Rock-Forming Minerals PDF

ROCK-FORMING MINERALS  Elements Versus Minerals ✓By the use of the 118 chemically elements known to exist on earth, it is theoretically possible to make millions of compounds. ✓However nearly 2500 minerals have been discovered so far. ✓The abundance of minerals depends on the availability of the...

ROCK-FORMING MINERALS  Elements Versus Minerals ✓By the use of the 118 chemically elements known to exist on earth, it is theoretically possible to make millions of compounds. ✓However nearly 2500 minerals have been discovered so far. ✓The abundance of minerals depends on the availability of the component elements that are available for its formation at or near the earth’s surface. 2  Elements Versus Minerals ✓Only 12 elements are sufficiently abundant. ✓These elements collectively make up 99.23% of the entire crust. ✓The fact that only 12 elements are abundant answer the discrepancy between the theoretically infinite number of minerals that should exist and the fact that only 2500 actually exists. ✓ Actually about 75% of earth crust is made of oxygen and silicon. 3 Abundant Elements on Earth Crust Elements Weight % Oxygen O 45.2 Silicon Si 27.2 Aluminum Al 8.0 Iron Fe 5.8 Calcium Ca 5.06 Magnesium Mg 2.77 Sodium Na 2.32 Potassium K 1.68 98% Titanium Ti 0.4 Hydrogen H.14 Phosphorus P.12 Manganese Mu.09 4  Actually because of the overwhelming abundance of oxygen and silicon, the silicate minerals which composed of those two elements are the most plentiful on earth. Thus silicates are called the ROCK- FORMING MINERALS. Minerals can be classified into: Silicates Minerals (rock-forming minerals) Non-Silicates Minerals Mineraloids 5 Silicate Minerals All silicate minerals posses the silicate oxyanion (SiO4)-4. The oxyanion resembles a tetrahedron as shown below because the four large oxygen ions are arranged so that their centers form the vertices of a tetrahedron. The small silicon ion sits among the four oxygen in the open space at the center of the tetrahedron. 6  Silicate mineral structures are therefore controlled by the ways tetrahedra packed together. Modern classification of silicate minerals is based on the way silicate tetrahedra are joined together as follows: 1. Framework Silicates All four oxygens in a tetrahedron are shared with other tetrahedra. Examples: Feldspars and Quartz 7 2. Sheet Silicates Three oxygen in a tetrahedron are shared with other tetrahedron. Examples: Muscovite (Micas) chlorite, clay minerals, Serpentine. 3. Chain Silicates There are single, double, triple chains. Two oxygens are shared. Example: Pyroxenes and amphiboles 8 4. Island Silicates The tetrahedra remain as discrete units sharing no common oxygens. Example olivine and garnet. Whether we have no, one, two, three or four oxygens shared is called the Degree of Polymerization. 9  By variations in the packing of the tetrahedra or by having another element besides the silicon (like Aluminum) we can have many hundreds of individual silicate minerals. Fortunately, just 11 common mineral groups account for more than 95% of all silicate minerals. 10 Important Silicate Minerals Quartz (framework) Pyroxene (Chain) Widely distributed Very large and complex mineral, most of sands group of minerals. and sandstone are composed largely of Amphibole (Chain) quartz. Very large and complex Feldspar (framework) group of minerals. The most abundant minerals (make up about Micas (sheet) 60% of earth’s crust). Parent of clay minerals. Olivine (Isolated) Occur mainly in igneous rocks. 11 NONSILICATE MINERALS Although there are many compositional families, only eight in addition to the silicates are important. 1. THE OXIDE (O2-)MINERALS The most widespread group of minerals after silicates. Ex. Magnetite, Hematite 2. THE HYDROXIDE (OH-)MINERALS Ex. Gibbsite 3. THE SULPHIDE (S2-)MINERALS Ex. Pyrite 12 NONSILICATE MINERALS 4. THE SULFATE (SO42-)MINERALS Ex. Gypsum, Anhydrite 5. THE CARBONATE (CO32-)MINERALS Ex. Calcite, Dolomite 6. THE PHOSPHATE (PO43-)MINERALS Ex. Apatite 7. THE HALIDE (Cl-,I-,Br-,F-) MINERALS Ex. Halite (rock-salt) 8. THE NATIVE ELEMENTS (C, Al, Au, Ag, Cu) Ex. Graphite, Native Metals 9. MINERALOIDS Ex. Glass 13 CLAY MINERALS One of the common silicate minerals with sheet structures are the clay minerals. They formed mainly by the alteration of the minerals, by the action of chemical weathering. The chemical process results in changes in the mineral form of the parent rock due to the action of water (especially if it contains traces of acid or alkali, oxygen, and carbon dioxide). 14 CLAY MINERALS Clay minerals can be defined as: They are crystalline particles (Predominantly silicates of Aluminum and/or Iron and Magnesium) resulted from the chemical weathering of parent rocks, mainly: Feldspars Mica (Muscovite). 15