558 Washington — Constitution of Some Salic Silicates. 



A closelv analogous substance is " sigterite,''' originally de- 

 scribed by Rammelsberg* as a new plagioclase, but which was 

 later shown by Tennef to be an intimate mixture of albite 

 and nephelite, though with well-defined cleavage and postsyn- 

 thetic twinning lamellae. Hammelsberg's analysis yields the 

 composition 26 orthoclase, 13 albite, 60 nephelite, or (f or 

 •J ab) 2 ne s . He assigns to the supposed feldspar the formula 

 (Na, K).,Al,Si 3 O 10 , and it is noteworthy that this is closely like 

 the formula which represents the composition of the Linosa 

 anemousite,i (Na 2 ,Ca)Al„Si 3 O 10 , which has been shown to be a 

 solid solution or mixed crystal of labradorite (Ab 4 An B )and car- 

 negieite, the triclinic form of Na 2 Al 2 Si 2 8 . 



The fact that the stable mixed crystals of anemousite contain 

 only 5*6 per cent of the carnegieite molecule, while sigterite, 

 with about 60 per cent of the hexagonal form of the same 

 molecule, evidently represents a mixed crystal which is unstable 

 at ordinary temperatures, is in harmony with the suggestion 

 made in the paper cited as to the limited miscibility of the iso- 

 dimorphous ]STa 2 Al 2 Si 2 O e and CaAl 2 Si 2 8 ; and serves to confirm 

 our views as to the comparative instability of the triclinic form 

 of the former. Taking these facts and the observed cleavage 

 and twinning into consideration, there is little doubt that sigter- 

 ite represents an original mineral, with triclinic plagioclase-like 

 form, to which the name sigterite would be applicable, but 

 which is unstable at ordinary temperatures. 



A somewhat similar case is the alteration product of spodu- 

 mene, the so-called yS-spodumene, described by Brush and 

 Dana.§ This is an intimate mixture of 67'C> albite and 32 - l 

 eucryptite, and may be expressed by the formula JSTaLi Al 2 Si 4 12 . 

 This does not seem to be a case of metastability, as with the 

 preceding, but rather due to replacement through weathering. 

 It will be discussed in a later paper on the constitution of the 

 pyroxenes. 



Isomorphism. — Isomorphism is the relation between two or 

 more crystalline compounds of identical type of chemical 

 formula and essential agreement in crystal form ; the con- 

 stituent atoms being replaceable by elements or atomic groups 

 of the same valency and similar chemical character and func- 



* Kamrnelsberg, Neues Jahrb., vol. i, J 890, p. 71. 



fTenne, Neues Jahrb., vol. ii, 1897, p. 206. 



JThis Journal, vol. xxiv, 1910, p. 57. Professor F. Zirkel (letter dated 

 10, III, 1910) kindly called my attention to the fact that the composition of 

 some of the feldspars of the Monte Amiata toscanites. described and analyzed 

 by J. F. Williams (Neues Jahrb. Beil. Bd., vol. v, 1887, p. 419-421), is closely 

 like that of the Linosa feldspar. The analyses are not satisfactory, having 

 been made on small amounts of material and the silica determined by differ- 

 ence, but the general resemblance is striking, and they all calculate out with 

 notable amounts, up to 13 per cent, of the Na 2 Al 2 Si 2 8 molecule, presumably 

 present as carnegieite. 



§Cf. Dana, Syst. Min., pp. 368 and 426, 1892. 



