Washington and Wright — Feldspar from Linosa. 57 



The ratios of the Linosa mineral are very close to whole 

 numbers, though there is a slight excess of silica. Calculating 

 the small amount of potash with the soda, the figures of the 

 analysis correspond to the formula Na 2 0.2Ca0.3Al 2 3 .9Si0 2 , 

 which, simplified, becomes (i^N"a 2 ,-§Ca)Al 2 Si 3 O 10 . The percent- 

 age composition of this molecule is given above. 



No anhydrous silicates with formulas corresponding to this 

 appear to be known independently, but several zeolites are 

 analogous, namely: wellsite, (K a ,Ca,Ba)Al 2 Si 8 O 10 +3H a O; eding- 

 tonite, BaAl 2 Si 3 'O 10 + 3H 2 O; natrolite, Na 2 Al 2 Si 3 O 10 +2H 2 O; 

 and a potassium natrolite observed by Pirsson"* in missourite 

 with the approximate composition (K 2 Ca)Al 2 Si 3 O 10 -f 2H 2 0. 

 Indeed, the composition of the Linosa mineral is exactly that 

 of a mesolite, (mNa 2 Al 2 Si 3 O 10 .2H 2 O+nCaAl 2 Si 3 O 10 .3H 2 O), with 

 !Na 2 : CaO — 1:2, and lacking the water. 



Compounds of this type can be regarded as salts of the 

 alumo-trisilicic acid (H 2 Al 2 Si 3 O 10 ) of Morozewicz,f the potas- 

 sium salt of which he considers as present in nephelite, 

 with varying proportions of the sodium alumo-disilicate, 

 (E"a 2 Al 2 Si 2 8 ). .Reduced to still simpler terms they would be 

 salts of the acid H 8 Si 3 O 10 , for which Vogt;j; proposes the name 

 pyrosilicic acid. He refers akermanite and gehlenite to this 

 simple formula, as does Tschermak § the mineral melilite. 



Comparing the composition of our mineral with those of the 

 two plagioclases, Ab x An a and Ab 3 An 2 , which it closely resembles 

 in its physical properties, it will be seen from the table above 

 that Ab 1 An 1 shows closely concordant figures for lime and 

 soda, while silica is distinctly higher and alumina lower ; and 

 that, on the other hand, Ab 3 An 2 shows much higher silica and 

 slightly higher soda, but lower lime and alumina. In fact a 

 composition satisfactorily close to that of the Linosa mineral 

 as regards all the constituents, and furnishing like ratios, 

 cannot be calculated from mixtures of the albite and anorthite 

 molecules. 



The relations may be better seen in the respective ratios, as 

 shown when the formulas are compared, as follows : 



Linosa mineral • = Na 2 0.2Ca0.3Al 2 3 .9SiO . 



Labradorite (A^AnJ = Na 2 O.2CaO.3Al 2 O 3 .10SiO 2 . 

 Andesine (Ab 3 An 2 ) = 3Na 2 0.4Ca0.7Al 2 3 .22Si0 2 . 



Discussion. 

 The data given in the preceding pages make it clear that the 

 physical and chemical characters of our mineral are at variance. 

 The crystal system, twinning laws, cleavage, and hardness are 



* Weed and Pirsson, this Journal, ii, p. 320, 1896. 

 f Morozewicz, Bull. Acad. Sci. Cracov., p. 999, 1907. 

 % Vogt, Mineralbildung in Schmelzmassen, p. 162, 1892. 

 § Tschermak, Lehrbuch der Mineralogie, p. 523, 1905. 



