of Chondrocyte, Ilumite and Clinohumite. 189 



It is evident from the above that the first three minerals 

 form a crystallographic series and that all of the forms occur- 

 ring on them could practically be referred to one system of 

 axes, but by so doing the parameter relations on the vertical 

 axis would become exceedingly complicated. Chondrodite 

 and clinohumite, although their inclination /? is 90°, are mono- 

 clinic both as regards the symmetry in the development of the 

 faces and their optical properties, chondrodite showing an 

 extinction of 26°-30° and clinohumite 7f°-12|- from the ver- 

 tical axis, while humite in all of its properties is orthorhombic. 



The chemical relations of the minerals have never been 

 satisfactorily determined. This is owing partly to the fact 

 that it has been difficult to obtain pure material in sufficient 

 quantity for analysis, while the analytical difficulties in the 

 accurate determinations of silica and fluorine have not always 

 been overcome. Water in the form of hydroxyl, which is an 

 unfailing constituent of the minerals, has either been over- 

 looked or incorrectly determined. Scacchi* regarded the min- 

 erals as representing three types of crystallization of one and 

 the same chemical substance, which he designated as humite 

 type I, type II and type III. Rammelsbergf and vom Rath;f: 

 have suggested for the whole group the formula Mg 6 Si 2 9 , 

 with part of the oxygen replaced by fluorine, although they 

 recognized that the percentages of silica varied in the different 

 types. Wingard§ also, from the results of his recent analyses, 

 concludes that the three minerals have the same chemical com- 

 position, expressed by the formula Mg 19 F 4 (OH) 2 Si 8 S8 , while 

 Hj. Sjogren,| largely from a recalculation of the older 

 analyses and a consideration that water had been overlooked 

 in them, derived a separate formula for each species, as follows : 



Clinohumite Mg 6 [Mg(OH. F)\ [Si0 4 ] 3 



Humite Mg 3 LMg(OH.F)] 2 [SiOJ, 



Chondrodite Mg 4 [Mg(OH . F)] 4 [SiOJ, 



Sjogren assumes that hydroxyl is isomorphous with fluorine 

 and calls attention to the fact, already suggested by Rammels- 

 berg and vom Rath, that the three minerals show a variation 

 in their silica percentages. 



In the present investigation we have been able to examine 

 the following materials : Chondrodite from Warwick and 

 Brewster's, New York ; Kaf veltorp, Sweden and Mte. Somma, 

 Italy. Humite and Clinohumite from Mte. Somma. 



* Loc. cit. 



f Mineralchemie, p. 434, 1875. 



j Pogg. Ann., cxlvii, p. 254, 1872. 



§ Zeitschr Anal. Chem , xxiv, p. 314, 1885. 



|Zeitschr. Kryst., vii, p. 354, 1883. 



