572 busch. SUBSURFACE TECHNIQUES [Ch. 31 



heavy liquid such as acetylene tetrabromide or bromoform. Ritten- 

 house (1948, p. 6) clearly summarizes the technique of heavy mineral- 

 separation : 



Acetylene tetrabromide of specific gravity 2.93 is placed in a glass funnel to 

 the stem of which is attached a piece of rubber tube closed by a pinch clamp. 

 The sand sample is introduced, and the mixture is stirred at intervals until 

 the heavy minerals have settled into the stem of the funnel. The pinch clamp 

 is opened and the heavy fraction is washed onto a filter paper in a second glass 

 funnel. After the excess heavy liquid has been filtered into a receptacle and 

 returned to the stock bottle, the filter paper containing the heavy minerals 

 is washed several times with alcohol. The light minerals and remaining heavy 

 liquid are then drained onto another filter paper, the heavy liquid is filtered 

 off, and the light minerals washed with alcohol. Both heavy and light minerals 

 are dried in an oven at 205° F. The alcohol-tetrabromide washings are saved 

 for recovery of the tetrabromide. 



The heavy minerals themselves may be further separated by the use of 

 still heavier liquids than acetylene tetrabromide and bromoform, but 

 it is seldom necessary to make more than the initial separation. Fields 

 of different magnetic and electrostatic intensity are also used to dif- 

 ferentiate the heavy-mineral fraction. 



The heavy minerals commonly isolated by a heavy-liquid separation 

 are andalusite, apatite, cyanite, epidote, tourmaline, zircon, rutile, 

 chromite, barite, magnetite, pyrite, hematite, garnet, ilmenite, stauro- 

 lite, topaz, muscovite, and biotite. The percentage of mineral types 

 in a random sampling of 100 or more grains usually suffices to char- 

 acterize the heavy minerals of one separation. In some samples the 

 variety of heavy minerals is of significance, whereas, in others, the 

 percentage of one or more of the mineral components of the assemblage 

 is more important from the standpoint of correlation. The percentage 

 composition of color varieties may be determined for a given forma- 

 tion. In addition, the percentage of euheclral, subangular, and round 

 grains of several of the heavy-mineral components may be determined. 

 These percentage values for one or more mineral characteristics may 

 be plotted and contoured on a map. Rittenhouse (1948, p. 14) suggests 

 such maps as "roundness maps, tourmaline variety maps, zircon round- 

 ness maps, zircon variety maps, and authigenic tourmaline maps." 

 From such maps, usually drawn for individual formations, progressive 

 changes of any physical feature and geographic variations in mineral 

 characteristics may be noted. Such maps are of considerable help in 

 interpreting the source of the sediment, conditions of accumulation, and 

 paleogeographic distribution of the sea in which it was deposited. 



