Chapter 2 - Minerals

Chapter 2 - Minerals

Rocks are aggregates of minerals. Many are silicate minerals. This granite, an igneous rock, has Quartz, an amphibole called Hornblende, a pink potassium feldspar, and a white Plagioclase feldspar Rockforming Minerals

Rock-forming minerals Common minerals that make up most of the rocks of Earths crust Only a few dozen members Composed mainly of the 8 elements that make up 98% of the continental crust Commonly formed Ion charges

often called oxidation state Metals can form more than one Ion. Fe+2 is name Ferrous, Fe+3 is named Ferric Classification of Minerals Silicates Most important mineral group Comprise most of the rock-forming minerals Very abundant due to large amounts of silicon and oxygen in Earths crust

Basic building block is the silicon-oxygen tetrahedron molecule Four oxygen ions surrounding a much smaller silicon ion The Component Atoms Oxygen has 6 electrons in its

valence shell Silicon has 4 electrons in Its outer shell Remember: atoms can gain or lose electrons They then combine with oppositely charged ions to form neutral molecules Ions

Anion (negative) Cation (positive) Silicate Molecule The Silicon-Oxygen Tetrahedron 2_25

O2 - Si4+ O2 - O2 The basis of most rock-forming minerals, charge - 4 O2 -

Silicate Bonding I Oxygen O atoms may obtain electrons from Si atoms, producing the SiO4 -4 Ion. The negative charge is balanced by positive metal ions. This occurs in Olivine, (Fe,Mg)2SiO4, a high temperature Fe-Mg silicate. Forms of this mineral are stable 100s of kilometers below Earths surface.

A type of Ionic Bond Example OLIVINE Positive ion Fe and Mg SiO4 -4 Ion Tetrahedron

facing down Tetrahedron facing up Independent tetrahedra Silicate Bonding II Alternately, the oxygen atoms may complete their outer electron shells by sharing electrons with two Silicon atoms

in nearby silicon tetrahedra. Mainly a covalent bond A Pyroxene Single chains weakly paired 2_26c An Amphibole

Positive ion Cleavages 56 and 124 deg Double chains (c) Example: Mica

Sheet silicates (d) Example: Quartz SiO2 2_26e Framework silicates

(e) (3-D, also the Feldspars) Summary Silicate Mineral Appearance Mica Feldspar

Olivine Quartz Pyroxene Classification of Minerals Common Silicate minerals Nesosilicates Independent Tetrahedra

Olivine (Mg,Fe)2SiO4 High temperature Fe-Mg silicate (typical mantle mineral - formed 100s km in Earth Individual tetrahedra linked together by iron and magnesium ions Forms small, rounded crystals with no cleavage

High interference colors No consistent cleavages Classification of Minerals Common Silicate minerals Pyroxene Group Single Chain Inosilicates for example (Mg,Fe)SiO3 Single chain structures involving iron and magnesium, chains weakly paired Two distinctive cleavages at nearly 90 degrees

Augite is the most common mineral in the pyroxene group Classification of Minerals Common Silicate minerals Amphibole Group Double Chain Inosilicates Ca2(Fe,Mg)5Si8O22(OH)2 Double chain structures involving a variety of ions Two perfect cleavages exhibiting angles of , e.g.

124 and 56 degrees in Hornblende. Hornblende is the most common mineral in the amphibole group Pleochroic in Plane Polarized Light Distinguish Hornblende from Pyroxene Group by cleavage Pyroxene Crystal

Two Cleavage Faces at about 90 degrees Hornblende Crystal 56 and 124 degree Cleavages Cleavage in Pyroxenes It isnt perfect in all slices

Cleavage in Amphiboles Classification of Minerals Common Silicate minerals Mica Group Phyllosilicates Sheet structures that result in one direction of perfect cleavage Biotite is the common dark colored mica mineral

Muscovite is the common light colored mica mineral KAl3Si3O10(OH)2 Muscovite In plane polarized light, Biotite is seen as dark brown to grey against the surrounding mostly colorless minerals. Under

crossed polars "bird's eye " = mottled = wavy extinction can easily be seen when the mineral is nearly extinct. Often, the mineral color masks the interference colors when the mineral is not extinct. Vkyyxjk

Pleochroic in PPL 3-D (Framework) Tectosilicates Quartz SiO2 Quartz Undulose extinction 1o grey for standard thin section thickness a thin section is 30 microns ( 3 hundredths

of a millimeter) Feldspars Common Silicate minerals Tectosilicates Feldspar Group Most common mineral group 3-dimensional framework of tetrahedra exhibit two directions of perfect cleavage at 90 degrees

K-spars (potassium feldspar) and Plagioclases (sodium to calcium feldspar solutions) are the two most common groups Pearly to vitreous Luster Potassium feldspar KAlSi3O8

Note Pearly Luster Tartan twins in Microcline. Microcline is the low TP version of K-spars KAlSi3O8 Microcline is Triclinic, Orthoclase is Monoclinic Perthitic Texture, Microcline plus exsolved Albite Plagioclase feldspar (Ca,Na)AlSi O 3

Note the Twinning, seems to have stripes Labradorite Albite 8

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