60 Washington and Wright — Feldspar from Linosa. 



Reckoning the small amount of potash as orthoclase with 

 the albite molecule, the ratios of albite, anorthite and sodium 

 alumino-metasilicate approximate closely to whole numbers, and 

 are almost exactly 8 : 10 : 1. The labradorite would thus have 

 the composition Ab 4 An 5 , while we have seen that the optic 

 data indicate that the mineral is actually somewhat more sodic, 

 from about AbjAnj to Ab 3 An 2 . 



The molecule Na 2 Al 2 Si 2 8 is that of potash-free nephelite, 

 which does not seem to occur in nature, but which has been 

 made artificially in small hexagonal crystals, much like those 

 of nephelite, and with a specific gravity of 2*555.* If this 

 mineral were present it would necessarily be as a mechanical 

 mixture or as what has been termed f an "anomalous solid 

 solution," since it is not crystallographically isomorphous 

 with the triclinic albite and anorthite, and true solid solution or 

 a mixed crystal, containing such an amount of the subordinate 

 mineral as shown above, would hardly be expected in such 

 dissimilar minerals. 



True solid solution could take place, however, if the mole- 

 cule Na 2 Al 2 Si 2 8 is dimorphous, and a second form exists whose 

 symmetry relations approximate those of anorthite and albite. 

 The two formulas 



Anorthite, CaO.Al 2 3 2Si0 2 

 Nephelite, Na 2 O.Al 2 3 .2Si0 2 



are identical, except that in the second Na 2 replaces the CaO 

 of the first, and it is not out of the range of possibility that 

 a soda anorthite should exist. This mineral is not yet 

 known to occur in nature, but its presumable characters 

 would harmonize the conflicting data. Thus, it should be' 

 triclinic and isomorphous with albite and anorthite, and there- 

 fore capable of forming mixed crystals with these analogous to 

 the ordinary plagioclase series. Similarly, it would presumably 

 possess optic characters more sodic, that is, more like those of 

 a soda-lime feldspar, than those of purely calcic anorthite ; so 

 that we would thus have an explanation of the fact that, while 

 the relations of CaO and Na 2 in our mineral are those of 

 AbjAnj, certain of the optic characters are those of a more 

 sodic plagioclase. Since the specific gravity of nephelite, and 

 presumably also of the soda anorthite, is less than that of 

 anorthite, the density of the mixed crystal should be less than 

 that of the equivalent plagioclase in which no soda anorthite 

 occurs, and we have seen that, while the normal plagioclase 

 present is about Ab 4 An 5 , which would have a density of 2*698, 

 the density of our mineral is that of AbjA^ or Ab 5 An 4 . 



* Cf . Hintze, Mineralogie, vol. ii, p. 97. 



f A. Johnsen, Neues Jalirbuch, 1903, ii, p. 93. 



