Some Aspects of Rocks and Isotopes

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Some Aspects of Rocks and IsotopesChristopher Chui, PhD

Acknowledgment: I am indebted to numerous sources in the Internet, and the Trilinear Chart of the Nuclides published by Mallinckrodt Nuclear.

http://ChristCenterGospel.org

ckchui1@yahoo.com

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Melting Points of Rocks Igneous rocks form through the

crystallization of magma. There is a considerable range of melting temperatures for different compositions of magma. All the silicates are molten at about 1200°C and all are solid when cooled to about 600°C. Often the silicates are grouped as high, medium and low-melting point solids.

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Melting Points of Rocks

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Approx Temp (°C) Minerals which are molten

1200 All molten

1000 Olivine, pyroxene, Ca-rich plagioclase

800 Amphibole, Ca/Na- plagioclase

600 Quartz, K-feldspar, Na-plagioclase, micas

Bowen's Reaction Series

Nature of Olivine The composition of most olivines can be

represented in the system Ca2SiO4-Mg2SiO4-Fe2SiO4 . The most abundant olivines occur in the system from forsterite (Mg2SiO4) to fayalite (Fe2SiO4). Most of the naturally occurring olivines are intermediate in composition to these two end-members and have the general formula (Mg, Fe)2SiO4. Members of the series monticellite (CaMgSiO4) to kirschsteinite (CaFeSiO4) are rare. Minor elements such as aluminum, nickel, chromium, and boron can substitute in olivine.

Nature of Pyroxenes The chemical composition of minerals of the pyroxene

group can be expressed by the general formula XYZ2O6, in which X= Na+, Ca2+, Mn2+, Fe2+, Mg2+, Li+; Y= Mn2+, Fe2+, Mg2+, Fe3+, Al3+, Cr3+, Ti4+; and Z= Si4+, Al3+. The range of possible chemical substitutions in pyroxene is constrained by the sizes of the available sites in the structure and the charge of the substituting cations. The Xcation sites in general are larger than the Ycation sites. Extensive atomic substitution occurs between the ideal end-member compositions. Most pyroxenes have only limited substitution of aluminum for silicon in the Z(tetrahedral) site. When a substituting ion differs in charge, electrical neutrality is maintained by coupled substitutions.

Chemical Divisions of Pyroxenes

General Plagioclase Information Chemical Formula: (Na,Ca)(Si,Al)4O8 Composition: Molecular Weight = 270.77 gm  Sodium      4.25 %  Na     5.72 % Na2O  Calcium     7.40 %  Ca    10.36 % CaO  Aluminum   9.96 %  Al    18.83 % Al2O3

  Silicon    31.12 %  Si    66.57 % SiO2

  Oxygen     47.27 %  O          ______         ______            100.00 %       101.48 % = TOTAL OXIDE Empirical Formula: Na0.5Ca0.5Si3AlO8

 Environment:Igneous and metamorphic rocks. Group name for Na, Ca feldspars.