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FREQUENTLY ASKED QUESTIONS ABOUT ROCKS

 

    What is a kimberlite?

    How do you classify a rock?  

    

What is a kimberlite?

     A kimberlite is a rock that is formed from magma that comes from the upper mantle of the earth. It is a serpentinized porphyritic phlogopite peridotite. A peridotite is a rock that consists primarily of olivine with some pyroxenes and does not usually contain feldspars. The olivine typically is altered to serpentine and the phlogopite is a mica. It is believed that the rock is extruded from great depth and reaches a fairly high velocity ( faster than the speed of sound). There is probably a lot of gas and water associated with the magma. The main reasons that this rock is studied extensively is that it occasionally contains diamonds and usually contains xenoliths ( foreign rock inclusions ) originating all the way from the upper mantle to the surface. This is one of the few places where scientists can obtain specimens that formed in the mantle.
     Surface outcrops of kimberlites do not cover large areas (usually only a few acres), but they have been found in numerous places in the US. The only know US diamond bearing kimberlites are from Arkansas and along the Colorado/Wyoming border.
      Diamond is a mineral that is formed and is stable only at the pressures and temperatures found in the mantle. The kimberlites bring along pieces of diamond when the magma comes to the surface. 

How do you classify a rock?  

     The classification of a rock depends on the ability to determine the various minerals in a rock and their percentage in the rock (around 50 to 100 common minerals). How the rock was formed also is necessary to know to properly classify the rock. There are always some rocks which do not fit well into a pigeonhole of classification.
     One needs to have a book (or web resource) that lists the various rock names. There are several beginner's guides to Rocks and Minerals such as the ones published by the Peterson guides or Simon and Schuster that provide some information on rock names and pictures. There are course books for petrology and its subdivisions that are taught on a university level if you want more complete information. You can search on the web under petrology and find a number of sites that are related to courses.  J. Butler maintains a list of petrology classes on the web.
     One rock ID key that lists some of the rocks and how to name them is here.
     To identify the minerals a low power (10X) hand lens is essential. One should also have a bottle of dilute hydrochloric acid (for calcite and other carbonates). It is best to work on a fresh surface of the rock (worn river pebbles are usually very difficult to properly name without breaking them).
     First one would decide what type of rock a specimen is ( sedimentary , igneous, or metamorphic ). 
      For sedimentary rocks, one would then determine if the rock was composed of particles brought to the deposition site (sandstone or conglomerate) or chemically precipitated (limestone or rock salt).
      For an igneous rock one would look at the size of the minerals and determine if it cooled at depth within the earth or from lavas at the surface. Then one identifies the feldspars in the rock and how much quartz it contains. One can usually come up with a name for the rock at this stage.
      Metamorphic rocks are categorized by the pressure and temperature to which the rock was subjected as well as the original rock type. A high temperature, low pressure condition is called contact metamorphism. Regional metamorphism is caused by rocks exposed to the normal geothermal gradient ( higher pressures and temperatures at greater depths). A shale will become a slate, schist, gneiss as the temperature and pressure are increased. Since water and other volatiles are driven off, rocks will equilibrate at the highest temperature to which they were exposed and not retrograde as they are cooled.
      This is just a brief description of the process since entire college geology courses are taught on this subject.
      One other quick way to identify a rock is to get a geologic map of an area and the rocks will be described on the map or accompanying report.
      Bauxite, it is a rock formed of various aluminum oxides and hydroxides. It forms due to the extreme weathering of rocks in tropical climates which removes the iron and silica in the preexisting rocks. 


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