402 K T. Allen, F. K Wright and J. K. Clement— 



Museum. Through the kindness of Prof. C. IT. Shepard, the 

 owner of these specimens, and the courtesy of Dr. Gr. P. Mer- 

 rill, the writers obtained *75 gr. of this material, which, on 

 microscopic examination, proved to contain not only enstatite 

 but also occasional intercalated lamellae of monoclinic pyrox- 

 ene identical in its optical properties with our artificial mono- 

 clinic variety. The Bishopville enstatite was analyzed by J. 

 Lawrence Smith* and shown to be almost pure magnesium 

 metasilicate. On this enstatite the following properties were 

 determined : cleavage 110 good ; 7 = P658d=-003, fc = 1 -653± 

 •003, a = l-650±-003 (by Schroeder van der Kolk's method of 

 refractive liquids) ; c = c ; optic axial angle 2-Y = 31°, average 

 of six determinations on different sections nearly normal to an 

 optic axis by the graphical method of Becke ; optical char- 

 acter, positive. The refractive indices of the interbanded 

 monoclinic lamellae were very close to those of the enstatite 

 and extinguished at an angle c : c = 21°*6, an average of 10 

 measurements on two different sections. 



Dr. G. P. Merrill has called the attention of the writers to 

 the fact that in his experience the pyroxenes of meteorites 

 often show extinction angles much too low for augite or 

 diopside, and are, moreover, very frequently twinned poly- 

 synthetically after the orthopinacoid. Time has not permitted 

 an extensive microscopic examination of natural intergrowths 

 of enstatite and monoclinic pyroxene to test the above infer- 

 ence, but the conclusion seems reasonable that not only those 

 of meteorites but those of rocks also, which are not uncommon, 

 are likewise aggregates of two polymorphs, and do not, as 

 generally believed, contain diopside or augite. 



The Transformation of Enstatite into Monoclinie Pyrox- 

 ene. — When heated to high temperatures, enstatite passes over 

 very slowly into monoclinic pyroxene, the time required 

 depending mainly upon the temperature, as will be readily 

 seen from the table below. 



Table V. 



Change of Enstatite into Monoclinic Pyroxene at Different Temperatures. 



Time. Temperature. Change. 



1 day 1200° None detected. 



2 days 1250 " " 



22 hours 1260 -1280° N Slight if any. 



3 days 1260-1290 Probably some change, 

 18 hours 1335 -1350 None detected. 



17 hours 1365-1415 Evident change. 



5 hours 1440 -1460 Much change. 



20 minutes 1500 max. Apparently complete. 



* This Journal (2), xxxviii, p. 225, 1864. 



