■New Occurrence of Rutile and Sapphirine. 273 



The ratios derived from the analyses of Lorenzen and Ussing 

 (VI and VII) lead to the formula given by them, Mg 5 A] 12 Si 2 27 , 

 although it is to be noted that there is a considerable deficiency 

 in the RO figure in both cases, and in VI of R 2 3 . The ratios 

 derived from the analyses of Stromeyer (III), Damour (IV), 

 and Schluttig (V) agree rather more closely with the formula 

 Mg 4 Al 10 Si 2 O 23 . The ratios of the St. Urbain sapphirine lie about 

 half way between these two formulae, while those of the Indian 

 mineral depart rather widely from both, although they perhaps 

 favor the second type. Ussing, in discussing the composition 

 of the mineral, attributed the relatively higher silica values of 

 the older analyses of Stromeyer, Damour, and Schluttig to the 

 presence of mica or other impurities. This criticism cannot 

 apply in the case of the St. Urbain mineral, nor apparently in 

 that of the Indian mineral ; and as the analysis of the mineral 

 presents no difficulties, it does not seem as if the differences 

 can be attributed to analytical errors. The composition is 

 that of a very basic silicate, and does not appear to conform to 

 any simple formula. It seems probable that there may be some 

 solid solution relation existing between the molecules which 

 make up the mineral ; at least this is the only apparent expla- 

 nation of the variations in composition which it shows. 



It may be noted that there are two types of the mineral from 

 Greenland (see Ussing), one of a light blue color, the other 

 having a deeper blue color and occurring in a more ferrugi- 

 nous association. Ussing suggests that the deeper blue color 

 may be due to the presence of more iron in this type, an idea 

 in keeping with the higher iron content of the St. Urbain and 

 Indian minerals. 



Origin of the Sapphirine. — In the case of the sapphirine 

 from Fiskernas in Greenland, the mineral appears to be of met- 

 amorphic origin, occurring, according to Steenstrup,* in small 

 ellipsoidal or irregular masses in mica-schist or gneiss, high 

 in MgO and A1 2 3 but low in silica. Walker, in the paper 

 alluded to, presents strong evidence to the effect that the Indian 

 sapphirine is also of metamorphic (contact) origin, having been 

 formed by the action of ultra-basic, spinel-bearing rocks of the 

 charnockite series, on a sillimanite schist. Its occurrence as a 

 metamorphic mineral in these two instances suggests strongly 

 that it may be also of metamorphic origin at St. Urbain, an 

 hypothesis which is perhaps also supported by the peculiar and 

 intimate association of the mineral with the feldspar. If so, it 

 must represent a reaction product between the feldspar and the 

 spinel, although in the absence of any lime-bearing silicate of 

 a secondary nature, such as epidote or hornblende, it is diffi- 



*Medd. om Gronl., Kopeixhagen, 1884, vii, 15. 



