Minerals hemimorphite

The rule reqiiires that in silicates the silicon tetrahedra share no elements with each other if the oxygen-silicon ratio is equal to or greater than four. This is found t o be vrue in general (topaz, zircon, olivine, other orthosilicates), Most of the few exceptional substances now known contain extra oxygen atoms in the form of hydroxide ions. These include the clay minerals, micas, and chlorites, discussed above, and also the mineral hemimorphite, Zn[Pg.561]

As for the lead oxide ore types, there are a variety of zinc oxide ores. There are three basic ore types of economic value (a) ores with hemimorphite as the predominant zinc oxide minerals, (b) ores with smithsonite as dominant zinc oxide minerals and (c) ores with a mixture of smithsonite and willemite. Typical mineral compositions of this ore are shown in Table 20.1. 

Research work conducted at the University of Liege was carried out on several ore types that contained hemimorphite and smithsonite. The mercaptan-type collectors were effective with both minerals in the presence of sulphidizer. Although mercaptans floated oxide zinc minerals, the recoveries were not satisfactory. Later in the research work, amine-type collectors were introduced to flotation of oxide zinc minerals with significant improvement in metallurgy over that obtained with mercaptans. 

The zinc oxide ores of any economic value are represented by smithsonite and calamine. Willemite, franklinitc and other zinc oxide minerals are quite rare. The gangue minerals are usually represented by calcite ferooxides, dolomite and hemimorphite. The composition of gangue minerals, however, varies considerably and may also contain clay, talk, Fe-hydro-xide and other minerals. 

This structure is similar to those of the isoelectronic species P2074-, S2072-, and C1207 (the pyrophosphate and pyrosulfate ions, and dichlorine heptoxide). The minerals thortveitite, Sc2Si207, and hemimorphite, Zn4(0H)2Si207, contain the Si2076- unit, which has the structure shown earlier. 

Zinc is a blnish-white metallic element (symbol Zn atomic no. 30) that is found in minerals such as frankli-nite, ghanite, goslarite, hemimorphite, smithsonite. 

Minerals containing the pyrosilicate or disilicate anion, Si O (Fig. 16.1b). are not common thortveitite, Sc SiiO. hemimorphite, Zn4(OH)2Si 07-H2O (does not contain discrete SijO ions), and barysilite, MnPbxISiiOy), as well as vesuvianite and epidote which contain both SiOj" and SiiO " ions. Linear tri- and tetrasilicates are almost unknown. 

Silicate Modifications. A method has been described in which silicate minerals are simultaneously acid-leached and trimethylsilyl end-blocked to yield specific trimethylsilyl silicates having the same silicate structure as the mineral from which these were derived (12). Olivine, hemimorphite, sodalite, natrolite, laumontite, and sodium silicates are converted to TMS derivatives of orthosilicates, pyrosilicates, cyclic polysilicates, etc, making it possible to classify the minerals according to their silicate structure. The same technique is used to analyze the siloxanol structure of aqueous solutions of vinyltrimethoxysilane (13). Certain anionic siliconates stabilize solutions of alkali silicates to give stable solutions in water or alcohols at any pH (14). Such silicate—siliconate mixtures are used as corrosion inhibitors in glycol antifreeze (15) (see Antifreezes and deicing fluids Corrosion and CORROSION CONTROL). 

This metal Stromeyer named cadmium, since cadmia is an old name for calamine or zinc carbonate, derived from the Lat. calamus reed, in allusion to its slender stalactitic forms. To avoid confusion it should be mentioned that American mineralogists know natural zinc carbonate under the name of smithsonite, after Smithson, who founded the Smithsonian Institute at Washington, and who analysed the mineral in 1803. Unfortunately, the Americans use the term calamine to designate our hemimorphite, ZnjSiCVHjO, or more probably Zn(0H)2"Zn,Sia07.Ha0, since one half of the... 

The abundance of zinc in the Earth s crust averages 70 g ton . Zinc is a chalcophUic element like copper and lead, and a trace constituent in most rocks. Zinc rarely occurs naturally in its metallic state, but many minerals contain zinc as a major component from which the metal may be economically recovered. The mean zinc levels in soils and rocks usually increase in the order sand (10-30 mg kg ), granitic rock (50 mg kg ), clay (95 mg kg ) and basalt (100 mg kg ) (Adriano 1986, Malle 1992). Sphalerite (zinc blende, cubic ZnS) is the most important ore mineral and the principal source for zinc production. Smithsonite (ZnGOj) and hemimorphite (Zri4(Si207) (0H)2 X H2O) were mined extensively before the development of the froth-flotation process (Jolly 1989, Graf 2002). The main impurities in zinc ores are iron (1-14%), cadmium (0.1-0.6%), and lead (0.1-2%), depending on the location of the deposit (ATSDR 1994). 

The main mineral from which zinc is extracted is the sulfide ZnS (sphalerite), although in some deposits the carbonate ZnC03 (smithsonite) and the silicate Zn4(OH)2Si20rH20 (calamine or hemimorphite) may also be abundant. 

How would you identify a mineral believed to be either smithsonite (ZnCOs) or hemimorphite (Zn3Si207) ... 

The relative abundance of zinc in the Earth s crust is about 75 mg/kg (i.e., ppm wt.), less than vanadium, nickel, and chromium. Major zinc minerals are the sulfide sphalerite or blende [ZnS, cubic], smithsonite [ZnCOj, trigonal], and hemimorphite or calamine [Zn SijO/OHjj.HjO, monoclinic] and franklinite [ZnFe O, cubic]. Of these mainly the sphalerite is used as zinc ore. Moreover, in ore deposits, sphalerite is often associated with galena and pyrite, and because it contains nonnegligible amounts of impurities such as Cd, In, and Ge, sphalerite is the principal source of these metals as byproducts. In 2002, the world zinc concentrate production totaled ca. 8.728 x 10 tonnes of contained zinc. With roughly 233 mines operating worldwide, the Asia-Pacific region, with 108 mines and 3.794 X 10 tonnes per annum, and the Americas, with 82 mines and 3.803 x 10 tonnes annualy,... 

This kind of structure is seen in the mineral thortveitite SCjSijO,. The Sc ions are octahedrally coordinated. Another example is hemimorphite Zn (OH)2SijO. ... 

As mineral examples, hemimorphite Zn,Si207(0H)2 (Figure M56), described in Chapter 17, and epidote (Figure M57) may be mentioned. 

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