ZIRCONIFEROUS SANDSTONE NEAR ASHLAND, VIRGINIA 281 



more abundant, and occurs under a variety of conditions and associations. 

 The mineral is one of the most resistant to ordinary atmospheric agents 

 and alters, so far as kno^vn, chiefly by hydration when it becomes isotropic 

 and of lower specific gravity. It then has a specific gravity of 3.905 and 

 is knowai as malacon. It sometimes alters through loss of silica. 



Zircon has been produced synthetically by several different methods, 

 as follows : By heating the oxide, zirconia (a) in a current of silicon fluoride, 

 (b) with quartz in the same gas, (c) a mixture of silica, zirconia, and lith- 

 ium molybdate heated to 800°, and (d) by heating gelatinous silica and 

 gelatinous zirconia under pressure to near redness. Clarke* remarks that 

 Deville's methods of s3Tithesis (a) and (b) above, "indicate a possible pneu- 

 matolitic origin for zircon in some instances; the other processes seem to be 

 unrelated to the ordinary occurrences of the mineral." 



Zircon is a product (pyrogenetic mineral) of igneous rocks, of regional 

 metamorphism,tof contact metamorphism as it is reported found sometimes 

 in contact limestones,! and of pegmatite veins or dikes. § Palache and 

 Warrenll have recently described zircon-quartz intergrowths of the Quincy, 

 Mass., pegmatites, which are referred by the authors to pneumatolitic 

 processes 



Alteration. 



Very little exact information on the alteration of zircon has been pub- 

 lished. This is probably due chiefly to the fact that zircon is one of the 

 most durable and resistant minerals to ordinary atmospheric agents, and to 

 the added fact that it generally occurs in small crystals and grains as a very 

 minor constituent of rocks. 



According to Dana° zircon assumes water, becoming isotropic and amor- 

 phous, accompanied ultimatelj^ by the loss of silica and the addition of 

 iron oxides and other impurities derived from infiltrating waters. Auer- 

 bachite, malacon, oerstedite, tachyaphaltite, calyptolite, and cyrtolite 



* Clarke, F. W., Op. cit., 1911, p. 336. 



t Emmons, W. H., A Genetic Classification of Minerals, Economic Geology, 1908, 

 vol. 3, p. 622. 



t Clarke, F. W., Op. cit., 1911, p. 678. 



§ Probably the most noted example is that in Henderson County, N. C. See 

 Pratt, J. H., Mineral Resources of the Untied States for Calendar Year 1903, pp. 1168- 

 1169. 



If Palache, C. and Warren, C. H., The Pegmatites of the Riebeokite-Aegirite 

 Granite of Quincy, Mass., U. S. A.; Their Structure, Minerals, and Origin, Proc. 

 Amer. Acad. Arts and Sciences, 1911, vol. xlvii, p. 131. 



° Dana, E. S., A System of Mineralogy, 1900, 6th edition, p. 486. 



