Six-sided, pyramidal
Quartz Crystals.
Minerals are the building blocks of rock and the solid earth. All minerals 1. have a
specific chemical composition, 2. have a specific atomic arrangement (crystal form),
3. are solid, 4. inorganic and 5. naturally occurring.
Basalt is composed of minerals that
crystallize from magma derived from a
partial melt of the asthenosphere. Crystals
are too small to see with the naked eye.
Minerals can form during
crystallization of a magmatic melt.
Andesite is composed of minerals that
crystallize from magma derived from a
partial melt of basaltic ocean crust.
The large plagioclase crystals cooled
slowly underground.
Minerals can precipitate directly from an aqueous (out of water) solution, such as
the salt deposits shown in the image above in the desert playa lake.
The arrangement of the constituent atoms will define the crystal form of a mineral,
providing the crystal can grow in an unrestricted environment.
Quartz crystal
Note that the quartz crystals in this granite do not form well-defined pyramidal shaped
crystals because quartz is the last mineral to crystallize and inhibited by space.
Chemical bond strength and the
fixed arrangement of atoms will
define whether weakness
(cleavage planes) will form in
minerals. The halite (salt) crystal
shown in the image on the left has
three cleavage planes at right
angles (90°) to one another.
The weakness planes develop
between the ionic bonds between
the sodium (Na) and chloride (Cl)
atoms in the crystal lattice.
Atoms are the smallest particles that define the chemical properties of matter.
They are composed of protons (+ yellow) and neutrons (neutral orange) in the
nucleus and electrons (-) surrounding the nucleus in defined energy levels.
+1
+2
+3
+4
-2
-1
I have provided the common ionic charges for elements that are common in
earth rocks. Note that N and P and metallic elements can have more than one
valency state.
+1
+2
+3
+4
-2
-1
I have provided the common ionic charges for elements that are common in
earth rocks. Note that N and P and metallic elements can have more than one
valency state.
Covalent Bond
Covalent Bond
+1
+2
+3
+4
-2
-1
I have provided the common ionic charges for elements that are common in
earth rocks. Note that N and P and metallic elements can have more than one
valency state.
The relative hardness of a mineral is controlled by its composition and bond
strength between its constituent atoms. What mineral do you think is found on a
dentist’s drill?
Graphite and diamond have the same composition, but different atomic
arrangements. The bonds between the carbon sheets in graphite are weak
Van der waal bonds and it is very soft.
Diamond is also composed of carbon atoms, but they are arranged in a more
compacted structure than graphite and have strong covalent bonds between the
carbon atoms. Diamond is the hardest natural mineral.
Streak is most diagnostic for metallic minerals. Hematite (Fe2O3) leaves a
distinct reddish-brown streak on a porcelain plate.
Silicon and oxygen are the dominant elements comprising earth
rocks. The other common elements are all cations (have
positive ionic charges.
The silicate tetrahedron is a complex anion with a charge of -4 (SiO4-4).
It achieves charge balance and fulfills valencies by ionic and metallic
bonding with available cations or covalent bonds between oxygen
atoms of adjacent tetrahedra.
The olivine structure satisfies its
tetrahedral valencies with ionic and
metallic bonds with magnesium
(Mg+2) and iron (Fe+2) atoms. It is
the first silicate mineral to crystallize
from a magmatic melt. Olivine has
no cleavage planes and will break
along fractures.
(Mg,Fe)2SiO4
Pyroxene is a single chain silicate
that share two oxygen atoms.
Pyroxene has two cleavage planes
(at right angles). Can you infer
which atomic bonds will define these
cleavage planes?
(Mg,Fe,Ca,Na)(Mg,Fe,Al)Si2O6
Pyroxene is one of the main
minerals comprising basaltic ocean
crust.
The amphibole structure shares electrons
between 2 or 3 of its oxygen atoms in its
crystal structure. It will form two cleavage
planes (124° and 56°) within the crystal
structure. Note that water is also present in
the crystal structure.
(Na,Ca)2(Mg,Al,Fe)5(Si,Al)8O22(OH)2
Biotite: K2(Mg,Fe)6Si3O10(OH)2
Muscovite: K2Al4(Si6Al2O20)(OH,F)2
Muscovite and biotite micas are sheet silicates and have one cleavage plane
at 180°. Based on the relative bond strength between constituent atoms
within the crystal lattice can you predict where this cleavage plane forms?
Biotite contains Fe and Mg which gives it a dark appearance. Both micas
have dissolved water present.
The 3-dimensional framework silicates
share electrons between four of the oxygen
atoms in each tetrahedron (covalent bonds
exist between all tetrahedral oxygen
atoms). Quartz has no cleavage planes
and will break by conchoidal fracture. Note
that the planes shown in the image
above are crystal faces and do not
repeat throughout the crystal.
(Ca,Na)AlSi3O8
Plagioclase
Feldspar
The feldspar structure is similar to
quartz except an aluminum (Al+3)
will exchange with a silicon atom
(Si+4) within the tetrahedron and
require an additional electron to
satisfy the extra valency. This
bond can be between calcium
(Ca+2), sodium (Na+1) and
potassium (K+1), depending upon
composition of the magma and
crystallization temperature.
KAlSi3O8
Potassium
Feldspar
Other important mineral groups include the carbonates of which calcite
belongs. Calcite comprises limestone and marble.
Halides (e.g., halite, NaCl) also form an important mineral group which sustains
our lives.
Galena (PbS) is a mineral in the
sulfide group. It is an important
source of the world’s lead.
Hematite (Fe2O3) is a mineral
in the oxide group. It is an
important source of the world’s
iron.