Vol. 4] Murgoci. — Classification of the Amphiboles. 



363 



As regards the chemical composition, the ideas of G. Tscher- 

 mak are generally admitted, namely: isomorphic mixtures of 

 Na 2 Al 2 Si 4 12) (Na 2 Fe 2 Si 4 12 ), and (MgFe)SiO,. A recalcula- 

 tion of all the analyses* of glancophane and related minerals 

 (Table I) shows that Al and Na do not behave as if they were 

 in a single formula of a double metasilicate. In the gastaldite 

 of Striiver, in the glancophane of Yoshida, etc., A1 2 3 enters 

 with the coefficient 21, Na.,0 with only 10. If Al and Na are in 

 one single formula, their coefficient must be equal, which in gen- 

 eral is the case with the other glancophane analyses. We find 

 again the independence of Na 2 Si0 3 from Al 2 Si 3 0,, in the abriach- 

 anite, crocidolite in the riebeckite series, etc. 



In the same way the consideration of FeO, MgO, CaO, forces 

 us to admit the direct and independent mixture of single for- 

 mulae FeSiOg, MgSi0 3 , CaSi0 3 , as well as H„Si0 3 , the importance 

 of which Berwerth and others emphasized years ago. 



Further, Table I shows that Striiver has defended with good 



reason the view that gastaldite (I) is different from glancophane, 



although the optic properties are almost the same. It is easy to 



see that the constitution of 



f Na.Si0 3 f Na.SiO, 



| FeSiO :i | FeSiO, 



Gastaldite is -{ 2Al.Si 3 0.,; and Glancophane is { 2(MgCa)Si0 3 



(MgCa)Si0 3 | Al..SiO„ 



Il.SiO, L ±ILSi0 3 



Mg: Ca = 5:2 Mg:Ca = 6:l 



or (E'») 2 R", (Al.)- (SiO 3 ) 10 or (E'..)„ (E") 33 Al„(SiO.) s 

 where E'= Na + H and R"= Fe + Mg( + Ca) 



Gastaldites or glaucophanes containing Al only are very rare; 

 they have faintly or moderately intense colors and absorption, 

 small angle of extinction (0°-6°), large (40°-60°) angle of the 

 optic axes, with negative acute bisectrix. 



In general some Fe replaces the Al and, as we shall see far- 

 ther on, the properties vary much and the varieties have been 

 given names, as crossite, rhodusite, abriachanite. etc.f 



* In table I, are represented the analyses by the molecular proportions 

 of the constituent oxides, obtained from the percentages divided by the 

 respective molecular weights of the oxides. 



t The alkali amphibole studied by A. Johnsen (Neues Jahrbuch 1901, 

 II. p. 117) is related more nearly to gastaldite than to glaucophane, as 

 Johnsen emphasizes (see XXI Table I). It is a lime free gastaldite with 

 Al:Fe = 4:3, Fe:Mn = 2:l, and poor in alkali, a member intermediate 

 between riebeckite and gastaldite. 



