These minerals are often diagnostic of the environment of sedimentation, source 

 areas of the sands, or the modes of transportation. Some of the minerals may 

 be diagenetic, and thus indicate the postdepositional history of the sandstone. 

 Some of the more common minor constituents of sandstones are biotite, mus- 

 covite, glauconite, pyrite, barite, siderite, cherts, coals, solid hydrocarbons, and a 

 variety of hard nonmetallic minerals. 



7. Color: The color of a sandstone may be caused by the colors of the 

 constituent grains, color of the cement, or by the staining of the entire aggregate. 

 Staining is most common in surface sections, but it may also be prevalent at or 

 near unconformities in the subsurface. Quartz sands may acquire a surface 

 film of coloration during a period of exposure as free sand before final deposition 

 either as a marine or non-marine aggregate. Color is an important character of 

 the rock and should always be noted. 



8. Structure: The structure of a sandstone as used in the description of well 

 cuttings refers to such characters as laminations, fractures and fracture patterns, 

 banding, and nodular or concretionary characteristics. 



9. Sorting {Texture) : The degree of sorting is one of the most important 

 features of a coarse clastic rock. In petroleum geology, the sorting of sands in 

 a potential oil reservoir has particular significance, for it determines the effec- 

 tiveness of porosity and permeability of the rock. A poorly sorted sand general- 

 ly has low porosity and permeability. The texture of the rock is determined, not 

 only by its coarseness, but also by the sorting and arrangement of the grains. 



The relative porosity and permeability of a sandstone can be estimated by 

 carefully placing a drop of water on a dry chip and observing under the micro- 

 scope how rapidly the water is absorbed into the rock. 



Fine Clastic Rocks 



(Plotted in natural colors of the rocks) 



The dividing line between sandstone and siltstone is somewhat arbitrary. 

 Wentworth and others make the distinction on the basis of a grain size of 

 1/16 millimeter. Although this division is entirely satisfactory for some types 

 of work, it is not so practical for low-power binocular determinations. Most 

 measuring devices are graduated in decimals of a millimeter, and for this 

 reason 0.1 millimeter is widely used as the upper limit of siltstone grain size. 

 In the application of this scale some consideration should be given also to 

 other characteristics of the rock. If the rock is poorly sorted and the maximum 

 grain size is 0.1 millimeter, it should be classed as a siltstone, or perhaps a 

 sandy siltstone. On the other hand, if the grains are well-sorted and the maxi- 

 mum size is 0.1 millimeter, the rock should be called a very fine sandstone, or 

 perhaps a silty sandstone. 



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