DALY. — ETCH-FIGURES ON AMPHIBOLES. 415 



pared ou account of the roughness of the general crystal-surface. They 

 are not pentagonal but triangular ; two edges, A and B C (Fig. 10), 

 meet at an angle of 80° or more, the apes of which points forward on the 

 crystal. Tlie edges A C and B O are quite straight, and their respective 

 figure-faces are plane. On the other hand, the figure-face A B B' A' is 

 curved and is predominant as a curve even when A A' C and BBC are 

 not well developed. The bottom plane of the pit is a plane parallel to 

 (TOl) and diminishing in size as the pit matures. AAG and BBC 

 are steeper (darker) than A BB A', but are of equal obliquity to (TOl), 

 thus agreeing with the other characteristics of the pit in supplying 

 perfect monosymmetry for the figure with respect to (010). 



The differences in the etclvpits on the positive unit-dome of the acti- 

 nolitic species as contrasted with the corresponding pits on hornblendes 

 is striking in face of the fact that we have already seen exemplification 

 of even greater contrasts between the mineral groups in the behavior of 

 the unit prism and the clinopinacoid during the same process of etching. 



Isomorphism in the Amphiboles. 



Chemical crystallographers are of different opinion regarding the value 

 of etch-figures in determining isomorphism. Arzruni, on the one hand, 

 denies any necessary relationship between them and the fact of isomor- 

 phic mixture ; * in this he is supported by Baurahauer, who regards the 

 pits on dolomite, calcite, and siderite as not indicating lack of isomor- 

 phism, although their orientation on corresponding faces of all three 

 species is widely different, f The opposite view has been strongly main- 

 tained by Retgers in his recent and valuable research on the subject. J 

 Retgers gives his three critei'ia of isomorphism as follows: — 1. Mix- 

 tures of the constituent salts of an isomorphic series must take place 

 in all proportions. 2. There must be a lack of chemical combination 

 in the mixture : thus diopside (CaMgSiOg) is not a member of the 

 isomorphic series (CaSiOg and MgSiOg), but an independent body. 

 Likewise manganese augite (MnSiOg) and the manganolime augite 

 (MnCaSiOa) are not isomorphic. 3. Etch-figures produced on the 

 same crystallographic plane of all members of the series will be alike 

 both in shape and symmetry. To support the last statement, he cites a 



* Phys. Chemie der Krystalle, 1893, p. 162 et seq. 

 t Resultate der Aetzmethode, p. 37. 



X Beitrage zur Kenntniss der Isomorphismus. Zeit. fiir phys. Chemie, 1895, 

 Bd. XVI. p. 36. 



