Physical Geology
Mineral Properties
Definition of a mineral
Mineral identification / mineral properties
hardness, cleavage, luster, crystal form, streak, color
Atoms/elements, protons, electrons, neutrons
Atomic weight, atomic number, ions
Atomic bonding: ionic, covalent, van der Waal’s
What is a mineral?
All minerals have distinctive physical properties. In addition to sometimes being useful to us, these properties allow us to identify different minerals.
A mineral example: Diamond
Named by Piny (77 AD), from the greek - ‘the invincible’.
Diamond as a mineral
Some diamond properties:
Diamond has a very high index of refraction. This means that light entering a diamond is bent at a high angle. If the diamond is cut correctly, most of the light entering the stone will be reflected back out the top of the stone. This is what gives diamonds their unusual brilliance.
Pure carbon can also form another mineral - graphite. If both diamond and graphite are made of the same element (carbon) why are they different minerals?
Graphite has a different crystal structure - rather than a tightly bonded network of atoms, the carbons in graphite are connected into sheets, which themselves are only loosly bonded to other sheets.
Graphite has the carbon atoms arrange in sheets
Diamond has carbon atoms arranged in a three-dimensional framework
Structure of diamond vs structure of graphite.
The different crystal structure of graphite gives it completely different physical properties from diamond.
Graphite is extremely soft - it is used as pencil ‘lead’ and as a dry lubricant.
Identifying Minerals
Elemental composition and crystal structure give minerals distinctive physical properties that can be used to identify each mineral.
Luster: The external appearance of the mineral–how it reflects light, what it ‘looks like’. eg. metallic, nonmetallic, vitreous (glassy), waxy, earthy, pearly, etc.
Hardness: As we saw with diamonds, the internal arrangement of the elements in a mineral has a great effect on hardness.
Hardness is measured on a scale of 1 ? 10 relative to a standard set of minerals. This scale was invented by a German mineralogist named Freidrich Mohs in the early 1800’s. On the Mohs Scale the softest mineral is talc (1), the hardest is diamond (10).
Hardness should not be confused with tenacity (what might commonly be called strength) which is a mineral’s resistance to being broken. Diamonds are difficult to scratch, but are very easily shattered.
Mohs Scale:
Practical Scale
Hard
6.0 - nail / knife (hard steel)
5.0 - window glass
Medium
2.5 - fingernail
Soft
Cleavage: When many kinds of minerals are broken, they tend to break preferentially in one or more directions called cleavage planes.
Minerals cleave because within the ordered arrangement of their atoms, the atomic bonds between some atoms are weak. If the internal arrangement of the atoms is such that the weak bonds form a continuous internal plane, then when stressed the atoms will separate along the plane.
Minerals can have from zero to six different cleavage directions. Minerals with no preferred direction of cleavage simply break randomly and are said to fracture.
If there are two or more cleavage planes in a mineral, then the angle between the cleavage planes can also be a distinctive characteristic of the mineral.
Halite structure and cleavage.
Salt (halite) NaCl has a regular geometric crystal structure of alternating large anions (Cl?) and smaller cations (Na+). Each ion is surrounded by 6 of its opposing ions. Notice that there are 3 planes along which the structure can be separated by breaking equal numbers of Na to Cl bonds. Thus Halite tends to cleave along 3 planes, each oriented 90° to the others.
Other distinctive properties
Streak: Some minerals, particularly those with a metallic luster, can appear very different in bulk specimens, but leave a distinctively colored streak of powder when rubbed against a ceramic plate.
Color: Some minerals have a distinctive color in bulk, but more often than not, minerals can be a variety of colors depending on the presence of various impurities in the crystal structure. A good example of this is quartz.
Crystal form: Crystal form is often mistaken for cleavage (and visa-versa). Crystal form is the characteristic shape that minerals have when they grow by solidification from melts or solution. Crystals are flat sided geometric figures (the flat sides are called faces). While the size and shape of the sides of crystals of a particular mineral are highly variable, the number of sides and the angles between them are generally constant.
A mineral’s crystal form can only develop if it grows unobstructed
Specific Gravity: This is the density of a mineral. If measured with a high degree of accuracy density can be very distinctive of different minerals.
A little chemistry review
Elements, the different kinds of individual atoms that compose the stuff of matter.
