206 BULLETIN 118, UNITED STATES NATIONAL MUSEUM. 



It must not be partly opaque on account of numerous cracks or fractures, must not 

 show any internal, iridescent, or rainbow colors due to incipient cracks along frac- 

 ture lines, nor any cleavage, nor twinning planes. Neither can there be any cap- 

 illary or larger tubelike cavities, nor cavities or bubbles of any shape, nor inclu- 

 sions, as isolated particles, veins, or clouds, composed of minute crystals of some 

 other mineral or of any kind of foreign substance. The spar should not be discolored 

 or stained by the presence of any clay, iron oxide, or other material. It should be 

 noted that many of the inclusions and imperfections of Iceland spar are not always 

 scattered irregularly through the mineral or even segregated in distinct masses, but 

 frequently lie in a distinct but very thin plane which can hardly be seen if looked 

 at on edge. In examining a piece of Iceland spar for defects the piece should there- 

 fore be turned in all directions while held to the light. 



The material suitable for optical uses naturally brings the highest prices, as it has 

 to be at least of the dimensions already given. Specimen material is generally of a 

 larger size. The material used for standardization, chiefly chemical, need be of 

 no special size, and the smaller pieces are as usable as the larger ones. ' 



Although calcite is, next to quartz, the commonest mineral, the only locality 

 outside of Iceland known to produce the variety Iceland spar in commercial quan- 

 tity is in Montana, about 9 miles from Gray Cliff, Sweet Grass County, on the main 

 line of the Northern Pacific Railway. The spar occurs in a nearly vertical fissure 

 vein from 3 to 8 feet thick, which strikes northwest, traversing a gneissic rock for 

 several miles. 



Jasper. — See Agate. 



Malachite.— Ore of copper, pigment for paint. 



Mariposite. — Pigment for paint. 



Meerschaum. — Pipe bowls; cigar and cigarette holders. 



Optical fluorite. — Fluorite, commonly called fluorspar, is a common mineral but is very 

 seldom found in pieces clear enough and large enough to be of special use in the 

 manufacture of certain optical lenses and prisms. Fluorite of the requisite quali- 

 ties, as described below, suitable for such use is known as "optical fluorite." Any 

 deposit of fluorite may yield a small quantity of such material, but at present about 

 the only localities known to produce it are southern Illinois; Meiringen, Switzer- 

 land; and Obira, Bungo, Japan. Optical fluorite is cut into lenses and placed 

 between glass lenses. It forms the apochromatic objective for microscopes and 

 similar optical instruments, the fluorite lens correcting the spherical and chromatic 

 errors of the glass lens systems. This result is due to the low refractive power, 

 weak color dispersion, and single refraction of fluorite. These apochromatic lenses 

 represent the finest type of microscope objectives made. The use of such a fluorite 

 lens greatly increases the value of a microscope and if optical fluorite were more 

 abundant many more microscope objectives would be equipped with such lenses. 

 Optical fluorite is also used in the lenses of certain telescopes, in making prisms 

 for spectrographs in ultra-violet work, and in other optical apparatus where trans- 

 parency in the ultra-violet and infra-red parts of the spectrum is necessary. 



i The optical variety of Iceland spar produced in the United States, sold, a pound, for $3 to $4 in 

 1914, about $S in 1915, and as high as $20 in July, 1918. The specimen variety sells for considerably less, 

 and material for standardization sells for from $1 to $2 a pound. 



The following firms are buyers of Iceland spar suitable for optical use: Bausch & Lomb Optical Co., 

 Purchasing Department, Rochester, New York; Central Scientific Co., 460 Ohio Street east, Chicago, 

 Illinois; Gilbert S. Day, Superintendent Optical Department, Eastman Kodak Co., Rochester, New 

 York. 



The market for specimen spar is irregular, as the demand is usually very light. The best market 

 will probably be found with some of the larger mineral dealers. 



Standardization material may be sold to large dealers in general chemicals as well as to mineral 

 dealers. 



