TURQUOISE 389 



TURQUOISE. 



This stone is referred to as oriental turquoise, true or mineral turquoise, and turquoise 

 de la vieille roche ; its little-used mineralogical name, calaite, is derived from a name used 

 by Pliny for a green stone supposed to be identical with the precious stone now under 

 consideration. Turquoise is always opaque and is usually of a green colour, the best 

 qualities only being blue. It never occurs in distinct crystals, in this respect differing from 

 all the valuable precious stones hitherto considered, with the exception of opal. It is 

 unique also in its chemical composition, since it belongs to the phosphates, a gi'oup of 

 minerals which includes no other precious stone of the first rank. 



Turquoise is a hydrous phosphate of aluminium, having the formula 2AI2O3.P2O5.5H2O, 

 which corresponds to a percentage composition of: alumina (AljOg), 47"0 ; phosphorus 

 pentoxide (PgOj), .32'5 ; water (HjO), 20'5 = lOO-O. In the analysis of actual specimens it 

 is found that these proportions are not invariable, a circumstance often observed in the case 

 of substances which do not occur as crystals. Besides these constituents turquoise always 

 contains small quantities of other substances. There is always from 1 to 4 per cent, of iron 

 oxide and from 2 to 8 per cent, of copper oxide, the presence of which is important, since it 

 is to this that the fine colour of the mineral is due. A blue oriental turquoise, probably 

 from Persia, was found on analysis by Hermann to contain : 



Per cent. 



Alumina (AljOj) 47-45 



Ferric oxide (FejOg) MO 



Cupric oxide (CuO) 2-02 



Lime (CaO) .... ... 1-85 



Manganous oxide (MnO) 0'50 



Phosphorus pentoxide (P2O5) 28 '90 



Water (H2O) 18-18 



100-00 



Professor S. L. Penfield has recently (1900) investigated the chemical composition of 

 turquoise, and has arrived at the conclusion that the copper and iron, which are always 

 present, are not accidental impurities but are essential constituents of the mineral. The 

 new formula he proposes, namely [A1(0H)2, 7e(0H.\, Cu(OH), H]g PO^, represents turquoise 

 as a derivative of ortho-phosphoric acid, in which the hydrogen atoms of the acid are 

 largely replaced by the univalent radicals, A1(0H)2, Fe(0H)2, and Cu(OH) in variable 

 amounts. 



When a fragment of turquoise is heated over a flame in a narrow tube closed at one 

 end, it decrepitates, that is to say, it flies into small fragments with a loud crackling noise ; 

 at the same time water is expelled and condenses on the cool parts of the tube. When 

 heated more strongly, for example, when ignited in a platinum crucible, a brownish-black 

 mass results, usually so incoherent that it falls to powder at the slightest touch ; in some 

 cases the fragment of mineral is directly converted by the intense heat into such a brown 

 powder. 



Turquoise by itself is infusible in the blowpipe flame; by virtue of the phosphoric 

 acid and copper oxide the mineral contains, it colours this, or any other colourless flame. 



