188 Penfield and Howe — Chemical Composition 



Art. XVIII. — On the Chemical Composition of Chondrodite, 

 Humite and Clinohumite • by S. L. Penfield and W. T. 

 H. Howe. 



Introduction. — These minerals, which are regarded collec- 

 tively as the humite group, have been the subject of repeated 

 crystallographic and chemical investigation. For our knowl- 

 edge of their crystallization we are indebted to such careful 

 and accurate observers as Hauy, Phillips, G. Rose, Levy, 

 Miller, Hausmann, Hessenberg, A. Scacchi, vom Rath, Nor- 

 denskiold, Ivokscharow, J. D. and E. S. Dana, C. Klein, Des 

 Cloizeaux and Hj. Sjogren, whose names are familiar to all 

 workers in crystallography and mineralogy. It is not the pur- 

 pose of this article to take up the details of the crystallization 

 of these minerals or to review the progressive steps by means 

 of which we have derived our present knowledge of their 

 highly modified and complicated crystals, but reference can be 

 made to the excellent historical sketch in a recent number of 

 Dr. Hintze's Mineralogy, page 370. In the description of the 

 crystals that were examined during the course of our investi- 

 gation we shall use essentially the same system of lettering 

 and of crystal notation adopted by A. Scacchi* and E. S. 

 Dana.f 



In the humite group three distinct species are at present 

 recognized, each characterized by the occurrence of certain 

 forms which are not found on the others and having the fol- 

 lowing axial relations : 



Chondrodite, Monoclinic, a:b:c = 1-08630 : 1 : 3-14472 (3 = 90j 

 Humite, Orthorhombic, a : b : c = i -08021 : 1 : 4-40334 /?= 90§ 



Clinohumite, Monoclinic, a:b:c= 1-08028 : 1 : 5-65883 (3 = 90| 



In the above the a axes are practically alike, while, as shown 

 by Scacchi and vom Rath, a simple relation exists between the 

 vertical axes, that of chondrodite being -f ths and that of humite 

 -|ths the length of the clinohumite axis. These relations are 

 shown in the following table, to which the axial ratio of 

 chrysolite, a closely related mineral, has also been added. 



Chondrodite a: 



Humite . a : 



Clinohumite a: 



Chrysolite b : 



*Pogg. Ann, Erg. B., iii, p. 161, 1851. 



f Mineralogy, Sixth Edition, p. 535. Trans. Conn Acad,, iii, p. 



% E. S. Dana, loc. cit. 



& A. Scacchi, loc. cit. 



|| vom Rath, Pogg. Ann., Erg. B., v, p. 373, 1871. 



b 



\c — 1-08630 



1 



0-62894 



b 



^c— 1-08021 



1 



0-62905 



b 



\G= 1-08028 



1 



0-62876 



2a 



c= 1-0735 



1 



0-6296 



