All matter is composed of ATOMS. Atoms are composed of PROTONS (sub-atomic particles with POSITIVE CHARGES), ELECTRONS (subatomic particles with NEGATIVE CHARGES) and NEUTRONS (subatomic particles with NO ELECTRICAL CHARGE).
Protons and neutrons are found in the NUCLEUS (center) of the atom. Electrons are found whirling in orbit around this nucleus (see Fig. 2.3, p.30 of your text).
It is the number of PROTONS IN AN ATOM which defines the ELEMENT. (For example, if you have an atom with 14 protons, this atom is the element SILICON).
Within certain limits dictated by laws of nuclear physics, variable numbers of neutrons can be added to an element without altering its chemical behavior. Atoms of an element containing DIFFERENT NUMBER OF NEUTRONS are called ISOTOPES. As an example, the element oxygen always contains only 8 protons in the nucleus, but commonly occurs with either 8, 9 or 10 neutrons.
The result of adding neutrons to an atom is to vary its atomic mass slightly.
ELEMENTS are substances which cannot be broken down by ordinary chemical methods. There are 106 ELEMENTS which have been discovered by chemists. However, of those 106 elements, only a few occur in abundance on Earth. All the others are relatively rare.
The table below lists the 8 MOST ABUNDANT ELEMENTS IN THE EARTH'S CRUST. Note that these 8 elements account for almost 98% of all matter in the crust!
ELEMENT ABUNDANCE ( %) OXYGEN 46.6 SILICON 27.7 ALUMINUM 8.1 IRON 5.0 CALCIUM 3.6 SODIUM 2.8 POTASSIUM 2.6 MAGNESIUM 1.5
Atoms with RESIDUAL ELECTRICAL CHARGE are called IONS.
Atoms which GAIN ELECTRONS (i.e. electrons have been added) will be observed to have a NET NEGATIVE CHARGE because electrons have negative charges associated with them. Adding an electron is adding a negative charge to the atom. IONS with NEGATIVE CHARGE are referred to as ANIONS.
Atoms which are DEFICIENT IN ELECTRONS (i.e. electrons have been removed from the atom) have a NET POSITIVE CHARGE. This is because particles carrying negative charge have been removed, leaving an excess of positively charged particles behind. IONS with POSITIVE CHARGE are referred to as CATIONS.
Generally, cations and anions will join in such a way as to cancel their net electrical charges, forming an electrically neutral COMPOUND (i.e. a substance with no net electrical charge). The resulting chemical bond is said to be IONIC (see Box 2.2, p.32 of your text).
MINERALS: Ch. 2
Of all the compounds that exist in nature, some of these compounds are classified as MINERALS. What is a MINERAL? Why is it different from any other compound? Geologists have a very strict definition of a MINERAL.
In order for a material to be considered a mineral, it must meet five criteria:
1. The material must be a COMPOUND (i.e. formed from two or more elements).
2. The compound must be NATURALLY OCCURRING. Any synthetic or man-made compounds are excluded.
2. The compound must be SOLID. Liquids and gases, while natural substances are not solid and thus excluded.
3. The compound must have a DEFINITE CHEMICAL COMPOSITION.
4. The compound must have a DEFINITE CRYSTALLINE STRUCTURE.
When atoms form either covalent or ionic bonds and join in an ORDERLY THREE-DIMENSIONAL ARRANGEMENT, the substance formed is said to be CRYSTALLINE.
Point to ponder: Is water a crystalline substance? Is ice a crystalline substance?
Points to ponder: Is water a mineral? Is ice a mineral? Is coal a mineral? Is petroleum a mineral? Are cubic zirconia minerals?
There are over 4500 known minerals on Earth.
The most important minerals are the SILICATES, formed primarily of the elements SILICON, OXYGEN and lesser amounts of ALUMINUM, IRON, CALCIUM, SODIUM, POTASSIUM, MAGNESIUM.
Coincidentally (or, perhaps not so coincidentally), the elements listed above are also the 8 MOST ABUNDANT ELEMENTS on Earth.
PHYSICAL PROPERTIES OF MINERALS: Ch. 2
In hand specimens, minerals can be identified by their PHYSICAL PROPERTIES. There are a variety of physical properties of minerals which are useful in identifying an unknown mineral.
FRACTURE is a physical property displayed by all minerals and is used to describe the manner in which a mineral breaks. Types of fracture include IRREGULAR or HACKLY, CONCHOIDAL, and FIBROUS.
CLEAVAGE is a special type of fracture exhibited by many (but not all) minerals. It is a fracture which will occur along a distinctive plane or planes through a mineral and is related to the crystalline arrangement of atoms within the mineral. Because minerals have a distinctive crystalline structure, cleavage is a diagnostic aspect of a particular mineral or mineral group.
