Download MINERALS

MINERALS
Minerals - Defined
1.
2.
3.
4.
Naturally occurring
Inorganic
Fixed chemical formula
Unique orderly internal arrangement of atoms (crystalline)
Atoms to Rocks (Figure) - Shows how the mineral is the basic building block of
the geologist. In order to build minerals the atoms must join together. The
process of the joining of atoms is called bonding.
There are several mechanisms through which bonding can occur
but from the geologic standpoint only two are important:


Ionic
Covalent
Figure for NaCl. All atoms attempt to achieve the stable
configuration of eight electrons in the outer most shell. To do this
they can gain or lose electrons. This gain or loss causes the atoms
to become charged since there is now an imbalance between
positive charges (protons) and negative charges (electrons). For
NaCl; chlorine gains an electron and hence a negative charge
while sodium does the opposite. Ionic bonds are generally weak
and many of the compounds resulting from these bonds are
soluble in water.
Covalent bonds result from the sharing of electrons. See the Cl-Cl
Figure. Each chlorine shares one of its outer most electrons with
an adjacent chlorine atom. This sharing results in stronger bonds,
particularly where multiple electrons are shared.
Crystals are built by this sharing or overlapping of electron
orbitals. Since only certain arrangements minimize the mutual
repulsive forces between electrons these are favored giving each
crystal/mineral its unique internal geometric arrangement.
Physical Properties
Properties of minerals that the eye can readily discern. All physical
properties are to a large extent a function of the orderly nature of atoms
making up the crystals and how those atoms are joined to build the crystal
structure. Let’s look at some physical properties:






Color - Probably the least reliable of the physical properties. Caused
by impurities or lattice defects in the crystals. Example the yellow of
sulfur or green of malachite.
Streak - The color of a powdered sample of a mineral. More reliable
than color since same mineral seems to have same streak regardless
of color of hand sample. Drawback is that many minerals have a
white or colorless streak. Example the red-brown streak of hematite.
Hardness - The ability of one mineral to scratch another or an object
of known hardness. Hardness is directly related to the strength of
the bonds.
Cleavage - (Figure) The tendency of a mineral to split along certain
preferred planes. A function of a weaker bonds in one or more
planes or directions. Can have as many as 6 directions or as few as
one. If a mineral does not have cleavage it is said to have fracture.
Example: the concoidal fracture of quartz.
Specific Gravity - The weight of a mineral compared to the weight of
an equal volume of water. Can use heft as a crude estimate of
specific gravity. Most silicates 2-3. Metallic minerals 4-10. A function
of atomic packing.
Luster - Appearance of a mineral when held up to the light. Terms
most commonly used are metallic and nonmetallic. Metallic luster a
function of metallic bonding. Other terms waxy, resinous, vitreous,
earthy. A function of the interaction of light with the outer most shell
of electrons.
Classification of Minerals
A classification of minerals is a necessity if we are to talk about them since
there are over three thousand different minerals. We use the anion
classification system in introductory classes to pigeonhole similar
minerals. This is because minerals with common anions share many
common physical properties. Before discussing the classification let’s
examine the abundance of elements in the earth's crust (Figure). We can
expect that the most common minerals will be dominated by the most
abundant elements (see below). Silicates (built from silicon and oxygen)
are by far and away the most important/common minerals. Common Rock
Forming Minerals table lists only the 10 most common minerals but they
comprise 98% (by volume) of all minerals at the Earth’s surface.
The Common Rock Forming Minerals
o
o
o
Feldspar (silicate)
Quartz (silicate)
Muscovite (silicate)
Ferromagnesians
o
o
o
o
o
Olivine (silicate)
Pyroxene (silicate)
Amphibole (silicate)
Biotite (silicate)
Muscovite (silicate)
o
Calcite (not a silicate)
Silicates
Consists of a small silicon atom with a +4 charge surrounded in tetrahedral
fashion by four larger oxygen atoms each having a -2 charge (Figure). Net
charge on the anion group is -4. To satisfy this charge deficiency the SiO4
tetrahedra can either bond with cations (Fe, Mg, Ca, K, Na) or join with
other SiO4 tetrahedra through oxygen sharing.
A) Simple silicates (Figure) Simplest structure in which each
tetrahedra bonds to cations, usually Ca, Fe or Mg. Olivine is an
example. Also the most dense of the silicates due to the close
packing of the tetrahedra. Due to cation-tetrahedra bonds there are
no stronger or weaker bonds and hence no cleavage.
B) Chain silicates (Figure) Can be either single or double chain
silicates. Single chains share two basal oxygen while the double
chain shares three. Two examples of this group are the pyroxenes
(single chain) and amphiboles (double chain). Since the Si-O bonds
are stronger than the tetrahedra-cation bonds this subgroup has
fairly good cleavage in two directions.
C) Sheet silicates (Figure) Involves sharing of all basal oxygens to
form a sheet of silicate tetrahedra. On top of this layer is a layer of
cations, then another sheet of silicates, etc. etc. This gives the well
developed basal cleavage in this group. Common sheet silicates are
the micas.
D) Framework silicates - All four oxygens are shared to build up a
framework of tetrahedra. Good example is quartz.
Other Anion Groups (See Slides in Class)
Carbonates - Consist of cation plus the carbonate anion (CO3-2).
Important minerals calcite (calcium carbonate) and dolomite
(calcium, magnesium carbonate). Calcite (limestone) the most
important constituent in cement.
Oxides - Consist of oxygen plus a cation, often Fe, Ti, Al, Cu, or Cr.
Important oxides include hematite (Fe oxide) magnetite (Fe oxide).
Major source of world’s iron, aluminum and chromium.
Sulfides - Sulfur plus a metallic cation. Most of our important ore
minerals are in this group. Includes galena (PbS), pyrite (FeS2) and
numerous copper sulfides. Another important group of ore minerals.
Other Groups
a) Sulfates (S04) - Gypsum used for drywall
b) Halides (Cl,F,Br) - Rock salt
c) Phosphates (P04) - Apatite for fertilizer