as far as the original dimensions of the grain will permit. Thus, a cylinder 

 with hemispherical ends is perfectly rounded. 



Sphericity, in contrast to roundness, is referred to the three dimensions of 

 the grain. A cube has a high degree of sphericity, but a low degree of roundness. 

 A sphere exhibits perfect roundness and perfect sphericity. 



3. Luster: This term is used here in a rather loose sense to describe the 

 condition of the surface of the sand grain. The luster or surface texture of sand 

 grains sometimes reveals the history of the grain before and possibly after, 

 deposition. Below are definitions of commonly used terms. 



COATED: Precipitated or accretionary material on the surface of the grain. 



Iron oxide, calcium carbonate, sulphates, clay6, pyrite, etc. 

 PITTED: Solution or impact pits, often of pin-point size. 

 FROSTED: Deeply etched, frosty, translucent, usually white. 

 SILKY: Lightly etched or scoured. 



OILY: Greasy or oily sheen; common in hematite and magnetite grains. 

 VITREOUS: Glassy, shiny. 



SHARP ANGULAR SUB-ANGULAR ROUNDED GLOBULAR 



Figure 3-3. Illustrating the 5 classes of sand-grain shapes. 



4. Induration: The induration of a sandstone is its resistance to physical 

 breakdown or disaggregation. Induration does not necessarily refer to the 

 hardness of the constituent grains, though they may have considerable influence 

 on the degree of induration. Common adjectives describing induration are 

 dense, hard (as in quartzites), medium hard, soft, spongy, friable. 



5. Cementation: The character and composition of cementing material 

 should be identified when possible. The cementing of a sandstone has a sig- 

 nificant bearing on its performance as an oil reservoir. The character of the 

 cementing material may reveal much of the rock's depositional history and post- 

 depositional alteration. One should note the relationship of the cement to the 

 constituent grains. Common cements are calcite, dolomite, sulphates, iron 

 oxides, silica, pyrite, clays, silts, and siderite. Some sandstones are compacted 

 into firm aggregates, yet have no discernible cement. 



6. Minor Constituents: In addition to the minerals constituting the bulk 

 of the rock, other minerals in very small amounts may also be present. Such 

 minerals are often called accessories. Although they constitute a negligible 

 portion of the aggregate, their importance is disproportionate to their bulk. 



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