Also, because minerals have a distinct arrangement of atoms within them, the EXTERNAL CRYSTAL FORM of a given mineral will be unique and is a useful physical property. However, it is not that common to find well formed crystals of minerals and this is why items such as the quartz crystals mined in Arkansas are prized by collectors.
The HARDNESS of a mineral is a relatively reliable physical property and is measured on a scale from 1 to 10 with 1 (TALC) being the softest and 10 (DIAMOND) being the hardest.
Other minerals on the MOHS HARDNESS SCALE are GYPSUM (2), CALCITE (3), FLUORITE (4), APATITE (5), FELDSPAR (6), QUARTZ (7), TOPAZ (8), and CORUNDUM (9).
LUSTER is a physical property of minerals describing their relative "shininess". Typical terms used to describe mineral luster are METALLIC, NON-METALLIC, VITREOUS (glassy), RESINOUS, SILKY, PEARLY, WAXY, DULL.
Mineral COLOR can sometimes be used to identify a specimen. However, color often varies in extreme ways due to small amounts of impurities in minerals and geologists learn early not to trust color as an identifying feature.
Mineral STREAK on the other hand, is a very useful physical property and does not vary nearly as much as color. Streak is the color of a mineral in its powdered form. To obtain a streak, geologists scratch the mineral over a small piece of unglazed porcelain called a STREAK PLATE.
Finally, SPECIFIC GRAVITY is a very consistent physical property used to identify minerals. In words, the specific gravity of a mineral is the RATIO OF A MINERAL'S WEIGHT IN AIR TO THE DIFFERENCE OF ITS WEIGHT IN AIR AND ITS WEIGHT IN WATER.
Mathematically, specific gravity is calculated by weighing the mineral on a balance in air, then weighing the same mineral on a balance in beaker of water. Then, these two weights are compared in the following way:
(WEIGHT IN AIR)÷(WEIGHT IN AIR - WEIGHT IN WATER) = SPECIFIC GRAVITY
The number represented by this quantity is the specific gravity of the mineral.
SILICATE MINERALS: Ch. 2
The basic building block of the silicate minerals is the SILICA TETRAHEDRON.
This is a pyramid-shaped molecule composed of 1 silicon atom (cation) surrounded by 4 oxygen atoms (anions). In the linked image above, the green spheres represent oxygen anions. The single silicon cation is not visible, but beneath the central oxygen anion (see Fig. 2.10, p.36 of your text).
The silica tetrahedron can be assembled in a variety of ways to produce a great number of SILICATE MINERALS which have been categorized into 5 CLASSES OF SILICATE MINERALS:
Some silicate minerals are formed of ISOLATED SILICA TETRAHEDRA loosely bound by cations (such as magnesium and iron) to form a mineral. The most common example of the isolated tetrahedral silicates is OLIVINE.
By recombining silica tetrahedra slightly, a new silicate structure can be formed. By allowing one corner of each tetrahedron to be joined to another tetrahedron, a chain of tetrahedra is formed (see Fig. 2.12, p.37 of you text). The family of silicates with this particular structure are known as the SINGLE-CHAIN SILICATES. The most common group of minerals with this structure are called PYROXENES and one of these which you will see in the lab is AUGITE.
Joining 2 rows of tetrahedral chains will produce the DOUBLE CHAIN SILICATE structure (see Fig. 2.10, p.37 of your text). Notice in this structure that there are large hexagonal spaces between the chains into which many larger cations (such as potassium or sodium) will fit. The most common group of minerals with this structure are called AMPHIBOLES and one of these which you will see in the lab is HORNBLENDE.
Increasing the complexity of the silicate structure by joining the bases of tetrahedra will produce the SHEET SILICATE structure (Fig. 2.10, p.37). This structure is characteristic of CLAYS and a group of silicates known as MICA. Common forms of mica include BIOTITE (shown here) and MUSCOVITE
Finally, the most complex silicate structure is that of the FRAMEWORK where each individual tetrahedron is joined at every corner to another tetrahedron (Fig. 2.10, p.37). Good examples of FRAMEWORK SILICATES are the minerals PLAGIOCLASE, POTASSIUM FELDSPAR, and QUARTZ.
Click on the button to view images of common rock-forming minerals
Many people become fascinated with the intricate world of minerals and crystals every year. The links below provide you with just a few sites so that you might discover the hobby of "rock hounding" for yourself. Enjoy! :-)
ROCK HOUNDING ARKANSAS BOB'S ROCK SHOP ARKANSAS DIAMONDS U.S. MINERALS MANAGEMENT SERVICE
GOOD LUCK ON YOUR FIRST EXAM!