Penfield — Chemical Composition of Turquois. 347 



In the new analysis the iron was found to exist wholly in 

 the ferric condition, and therefore the iron in Church's analysis, 

 given as FeO in the original article, has been calculated to 

 Fe 3 3 to agree with the observations of the author and other 

 investigators. 



It is evident from an examination of the foregoing analyses 

 that turquois is a material which is quite uniform in its chem- 

 ical composition, so uniform in fact that it does not seem 

 reasonable to consider it as an accidental mixture of an alu- 

 minium phosphate and a copper phosphate. The presence of 

 the bivalent element copper, however, in somewhat variable 

 amounts, is not so easily accounted for if we are to consider a 

 copper phosphate as isomorphous with an aluminium phos- 

 phate. The small amount of iron is probably isomorphous 

 with the aluminium, and it is to be expected that the iron 

 phosphate would have little effect upon the color of the stone, 

 for the hydrated ferric-phosphate, strengite, and the hydrated 

 ferric-arsenate, scorodite, are both light-colored minerals. The 

 idea that the iron is present as the hydrated oxide, limonite, 

 can scarcely be entertained. 



Clarke,* in discussing the composition of turquois, states 

 that if the alumina is combined with the phosphoric acid and 

 water to form a molecule 2A1 2 3 . P 2 6 . 5H 2 0, there remains 

 an excess of phosphoric acid and water which forms with the 

 copper a salt of the composition 2CuO. P 2 5 . 4H 2 0. Turquois 

 is considered therefore by him as consisting of variable mix- 

 tures of the foregoing salts. He regards normal turquois as 

 the aluminium salt, 2A1 2 3 . P 2 5 . 5H 2 0, which he also expresses 

 as Al 2 HP0 4 (OH) 4 , and u the copper salt, to which the mineral 

 owes its color, is to be considered merely as an impurity." By 

 means of ratios it is quite easy to apply Clarke's theory to the 

 analyses as tabulated on page 346. Taking Al and Fe as a 

 basis, and establishing a ratio of P : Al + Fe : H = l : 2 : 5, as 

 demanded by Clarke's formula for normal turquois, A1 2 UP0 4 

 (OH) 4 , the ratio of the excess of Phosphorus : Cu : the excess of 

 Hydrogen can then be found. The results of the calculation 



are as follows: 



















I. 



II. 



III. 



IV. 



V. 



VI. 



VII. 



Normal ) P 

 turquois, [■ Al + Fe 

 Clarke. ) H 



•352 



•408 



•373 



•371 



•387 



•376 



•396 



•704 



•816 



•746 



•742 



•774 



•752 



•792 



1'760 



2-040 



1-865 



1-855 



1-935 



1-880 



1-980 



/ P 



•130 



•054 



•111 



•097 



•063 



•08S 



•008 



Residues. > Cu 



•108 



•066 



•096 



•098 



•080 



•094 



•083 



Sh 



•394 



•108 

 * Loc, 



•201 



, cit. 



•365 



•365 



•300 



•074 



