Minerals of the Composition MgSiO r 403 



The above data show that in a few days time the change is 

 first apparent between 1260° and 1290°, and that it is here 

 extremely slow. Comparing this table with the one on p. 399, 

 it is clear that the glass crystallizes directly to monoclinic 

 pyroxene about 100° lower. By the use of a solvent, the same 

 change proceeds as low as 800° and probably even lower. 

 Calcium vanadate, or still better, magnesium chloride, under 

 the conditions described on p. 389, yields well-shaped crystals of 

 the monoclinic variety at 800°. 



The natural enstatite of the Bishopville meteorite showed, 

 an identical behavior when heated to 1450° ; the enstatite with 

 its parallel extinction disappeared completely and was replaced 

 by the twinned monoclinic lamellae, the latter still preserving 

 the original prismatic direction of the enstatite and also appar- 

 ently the cleavage, the size of grain of the two phases being 

 about the same and the original outline of the enstatite frag- 

 ments being still preserved after the paramorphic change. A 

 specimen of enstatite from Webster County, ^orth Carolina,* 

 containing ferrous silicate also passed into the more stable 

 monoclinic form when it was heated. Here magnetite was 

 observed as a by-product. 



III. Monoclinic Amphibole. — In several of our prepara- 

 tions of magnesium silicate which had been crystallized by 

 more or less rapid cooling, small quantities of a twinned 

 monoclinic form were observed, whose properties, with the 

 exception of the extinction angle, were similar to those of the 

 orthorhombic amphibole (the fourth form which is described 

 under IY). Later, in studying the action of water on the lat- 

 ter substance at 375°-475°, we observed the constant recur- 

 rence of this monoclinic amphibole in larger amounts than in 

 the original preparations; when, however, the amphibole was 

 heated alone to considerably higher temperatures (above 900°) 

 no such transformation took place. The change with water 

 was always quite incomplete, but further experiments, it is 

 hoped, may point the way to satisfactory conditions of forma- 

 tion. 



Properties of Monoclinic Amphibole. — The crystallites of 

 the monoclinic magnesian amphibole were invariably micro- 

 scopic in size and too small to admit of a satisfactory study of 

 their optical constants. Their direction of elongation is the 

 prism axis, with which they extinguish at a small angle. In 

 polysynthetically twinned fragments, a maximum extinction 

 angle c : c = 11° was observed, though smaller angles, 4°-8°, 

 were noticed more frequently. The average refractive index 



* The writers are indebted to Dr. G. P. Merrill of the National Museum 

 for these specimens. 