Element - substance that cannot be further subidvided by ordinary chemical or physical means
There are curently 92 naturally occurring elements..
Atoms are the smallest particles of an element that retain the identity and properties of the element.
OVERHEAD:Fig 2-1
Granite is made up of minerals. A mineral is composed of atoms arranged in a very orderly, 3-dimensional structure which makes it crystalline
OVERHEAD: Fig 2.2
The formula for halite is NaCl. This means that it is composed of equal numbers of sodium and chlorine atoms. Each sodium atom is surrounded by 6 chlorine atoms and each chlorine atom is surrounded by 6 sodium atoms. Billions of these atoms would be needed to form a salt crystal the size of a pinhead. Halite crystals tend to be cubic because of the way the atoms are packed together.
OVERHEAD:Fig 2.3
Atoms are composed of a nucleus surrounded by ‘clouds’ or ‘orbitals’ of orbiting electrons.
The nucleus of the atom:
Protons + charge, these give the element its distinctive properties
The atomic number of an element is its number of protons.
All atoms of any particular substance have the same number of protons and therefore the same atomic number
The atomic mass of an element is its total of protons plus nuetrons
Neutrons 0 charge, these stabilize the nucleus and lend weight to the atom. The same element can have different numbers of neutrons. These atoms are called isotopes
Isotopes of a particular element have the same number of protons but a different number of neutrons. Atomic number remains the same.
OVERHEAD: Fig. 8-18
Certain isotopes are subject to radioactive decay because the nuclei are unstable.
U-238 disintegrates radioactively to Pb-206
Radioactive decay is the spontaneous nuclear disintegration of isotopes with unstable nuclei.
As protons and neutrons leave these atoms, energy is produced.
As they emit subatomic particles and energy they become stable
OVERHEAD: Fig. 8-19
Radioactive decay is given in terms of Half-life. Half-life is the time it takes for a given amount of a radioactive isotope to be reduced by one-half. The other half disintegrates into daughter products.
TABLE 8.3
Naturally occurring isotopes that we find in rocks have very long half-lives
OVERHEAD: Fig 2.5
Electrons ? charge, these go whizzing around the nucleus in a series of orbits or shells.
Each orbit or shell of electrons around the nucleus can also be thought of as an energy level. Each particular orbit can hold a set maximum of electrons, and each orbit is most stable when it is filled to capacity.
The innermost shell holds a maximum of 2 electrons
Outermost shell can hold no more than 8 electrons
The number of electrons an atom can have is variable within limits.
Atoms have two general tendencies.
Atoms generally like to have as many electrons as protons so that the +’s and ?’s are balanced, that is so they are electrically neutral.
An ions is an elecrically charged atom or group of atoms.
Atoms with excess electrons relative to protons are called anions and are abbreviated with a negative subscript (eg. O2-).
Chlorine, with an atomic number of 17, has a complete inner shell with 2 electrons and a complete second shell of 8 electrons. A neutral chlorine atom would only have 7 electrons in the 3rd shell, but this shell requires 8 electrons so an extra electron is captured and incorporated in it. 18 electrons (18-) plus 17 protons (17+) gives the ion a single excess negative charge (Cl-)
Likewise, atoms that have a deficit of electrons relative to protons are called cations, and are abbreviated with a positive subscript (eg. Fe2+).
Sodium, with an atomic number of 11, has a comple inner shell and 2nd shell. An 11th electron alone in the outer shell would make theis atom very unstable so the sodium atom normally does without it. Therefore, 11 protons ( 11+) plus 10 electrons (10-) add up to a single excess positve charge (+1)
Ions are either larger (?) or smaller (+) than their respective uncharged atoms.
Atoms also like to have their outermost orbital either empty, or filled to capacity. However, this is often not compatable with their tendency to remain neutral.
The solution to this problem is to combine with other atoms and share electrons, this fills the orbitals without greating too much excess charge. - Bonding
OVERHEAD: 2.1:2
Carbon has an atomic number of 6 which means that the inner shell has 2 electrons and the outer shell only has 4. 4 more electrons are required to maintain electrical neutrality. Neighboring carbon atoms are so close together that each of the outer shell electrons spends half its time orbiting one atom and half orbiting the other.
Bonding may occur between atoms of two different elements OR between atoms of the same element
Groups of bonded atoms are called molecules.