What was limestone before it changed

Calcite is colourless or white when pure, but it may be of almost any colour - reddish, pink, yellow, greenish, blueish, lavender, black, or brown, owing to the presence of diverse impurities.

It may be transparent, translucent, or opaque. Its luster ranges from vitreous to dull; many crystals, especially the colourless ones, are vitreous, whereas granular masses, especially those that are fine-grained, tend to be dull. Calcite is number 3 on the Mohs hardness scale; it can be scratched readily by a knife blade or geologic pick. It has a specific gravity of 2. Three perfect cleavages give calcite its six-sided polyhedrons with diamond shaped faces; the angles defining the faces are 78 and degrees.

When light passes through some minerals, it is split into two rays that travel at different speeds and in different directions.

This phenomenon is known as birefringence. Calcite consequently exhibits double refraction that can be observed with the naked eye. Iceland spar, first produced in the 17th century from the east coast of Iceland, has been used for nearly two centuries for optical instruments.

William Nicol in found that by cutting the crystals in the appropriate direction one could make an optical device which eliminated one of the rays and permitted the other to emerge as plane polarised light.

Polaroid sunglasses use this same property to cut down glare on a sunny day. This technology was used in microscopes which are utilized in the study of rocks and minerals.

Hay developed a theory of crystal structure that played an important part in the evolution of modern structural crystallography. He continued to break other pieces of calcite and was able to determine the idea of crystal structure which came from the calcite breaking in a similar manner each time. Limestone is a rock made of calcite. Most limestone is grey, but all colours of limestone from white to black have been found. Limestone gives off bubbles of carbon dioxide.

Most fresh water and sea water contain dissolved calcium carbonate. All limestones are formed when the calcium carbonate crystallizes out of solution or from the skeletons of small sea urchins and coral. All the different kinds can be divided into two groups. The first group includes limestones that formed almost completely without the aid of organisms. This type of limestone is forced out of a solution when the water evaporates. If carbon dioxide is removed from this water by warming, agitation or photosynthesis, there is a tendency for calcium carbonate to be precipitated.

For example, the beach rock commonly found along tropical beaches is a limestone that has been formed in the intertidal zone in this way. Some limestones are formed in freshwater environments associated with caves stalactites and stalagmites , springs tufa and travertine and lakes. Stalactites and stalagmites form in caves as water evaporates from calcium carbonate-rich waters that drip from the roof of the cave and onto the floor.

Travertine forms as a result of calcium carbonate precipitation from ground and surface waters and especially geothermal waters. Oolites or ooids are small rounded spheres of calcium carbonate that form in warm, supersaturated, shallow, highly agitated tropical marine water. The oolites form by chemical precipitation of a layer of aragonite crystals about some hard particle — often a shell fragment.

Strong currents continually shift the oolites around so that new concentric layers of aragonite grow on top of the earlier layers. The oolites are alternately exposed to pick up a concentric layer and then buried to set the layer. Marble is a hard crystalline rock that takes a high polish and is used for building and sculpture. Chalk is a special form of limestone mainly formed in deeper water from the shell remains of microscopic marine plants and animals such as coccolithophores and foraminifera.

Unless deeply buried, most chalks are relatively soft rock with a high calcium carbonate content. In the rock cycle , there are three different types of rocks: sedimentary, igneous, and metamorphic. Sedimentary and igneous rocks began as something other than rock.

Sedimentary rocks were originally sediments, which were compacted under high pressure. Igneous rocks formed when liquid magma or lava —magma that has emerged onto the surface of the Earth—cooled and hardened.

A metamorphic rock , on the other hand, began as a rock—either a sedimentary, igneous, or even a different sort of metamorphic rock. Then, due to various conditions within the Earth, the existing rock was changed into a new kind of metamorphic rock.

The conditions required to form a metamorphic rock are very specific. The existing rock must be exposed to high heat, high pressure, or to a hot, mineral-rich fluid. Usually, all three of these circumstances are met.

In order to create metamorphic rock, it is vital that the existing rock remain solid and not melt. If there is too much heat or pressure, the rock will melt and become magma. This will result in the formation of an igneous rock , not a metamorphic rock. Consider how granite changes form. Granite is an igneous rock that forms when magma cools relatively slowly underground. It is usually composed primarily of the minerals quartz, feldspar, and mica. When granite is subjected to intense heat and pressure, it changes into a metamorphic rock called gneiss.

Slate is another common metamorphic rock that forms from shale. Limestone, a sedimentary rock , will change into the metamorphic rock marble if the right conditions are met. This happens due to geologic uplift and the erosion of the rock and soil above them.