Cassiterite

In order to find the impurity responsible for the yellow emission, artificial Sn02 have been studied activated by different impurities such as Ti, Nb, Ta, W, Fe (Gaft et al. 1982). From crystallochemical positions such ions are capable of substituting for Sn. Nevertheless, the correlation of luminescence with 

We studied laser-induced time-resolved lirminescence of synthetic BaS04 artificially activated by Sn and foimd several intensive UV and blue bands evidently connected with Sn emission (Fig. 5.55). Similar bands have been also found in natural barite laser-induced luminescence spectra and we think that their connection with an Sn center is quite possible. 

This luminescence center is not detected in minerals yet, but is well known in synthetic phosphors, for example in halophosphates, which are closely related to hydroxyapatite (Blasse and Grabmaier 1994). The Sb doped calcium halophosphate is a very efficient blue-emitting phosphor under short wave 254 excitation (Fig. 5.56). When the halophosphate host lattice contains not only Sb, but also Mn, part of the energy absorbed by the Sb ions is transferred to Mn , which shows an orange emission. By carefully adjusting the 

Another example is the luminescence of in YPO4 with tetragonal zircon structure (Oomen et al. 1988). The emission consists of two bands, one in the UV region and one in the visible part of the spectrum. The intensity ratio of these bands is strongly temperature dependent (Fig. 5.57). 

The possibility of Cr emission may be excluded because octahedral coordination is absent in barite structure. Luminescence of Fe is crystallographi-cally possible because tetrahedral surrounding presents in barite structure but Fe -S substitution is very difficult to suppose. The iron presence in ICP data is evidently connected with micro-impurities of iron minerals, which is usual for natural barite. Other ions, such as Ti and Ni with possible red luminescence, have ionic radii of 81 and 83 pm, respectively, which are small compared to the 156 ppm of Ba . ICP data confirm the absence of Ni in barite, while the minor quantities of Ti maybe connected to Ti .  

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The common ore of tin is tinstone or cassiterite. Sn02, found in Cornwall and in Germany and other countries. The price of tin has risen so sharply in recent years that previously disregarded deposits in Cornwall are now being re-examined. Tin is obtained from the tin dioxide, Sn02, by reducing it with coal in a reverbatory furnace ... 

Before this treatment, the cassiterite content of the ore is increased by removing impurities such as clay, by washing and by roasting which drives off oxides of arsenic and sulphur. The crude tin obtained is often contaminated with iron and other metals. It is, therefore, remelted on an inclined hearth the easily fusible tin melts away, leaving behind the less fusible impurities. The molten tin is finally stirred to bring it into intimate contact with air. Any remaining metal impurities are thereby oxidised to form a scum tin dross ) on the surface and this can be skimmed off Very pure tin can be obtained by zone refining. 

Tin is found chiefly in cassiterite (Sn02). Most of the world s supply comes from Malaya, Bolivia, Indonesia, Zaire, Thailand, and Nigeria. The U.S. produces almost none, although occurrences have been found in Alaska and California. Tin is obtained by reducing the ore with coal in a reverberatory furnace. 

As the temperature rises, siUca (which is present in nearly all concentrates) also reacts with cassiterite under reducing conditions to give staimous siUcate ... 

In the field, cassiterite ore is usually recognized by its high density (7.04 g/cm ), low solubiUty in acid and alkaline solutions, and extreme hardness. Tin in solution is detected by the white precipitate formed with mercuric chloride. Stannous tin in solution gives a red precipitate with toluene-3,4-dithiol. 

Stannic Oxide. Stannic oxide tin(IV) oxide, white crystals, mol wt 150.69, mp > 1600° C, sp gr 6.9, is insoluble in water, methanol, or acids but slowly dissolves in hot, concentrated alkaH solutions. In nature, it occurs as the mineral cassiterite. It is prepared industrially by blowing hot air over molten tin, by atomizing tin with high pressure steam and burning the finely divided metal, or by calcination of the hydrated oxide. Other methods of preparation include treating stannic chloride at high temperature with steam, treatment of granular tin at room temperature with nitric acid, or neutralization of stannic chloride with a base. 

The chemistry of these materials is complex (23). If one mixes about 90% of tin oxide with small amounts of Cr202 and either CaO or Ce02, together with 2 3 minerali2er, one obtains a purple or orchid shade. The crystal stmcture is cassiterite. This pigment is a soHd solution of chromium... 

Tin vanadium yeUows are prepared by introducing small amounts of vanadium oxide into the cassiterite stmcture of Sn02 (28). Tin vanadium... 

Tin occurs mainly as cassiterite, Sn02, and this has been the only important source of the element from earliest times. Julius Caesar recorded the presence of tin in Britain, and Cornwall remained the predominant supplier for European needs until the present century (apart from a minor flourish from Bohemia between 1400 and 1550). Today (1990s) world production approaches 200 000 tonnes per annum (see next section), of which the UK contributes less than... 

Sn02, cassiterite, is the main ore of tin and it crystallizes with a rutile-type structure (p. 961). It is insoluble in water and dilute acids or alkalis but dissolves readily in fused alkali hydroxides to form stannates M Sn(OH)6. Conversely, aqueous solutions of tin(IV) salts hydrolyse to give a white precipitate of hydrous tin(IV) oxide which is readily soluble in both acids and alkalis thereby demonstrating the amphoteric nature of tin(IV). Sn(OH)4 itself is not known, but a reproducible product of empirical formula Sn02.H20 can be obtained by drying the hydrous gel at 110°, and further dehydration... 

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