Limestone, one of the common forms of sedimentary rocks, is predominantly composed of the so-called calcite, a mineral that forms 10 % of the Earth’s crust. It is regularly formed either through the process of direct crystallization from water or through the accumulation of shell and its fragments. All limestone rocks originate from the precipitation of calcite from water. Using seawater as its source, limestone usually develops in shallow water below 20 meters. Oftentimes, limestone rocks are embedded with caves. Such unique structure makes it famous for cements and other concrete materials.
Composing predominantly of calcite, limestone rocks are usually of clear or white in color. However, because of the existences of some impurities, a variety of colors may come out from the mineral. Most often than not, limestone is in light colored of gray. But the impurities make the limestone exist in almost all colors. These impurities were mostly identified as sands, clay, hydroxides, and other organic materials.
In a case where a drop of diluted hydrochloric acid is placed over a limestone, the acid normally reacts along with the calcite and eventually develops into bubbles of carbon dioxide. Such bubbly reaction normally describes the entirety of a limestone. Thus, for most geologists, they always bring with them a small container of a diluted hydrochloric acid so they can easily identify which rocks are considered limestone and which are not.
On the other hand, sandstone also exists in some parts of the earth. However, it comes in three categories namely: Quartz sandstone, Arkose and the commonly called greywacke. Each of these sandstones has its respective dispositional distinctions despite the fact that they belong under the same group. Quartz sandstone implies longer time over a dispositional basin. While Arkose implies shorter time and rapid erosion, Greywacke too, holds rapid erosion and a temperate climate. Similarly, all these three categories are composed of grains. However, they are dominated by different types of grains. Quartz sandstone is dominantly composed of quartz grains while Arkose and Greywacke are composed of feldspar grains and rock fragments grains respectively.
In order to further distinguish one from the other, it is better to describe each category of sandstone. Firstly, Quartz Sandstone results from an extreme weathering of sediments until the very last thing that needs to be taken away. Such removal is completed through the so-called chemical weathering. Whilst, tidal sand bars that dominantly composed the quartz is another process considered to help in the transformation of rocks into sandstone. Arkose, on the other hand, is a sandstone that comes from the breakdown process of granite. It is usually characterized by extreme existence of feldspar. Arkose are famous for its building stones and millstones which are commonly used for grinding the corn. Lastly, Greywacke, of which is also known by the name of Litharenite, predominantly contains fragments of dark colors and those which are in the size of a sand. There are however few micas, quartz, and feldspar grains that remained to be visible in the matrix of a Greywacke.
“Major Types of Sandstones.” Indian Sandstone. 9 November 2008. <http://www.indian-sandstone.com/types-of-sandstone.html>