Subsurface Laboratory Methods 135 



Large apparent differences in heavy-mineral composition may be due 

 to the selective removal of certain minerals. In the example below re- 

 moval of the less stable minerals would leave a very different mineral 

 suite. 



Before removal After removal 



(percent) (percent) 



Hornblende 20 



Garnet 30 



Staurolite 25 



Tourmaline 15 60 



Zircon 5 20 



Rutile 5 20 



The types of minerals present in a sediment are indicative of the 

 sources from which they were derived. Andalusite, kyanite, staurolite, and 

 sillimanite probably indicate a metamorphic source. Ilmenite, zircon, ru- 

 tile, apatite, olivine, titanite, and some varieties of tourmaline probably 

 indicate an igneous source. Any heavy minerals that are very well rounded 

 suggest a sedimentary source. 



Errors in Heavy-Mineral Analysis 



Heavy-mineral analysis, like other types of analyses, is subject to 

 various errors that must be considered in interpreting the results. These 

 errors are due to (1) the composite nature of the samples, (2) contamina- 

 tion, (3) grain breakage, (4) the misidentification of the grains, and (5) 

 the size of the final sample. These errors have been discussed by Ritten- 

 house.^^ 



Grain Roundness 



The roundness of detrital grains may be an important criterion for 

 identifying producing horizons, outlining petrographic provinces, and 

 deciphering geologic history. A short discussion of roundness, giving 

 particular emphasis to its interpretation, is given here. 



Roundness and sphericity (shape) are independent properties of sedi- 

 ment particles and must not be confused. Roundness is a measure of the 

 angularity of the corners and edges of a grain. In contrast, sphericity is a 

 measure of the approach to spherical form and may be expressed roughly 

 as a ratio between the length and breadth of a grain. Thus in figure 55, 

 grain A has high roundness and high sphericity. Grain B has high round- 

 ness but the sphericity is much lower; the length of the grain is much 

 greater than its breadth. Grain C has low roundness — the corners are 

 very sharp; but it is nearly equidimensional and consequently has a fairly 

 high sphericity. 



*^ Rittenhouse, Gordon, Analytical Methods as Applied in Petrographic Investigations of Appalachian 

 Basin: U. S. Geol. Survey Circ. 22. 20 pp., 1918. 



