Limestone is a very common sedimentary rock composed largely of mineral calcite. Limestone is mostly grey. Sometimes it is composed of pure calcite, but in most cases, limestone is filled with some impurities such as clay, sand, iron oxide and other materials, which make it show other colors: iron oxide makes it brown, yellow, or red; carbon makes it blue, black, and gray. Therefore, the color of limestone can vary largely. One thing should be mentioned is: because limestone is an extremely porous rock, it is susceptible to staining.
Depending on the methods of formation, limestone can be crystalline, clastic, granular,
or massive. The calcite comes from the skeletal remains of marine life like fish, clams, brachiopods, and corals and other marine micro-organisms. These shelly remains, sand, and mud are deposited at the bottom of oceans and lakes, over thousands and millions of years, layer after layer is built up. The heat and pressure from the buildup of materials causes chemical reaction at the bottom and the sediments turn into solid stone-the limestone. Because of its biogenic origins, limestone is the best rock for finding fossils. Sometimes a limestone may be completely composed of fossils.When it comes to limestone, one may think of caves, which is formed by the dissolution action of natural acid in the groundwater on soluble rocks. Most such caves are formed in limestone. The
dissolution process produces a distinctive landform known as karst. The most beautiful decorated cave is Lechuguilla Cave in New Mexico.Generally speaking, limestone is soft rock whose surface can be scratched easily and fine-grained. Its hardness ranges from 3 to 4 on Moh’s scale, and its density varies from 2.5 to 2.7 kg/cubic centimeter. Its compressive strength is 1800 to 2100 Kg/cm2. And it has less than 1% water absorption.
Difference between limestone and marble
Some people maybe confuse limestone with marble. Marble is formed through metamorphosis of pure limestone. The transformation is completed under high pressure and high temperature. Though limestone and marble are same chemically, but different physically. The word “marble” comes from the Latin word “marmor”, which means “to shine”, therefore, marble is more suitable to be polished. The fragments of fossils and shells comprising limestone are sometimes visible to the naked eye. Limestone is more porous because of the small openings between fossil fragments. Marble tend to have a lighter color and consists of crystals of calcite locked together like pieces of a jigsaw puzzle.
References:
http://www.galleries.com/rocks/limestone.htm
http://www.mineralszone.com/stones/limestone.html
http://en.wikipedia.org/wiki/Limestone