362 Mr. T. S. Hunt's Examinations of some Felspathic Rocks. 



ore is coarsely crystalline ; its colour and streak are iron-black, 

 and its lustre submetallic ; it affects the magnetic needle very 

 feebly. Hardness, 6 ; density, 4-56 to 4'66. Its analysis gave 

 me titanic acid, 48-60; protoxide of iron, 37-06; peroxide of 

 iron, 10-42; magnesia, 3-60 = 99-68. Disseminated through 

 portions of the ore, are small, garnet-red, translucent grains, 

 which have an adamantine lustre, a conchoidal fracture, and a 

 hardness of 6. They are found by analysis to be pure oxide of 

 titanium, and are to be referred to the species rutile or brookite. 

 We have in the rocks which have been the subject of these 

 examinations, a series of felspars in which the amount of silica 

 varies from 47-40 to 59*80 per cent., and that of the lime from 

 7-73 to 14-24 per cent., the amount of the alkalies decreasing as 

 that of the lime augments. These results only help to confirm 

 the conclusion which may be drawn from all the previous analyses 

 of triclinic felspars, that there are no defined limits for those 

 species which, like vosgite, labradorite, andesine, and oligoclase, 

 have been created between albite on the one hand, and anorthite 

 on the other. I therefore proposed some time since to regard 

 all of the intermediate felspars as mixtures of these two species, 

 which, being homceomorphous, may be supposed to crystallize 

 together in indefinite proportions. Multiplying and expanding 

 the received formulae of albite and anorthite, I represented them 

 as follows (silica being SiO, and alumina alO = (Al a O 3 ) 4-3)* : — 



Eq. wt. Density. Eq. vol. 

 Albite . . (Si 48 al 12 Na 4 )0 6 ' 4 = 1054-4^-2-62 =402-4 

 Anorthite . (Si 32 al 24 Ca 8 )O 64 = 1118-4-H2'72=405-0. 

 The composition and density of the intermediate felspars per- 

 mit us to regard them, for the most part, as mixtures of a soda- 

 albite and a lime-anorthite. In the analyses of many albites and 

 anorthites, however, we have evidence of similar admixtures ; for 

 some albites contain from 1 to 25 per cent, of lime, and anor- 

 thites from 3 to 4 per cent, of alkalies. Of a like significance is 

 the constant presence of a small amount of potash with the soda 

 of these felspars, and the magnesia, sometimes amounting to 5 

 per cent, in anorthite, leading us to infer the existence of lime 

 and potash-albites (orthoclase ?), and soda and magnesia anor- 

 thites. The difficulties presented by the varying composition 

 of these felspars are obviated by admitting such a mixture of 

 species as constantly takes place in the crystallization of homoeo- 

 morphous salts from mixed solutions, and this consideration 

 should never be lost sight of in the study of mineral chemistry. 

 It was not until after I had published this view of the consti- 

 tution of the triclinic felspars (a view which must also be ex- 



* American Journal of Science, 2nd series, vol. xviii. p. 270. 



