Sedimentary rocks are rocks which are formed by the “turning to stone” of sediments and those sediments, in turn, are formed by the breakdown of yet-older rocks [1]. There are 8 types of sedimentary rocks, those are breccia, conglomerate, sandstone, shale, limestone, saltstone, gypsum, and coal.
Types of sedimentary rocks have a different density. The meaning of density itself is a physical property of matter, as each element and compound has a unique density associated with it [2]. So by the definition of density, density has a function to be able to determine what kind of a matter. Then when a matter has a high density, it will have a lower mass than a matter that has a low density.
Measuring density of sedimentary rocks needs to know volume of that sedimentary rocks. For knowing volume of the rocks, it uses Archimedes’ law. The use of Archimedes’ law is because sedimentary rocks have irregular shape. Sound of Archimedes’ law is the buoyant force is equal to the weight to the displaced water.
Rocks that have the same density will have the different of porosity. The meaning of porosity itself is the ratio of pores (micro-voids) in the stone, to its total solid volume [3]. Porosity divides to 2 types based on origin, those are original porosity and induced porosity. Original porosity is porosity that formed when rocks making progress. And induced porosity is porosity that form after rocks making progress.
There are some factors that affect porosity, that are structure of rocks, distribution of rocks, cementation, compacting, and angularity.
Besides rocks have density and porosity, rocks have also matrix rocks. Matrix rocks is physical structure that formed a rock. Things that can characterize characteristic rocks matrix are grain shape, roundness, and grain size.
Grain shape based on comparison of long diameter (l), intermediate diameter (i), and short diameter(s), there are 4 grain shape in sedimentary rocks, those are oblate (l = i s), equant (l = i = s), bladed (l i s) , and prolate (i = s l).
Based on roundness or sharpness sedimen grain, there are 6 level of roundness. 6 level of roundness that are very angular, angular, subangular, subrounded, rounded, and well-rounded.
[1] J. E. Sanders. Principles of physical geology (John Willey, New York, 1981)
[2] Charles E. Ophardt. What is density. 2003. URL : http://www.elmhurst.edu/~chm/vchembook/120Adensity.html (accessed 11 May 2012)
[3] Porosity. URL : http://www.stonecaretechniques.com/6_m.htm (accessed 11 May 2012)
