Minerals : Earth Materials
quartz
    Minerals are naturally occuring, solid, chemical compounds with crystalline structures.  The term crystalline refers to the ordered, symetrical, arrangement of the atoms that make up the structure.

Although, the 91 naturally occuring elements in the Earth form or are hosted by thousands of minerals, just eight elements, make up over 98% of the Earth's crust. To get the relative abundances of these elements click here.


quartz with rutile inclusions (Paris School of Mines)

An ion is a charged atom.  Cations have a positive charge and anions have a negative charge. The value of the charge is a small integer; the unit charge being that of an electron.  A simple model of the chemical bonds that hold atoms together in minerals is that of the ionic bond.   Because of their unlike electrostatic charges, cations are bonded to anions.  While much of the chemical bonding in minerals is only partially ionic in character, the ionic model is simple and allows us to understand some things about coordination numbers and why there is atomic or ionic substitution of some elements for others in mineral stuctures.

With the redox conditions that exist in most of the outer portions of the Earth, the ionic states of the elements O, Si, Al, Fe, Ca, Na, K, Mg, are
  • Si+4
  • Al+3
  • Fe+2
  • Mg+2
  • Ca+2
  • Na+1
  • K+1
  • O-2  
The coordination number of a cation is the number of anions that are its closest neighbors, i.e. how many anions are surrounding the cation. In minerals where O is the only anion, the coordination numberof cations depends on their size. Those with the larger ionic radii have the higher coordination numbers. Common coordination numbers of the common cations are:
  • Si (4)
  • Al (4, or 6)
  • Fe (6)
  • Mg (6)
  • Ca (8)
  • Na (8)

    • K (8, 12)
The charges and relative sizes of the common ions of these eight elements are shown here.

Pairs of elements that commonly substitute for one another in silicates are:

  • Si and Al
  • Mg and Fe (and also Al)
  • Na and K

    • Na and Ca
Silicate Minerals
Oxygen and silicon are the most abundant elements in the Earth's crust, and the Si-O bond is among the strongest in minerals. Consequently, most of the crust is composed of silicate minerals.

The silicaon ion so small that, under ordinary conditions, only 4 oxygen ions can fit around it.
The silicon teterahedron is the basic building block common to various groups or classes of silicate minerals.

You can see the different structures developed by the tetrahedra here, and here. These different arrangements of silica tetrahedra gives rise to the structural classes A. independent tetrahedra, B. single chain silicates, C. double chain silicates, D.sheet silicates, and E. framework silicates


some important silicate mineral:  compositions and structures
  • olivines - (Mg,Fe)2SiO4 ; independent tetrahedra
  • pyroxenes - (Mg,Fe)2Si2O6) ; single chain
  • amphiboles - (W,X,Al)7-8(Z4O11)2(OH)2 ; double chains
  • biotite mica - K(Mg.Fe)3(AlSi3O10)(OH)2 ; sheet silicates
  • muscovite mica - KAl2(AlSi3O10)(OH)2 ; sheet silicates
  • alkali feldspars - KAlSi3O8---NaAlSi3O8 (solid solution series) ; framework silicates
  • plagioclase feldspars - NaAlSi3O8---CaAl2Si2O8 (solid solution series) ; framework silicates
  • quartz - SiO2 ; framework silicates
The composition of olivine and pyroxene minerals are variable with respect to their Mg and Fe contents.  Terrestrial olivines contain both Mg and Fe in varying proportions.  In the general formula for amphiboles given above W represents the large cations Ca, Na, and K (that can substitue for one another), X represents the smaller Mg and Fe, and Z represents the cations in the tetrahedral sites, Si and Al.

In the figure below, the compositions of known feldspars are represented by the blue area of the equilateral triangle.  There exist a complete solid solution series between K and Na  endmembers (alkali feldspars) and between Ca and Na endmembers (plagioclase feldspars), but only limited solid solution betwee K and Ca.

Would you like to see pictures of the silicates?
  • olivine - common independent tetrahedra silicates  12
  • plagioclase - common framework silicates   1   2
  • amphibole - common double chain silicates   1     2
  • pyroxene - common single chain silicates   1   2
  • biotite - common sheet silicate      1 2
  • alkali feldspar - common framework silicates   1 2
  • quartz - common framework silicate   1 2
anyone interested in extra information and images of minerals can check out

Non-silicate minerals

While silicate minerals constitute most of the outer part of the solid Earth, many other minerals are both geologically and economically important.  These include native elements such as gold, silver, and copper; oxides such as magnetite (Fe), bauxite (Al), and rutile (Ti) ; sulfides such as pyrite (Fe) and galena (Pb); sulfates such as gypsum (Ca) ; carbonates such as calcite (Ca) and malachite (Cu).

 summary