526 AUSTIN F. ROGERS 



recognized as a distinct species. The properties of opal are so 

 well known that a description is unnecessary. Attention , however, 

 should be called to the fact that hyalite opal, the purest and most 

 typical form of opal, usually shows double refraction due to strain. 



(Lechatelierite) ? — The amorphous constituent of fulgurites 

 and of some inclusions in volcanic rocks has recently been named 

 lechatelierite by Lacroix. 1 This material approaches silica glass in 

 composition and is also very similar to opal in properties. The 

 only difference between lechatelierite and opal is due to their 

 previous history, but, as Miers says, ". . . . the essential character 

 of a mineral, moreover, is quite independent of its source or previous 

 history." 2 Yet these two substances are so different in occurrence 

 and origin that one feels inclined to consider them as distinctive 

 minerals. Then we are confronted with the question whether we 

 are ever to recognize more than one amorphous mineral for a 

 given crystalline equivalent or not. 



Now, lechatelierite is a glass and may be considered along with 

 other natural glasses as a mineraloid (see p. 540) rather than as a 

 mineral proper. As an argument for this, I give the results of my 

 examination of a fulgurite found in the sand dunes along Lake 

 Michigan in Van Buren County, Michigan, and obtained from 

 Ward's Natural Science Establishment. The fulgurite is a hollow 

 tube of glass with small grains of white sand adhering to the 

 exterior surface. The sand grains are quartz and orthoclase. The 

 glass is colorless and perfectly isotropic with an index of refraction 

 of 1 .46 2 ±0.003. It is fusible on the edges and gives a small 

 amount of water in the closed tube. From these tests it can be 

 seen that the glass is not pure silica, but a glass high in silica. 

 The lechatelierite described by Lacroix is almost pure silica, as 

 its index of refraction is 1.458. Whether a distinctive name is 

 desirable or not, the glass of fulgurites may be considered simply 

 as a mineraloid, which approaches pure silica glass in composition. 



Hydrocuprite. Cu 2 0(H 2 0) :j; (hydrocuprite, ziegelite, zigueline, 

 tile-ore). — Two kinds of cuprous oxid occur in nature: (1) iso- 

 metric cuprite and (2) amorphous tile-ore, for which the name 



•M. Soc. Fran, de Min., XXXVIII (1915), 182-86. 

 2 Mineralogy, 1902, p. v. 



