The Formation of Non-Clastic Chemical Sedimentary Rocks

• Non-clastic, Chemical Sedimentary Rocks are not composed of weathered rock fragments.
  Chemical Sediments form when a solid comes out of solution either by evaporation or precipitation.
  
  
• On Sedimentary Rock Reference Tables that are used by students in high School and college course (See image below), there is often considerable confusion concerning this chemical/crystalline rock classification. The reason for this is because evaporites and precipitates are commonly monomineralic, meaning they are composed of one mineral.
  For example: Halite, common table salt is a mineral. As a hand specimen in the classroom, it is one of the minerals commonly observed by students when studying a Mineral Unit in Earth Science or Geology.
  
  
• However, in the field, when halite is found in large quantities over broad regional areas, it is termed Rock Salt and it is classified as a Sedimentary Rock. Are Halite and Rock Salt one in the same? Is the composition the same? Essentially yes!
  
  
• The same is true of the mineral Calcite. When it occurs in large quantities on Earth's surface it is termed Chemical Limestone.
  Similarly, the mineral Gypsum is called Rock Gypsum and the mineral Dolomite is called Dolostone.
  
  
• This apparent contradiction in terms often results in Sedimentary Rocks being the Rock Type least favored by students. But, if one remembers the simple "mineral name in the hand/rock name when in the field rule" this classification is not all that difficult to understand.

The diagram below is the Reference Chart used by New York State public school students who are studying Regents Earth Science. Though Sedimentary Classification charts vary slightly from one publisher to another, they all contain the same basic information.
On the chart, the Chemical Sedimentary Rocks, those formed by the processes of Evaporation and Precipitation, have been outlined in red.
Observation of the rocks that are so outlined clearly shows their monomineralic composition and one can see how the mineral is renamed when it is classified as a Sedimentary Rock.

Limestone is lumped under one group termed Bioclastic. Having used this chart in the classroom, this is where Geoteach is in disagreement with the diagram's format. In fact, Limestone may be composed of organic remains or it may be a Chemical Limestone formed only by the precipitation of solids out of a watery solution with absolutely no organic remains as part of its composition. Therefore, "precipitates" has been outlined in red and an arrow points to the Crystalline classification, indicating that Limestone may belong to both categories.

It would have been better if Limestone was shown twice, belonging to 2 distinct classification groups: the Bioclastic (organic) group and the Chemical (Crystalline) Precipitates group. Such a presentation would also eliminate a lot of student confusion and misunderstanding.
A second illustration, showing the chemically formed rocks, has been reworked by Geoteach.com to facilitate an understanding of Limestone formation.

Limestone is now grouped into 2 distinct categories.

Formation of Sedimentary Rocks Lesson, illustrations and photographs, unless stated otherwise, are © L. Immoor Geoteach.com, Geolor.com 2006; All Rights Reserved.
Sedimentary Rocks Reference Table is © The University Of the State of New York; Board of Regents; Earth Science Regents Examinations; 2001 Edition

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