TOPAZ CKYSTALS IN THE MINERAL COLLECTION OF THE 

 U. S. NATIONAL MUSEUM. 



By Arthur S. Eakle, Ph. D., 



T)epav 1)11 (lit of Mhivralofiii, Harvard UnircrxHi/. 



The U. S. National Museum collection of miueral.s contains many 

 excellent crystals of topaz, representing most of the localities from 

 which this mineral has been obtained in crystal form. A large number 

 of the best ones were a part of the Leidy collection, while the balance 

 have been accpiired through individual gifts or from dealers. 



Topaz has been such a very attractive mineral to investigators, 

 owing to its rich variety of foruis, its varying axial ratios, and its 

 physical and optical characteristics, that very little that is wholly new 

 can be added toour seemingly complete knowledge of the mineral, con- 

 sequently the present article, while adding a little to the crystallography 

 of the mineral from some of the localities, is mainly a description of 

 the collection. 



A wide range naturally exists in the perfectuess of develoi^ment of 

 the crystals, but the majority of them have good bright faces and are 

 easily measurable. The Russian crystals are superior to the others 

 in size, beauty, and perfectness. 



Many of the crystals have well-defined natural etch figures, especially 

 on the prismatic and brachydome faces, and a few possess "Prarosion" 

 faces. 



ALABASHKA. 



A larger part of the Russian crystals are credited to Alabashka. 

 They are mounted either as single crystals or shown as group speci- 

 mens, associated with quartz, feldspar, and mica. They are short, 

 prismatic, with but one termination, and vary in macrodiagonal width 

 from U to 5 cm. Owing to the predominance of the brachyprism they 

 have a tetragonal appearance, with the prismatic faces usually striated. 

 The characteristic color is i)ale blue or green, and a few have a beauti- 

 ful aquamarine shade. 



Two general types of the Alabashka crystals have been described by 

 Kokscharow' ; a simple and more frequently occurring type, consisting 



'Miu. Ru8s., 1854, II, p. 198. 



Proceedings U. S. National Museum, Vol. XXI— No. 1 148. 



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