W. F. Hillebrand — Composition of Yttrialite. 145 



Art. XIII. — The Composition of Yttrialite with a Criticism of 

 theformtda assigned to Thalenite ; by W. F. Hillebrand. 



In a paper describing the new yttrium silicate thalenite,* 

 the author, C. Benedicks, casts doubt on the formula assigned 

 by Hidden and Mackintoshf to their mineral yttrialite from 

 Llano Co., Texas, for which they deduced the formula B 2 3 

 2Si0 2 or B 2 Si 2 7 , in which B 2 3 includes the sesquioxid 

 equivalents of very considerable percentages of monoxides and 

 dioxides. Benedicks would assign yttrialite to a group of 

 which rowlandite is the prototype and to which he believes 

 thalenite and kainosite belong, being either plain basic salts of 

 H 6 Si 2 7 of the. type B" B //r 4 Si 4 15 or derivatives in which 

 the fifteenth oxygen atom is replaced by its equivalent in fluor- 

 ine (rowlandite) or C0 3 (kainosite), as shown below. 



H 2 < > 

 Y = Si 3 0,-Y/ 



Thalenite. 



Fe< 2 ' \o 

 Y = Si g O,=Y/ 



Yttrialite. 



Y = Si 8 7 =Y-Fl 

 Fe< 



Y = Si g O,=Y-Fl 



Rowlandite. 



Ca = Si 2 G 7 =Y x 

 H 4 ^ ' )C0 3 

 Ca = Si 2 0,^Y/ 



Kainosite. 



Benedicks says : " Dem Yttrialit, von Hidden and Mackin- 

 tosh beschrieben, sollte die Formel B 2 3 , 2Si0 2 zukommen, 

 worin B hauptsachlich Ytter-und Thorerde ist. Dabei wird 

 aber ca. 4$ Eisenoxydul in der Analyse vernachlassigt. Wird 

 dies nebst etwas Kalk und Bleioxyd mitgerechnet, so bekommt 

 man die Formel Fe"0, 2B 2 3 , 4Si0 2 , analog mit der des 

 Tbalenits, welche besser die Zusammensetzung des Yttrialits 

 wieclergibt, obgleich die Ubereinstimmung gar nicht gut ist." 



It is, however, not true that Hidden and Mackintosh neg- 

 lected to take account of the ferrous iron, etc., of their analysis. 

 It was regarded in the derivation of their empirical formula 

 B 2 Si 2 7 . How wholly unwarranted was the substitution by 

 Benedicks of his formula for yttrialite is shown by the molecu- 

 lar ratio for BO : B 2 3 : Si0 2 , which he gives as 1:2:4 

 instead of 1 I 3*25 : 7*42 as calculated by me from Mackintosh's 

 figures, wherein for a sound reason I have converted his U0 3 



* Bull. Geol. Inst. Upsala, iv, 1, 1898. f This Journal, xxxviii, 477, 1889. 



A.M. Jour. Sci. — Fourth Series, Vol. XIII, No. 74. — February, 1902. 

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