of Chondrodite, Humite and Clinohumite. 205 



related to the members of this group and, as shown by vom 

 Hath,* a few of its forms are almost identical with those of 

 humite. Their relation is shown on page 188 where the axial 

 ratio o : 2a : c of chrysolite is similar to a : b\c of clinohumite. 

 As chrysolite contains no fluorine or hydroxyl it deviates con- 

 siderably in its chemical type from the members of the 

 humite group and its crystalline habit is also different, as the 

 majority of its common forms do not correspond to any of the 

 forms of humite. Of the three species constituting the 

 humite group chondrodite has the simplest composition and is 

 the most common, clinohumite has the most complicated com- 

 position and is the rarest, while humite occupies an interme- 

 diate position. At Mte. Somma chondrodite, which is the 

 most basic, occurs usually with calcite, while clinohumite, 

 which is the most acid, is usually associated with the silicate 

 forsterite. 



Of the formulae proposed by other investigators that sug- 

 gested by Eammelsberg and vom Rath, Mg 5 Si 2 9 , is similar to 

 the one deduced by us for chondrodite. They considered that 

 part of the oxygen was replaced by fluorine, while we have 

 shown that one atom of oxygen is replaced by the isomorphous 

 fluorine and hydroxyl. The formula proposed by Wingard, 

 Mg 13 [MgF] 4 [Mg(OH)] 2 [Si0 4 ] a or Mg 19 (F . OH) 6 (Si0 4 ) 8 , is not 

 very different from ours for humite, it being equal to three 

 times the latter, Mg 21 (F. OH) 6 (Si0 4 ) 9 , less Mg 2 Si0 4 . Wingard 

 derived his formula from the analysis of humite given below 

 which, like the accompanying analyses of chondrodite, have 

 been changed from their original shape only by recalculating 

 FeO as MgO and bringing the total to 100 per cent. His 

 humite analysis, naturally, agrees very well with his theory, 

 but it also agrees quite well with the author's, while his chon- 

 drodite analyses agree very indifferently with his theory. 



Humite. Chondrodite. 



Author's theory for Wingard's theory. Anal. IV, Anal. Ill, Anal. IV, 

 humite F: OH = 2: 1. F : OH =2:1. p. 200. p. 197. p. 197. 



Si0 2 37-45 36-87 36'63 34-21 35-01 



MgO 58-25 58-38 58-62 61'33 60-29 



F 5-27 5-84 5-77 5-29 5'81 



HO 1-25 1'37 1-41 1-40 1-36 



102-22 102-46 102'43 102-23 102-47 



Oeq. toF_. 2-22 2'46 2-43 2*23 2-47 



Our formulae for the different members of the series do not 

 agree in any single instance with those proposed by Sjogren, 

 and yet his formulae for the two upper members, humite and 

 clinohumite, are identical with ours for chondrodite and 

 humite respectively, as shown by the following : 



*Pogg. Ann. Erganz., Band V, p. 412, 1871. 



