total lithogeochemical composition (cassiterite, rutile, monazite, zircon, sphene, gahnite, chromite and barite are partially dissolved). Gold analyses were done by Fire Assay with Atomic Absorption finish on 30g samples and have a detection limit of 5 ppb. 

Zinc occurs in nature, widely distributed. The principal ores are sphalerite (and wurtzite) known as zinc blende, ZnS gahnite, ZnAl204 calamine smith-sonite, ZnCOs franklinite, ZnFe204 and zincite, ZnO. Abundance in earth s crust is about 70 mg/kg and average concentration in sea water is about 10 pg/L. 

According to J. Papish and C. B. Stilson, the zinc minerals sphalerite, gahnite, hopeite, parahopeite, and adamite all contain gallium (44, 46). It has also been detected spectroscopically in certain French, Spanish, and Japanese mineral waters (47, 48). Germanite from Tsumeb, Southwest Africa, contains from 0.57 to 1.85 per cent of gallium and is thus a rich source of this rare metal (44,49). 

Sulphate op Zinc,—Goalarite, mkvdriol, while. vitriol, aluminate of nine, gahnite epinelU, eincif re, and a few other minerals of zinc exist but their importanoe in relation to their abundance, or for their application in the metallurgy of -zinc, is not so great as to require a description here. 

Fig. 5.24. Variation of hardness Hy with volume fraction Vf of gahnite crystals in glass-ceramic.

The powdered crystalline samples of Cr " doped ZnAl204 gahnite were prepared for comparison from stoichiometric ratios of Zn(N03)2, Cr(N03)3 and A1(N03)3.9H20 precipitated by aqueous ammonia, and the mixed hydroxides calcined. The lattice constant a of the crystalline ZnAl204 was found by X-rays to be 8.085 A. 

The excitation spectra of gahnite glass-ceramics due to A2 T2 broad-band... 

The variety of emission lines in the gahnite-like glass-ceramics and the possibility to ascribe these lines to various Cr " centers can be a useful tool in following the mechanisms of the nucleation of glass-ceramics doped by Cr(III). The high quantum efficiency of Cr(III) emission in glass-ceramics and the proximity of T2 and levels in the distorted sites may have importance in designing tunable lasers. 

Fig. 6. Time-resolved spectra of Cr in gahnite glass-ceramics at Xh = 570 nm and temperature 4.4 K...

Therf-spacing of standard sample (JCPDS 71-0968) signed hyds- The lattice parameters a of the cubic gahnite have been calculated from the experimental lattice spacing with the expression relative to the cubic system ... 

Zinc is widely distributed in nature. Its abundance in the earth s cmst is 0.02%. Principal ores are zinc blende (ZnS), gahnite (ZnAl204), franklinite ([(Zn,Mn,Fe)0 (Fe Mn2)03]), calamine (ZnCOs), willemite (Zn2Si04), and smithsonite (2Zn0-Si02 H20). Soil levels range from 10 to 300 (JLg/g [4]. Reported levels in air are 0.01-0.2 xg/m in nonurban and 0.01-0.84 xg/m in urban areas [5]. 

From 20 to 1200 °C franklinite, gahnite, spinel and chrysoberyl show no thermal events. 

---