Atoms, Elements and Minerals

Introduction

Minerals - naturally occurring inorganic, solids with crystalline structure ,have certain chemical composition and physical properties.

Minerals - building blocks of rocks, example: granite (Figure 2.1, p30), made up of four minerals: quartz (transparent, light gray), potassium feldspar (pink), plagioclase feldspar (white) and biotite (black).

Approximately 4,500 different kinds of minerals

Every mineral - orderly arrangement of atoms, definite chemical composition gives unique set of physical properties.

Physical properties - hardness (slide 189), cleavage (slide 195), luster (slide 178), color (slide 185), streak (slide 187), fracture (slide 224), specific gravity (slide 228), taste (slide 230), magnetism (slide 232).

Geologists use minerals to classify rocks.

Mineral - distinctive physical properties based on particular crystalline arrangement of its atoms and chemical composition.

Chemical formula mineral - type of atoms present in mineral and relative proportions expressed in whole number ratios.

Example: Quartz ( oxygen and silicon atoms). Quartz contains twice as many oxygen (O) atoms as silicon (Si) atoms. Chemical formula SiO₂ , indicating specific composition.

Example: Halite (rock salt), chemical composition NaCl. Meaning composed equal numbers of sodium (Na) and chlorine (Cl) atoms. Atoms arranged in orderly, three dimensional lattice (figure 2.4) creating box-like grid, cubic form halite crystals seen hand samples.

Atoms & Elements

Element substance cannot be broken down other substances except by radioactive decay.

Each atom element possesses same number of protons.

Atom - smallest possible particle of element, retains properties element. Atoms consist neutrons (subatomic particles contribute mass to an atom and electrically neutral (positive & negative charge)), protons (subatomic particles contribute mass and single positive electrical charge to atom) and electrons (single negative electric charge contributes virtually no mass to atom).

Protons and neutrons form nucleus of an atom, although nucleus occupies very small fraction of the volume of entire atom, almost all the mass is concentrated in nucleus (slides 1366, 1368).

Number of protons in an atom, element’s atomic number, example each atom of oxygen has 8 protons giving atomic number of 8 (Figure 2.5, p32).(slide 1369)

Atomic mass number, total number of neutrons & protons in atom. Example: atomic mass number of oxygen is 16 (8 protons + 8 neutrons). Heavier element more neutrons & protons compared to lighter elements (gold – atomic mass number 197, helium atomic mass number 4).

Isotopes of an element – atoms containing different number of neutrons, same number of protons (slides 1372 – 1374). Isotopes either stable or unstable. Unstable or radioactive isotope, protons or neutrons, lost or gained by nucleus over period of time.

Unstable isotopes – radioactive hazardous in high doses. Unstable isotopes of Uranium are very important to geology, used to determine the ages of rocks, radiometric age dating of rocks (cover later in semester in Chapter 8).

Element’s atomic weight, weight of average atom of element given in atomic mass units. Example: sodium, only one naturally occurring isotope, its atomic mass number and atomic weight are the same (23).

Bonding

Linking together or bonding of atoms to form minerals occurs as a result of an imbalance of electrons (deficit or surplus) in their outermost energy levels.

Electrically charged atom called an ion, tiny spheres behave like magnets. Positively charged ions attract negatively charged ions much like a magnet so that their electrical charges can be neutralized. Example: chlorine ion and sodium ion fixed in place by electrical attraction to each other, called ionic bonding (Figure 2.7, p34).

Ionic bonding most common type in minerals, atoms commonly bonded together by covalent bonding, adjacent atoms share electrons. Diamond composed exclusively of covalently bonded carbon atoms (Figure 2.8, p34). Covalent bonds diamond are extremely strong, diamond hardest natural substance on our planet.

Graphite, like diamond pure carbon polymorph, different crystal structure same chemical composition. Graphite used in pencils and as lubricant.

Third type bonding, metallic bonding found in metals, iron and copper. Atoms closely packed, electrons move freely throughout crystal keeping atoms together. Ease electrons move accounts for high electrical conductivity of metals.

Ions and Crystalline Structures

Atom most stable each energy levels completely filled with electrons.

Example, consider two ions (sodium & chlorine) bond form halite (rock salt) electrical attraction each other.

Na⁺ inner energy level filled 2 electrons and second energy level filled 8 electrons. One more electron neutralize positive charge 11 protons in nucleus. Eleventh electron alone energy level unstable, sodium atom gives up electron taken up other electron-deficient atoms.

Each sodium ion, 11 protons (11⁺ ) and 10 electrons (10^-) add up single excess positive charge (Na⁺) called cation.

Chlorine atomic number 17, complete inner energy level 2 electrons and complete second energy level 8 electrons around inner level. Neutral chlorine atom 7 electrons third level, level needs 8 electrons be stable, extra electron captured fill third level. Chlorine ion now contains 18 electrons (18^-) and 17 protons (17⁺) single excess negative charge (Cl^-) called anion.

Ions form stable mineral structure, become “glued” into place by bonding with ions opposite charge.

Silicon, second most abundant element in crust, most minerals contain silica (oxygen combined silicon). Example mineral quartz (SiO₂), pure silica in crystal form. (slide 138)

The Silicon – Oxygen Tetrahedron

Silicon and oxygen combine form atomic framework most common minerals. Basic structural unit consists consists 4 oxygen atoms (anions) packed together around single, smaller silicon atom, forming four sided geometric shape called silicon-oxygen tetrahedron (slide 139). Within silicon-oxygen tetrahedron, more negative charges than positive charges, formula SiO₄^-4 (slide 140). Silicon-oxygen tetrahedron stable within crystal structure, must balanced by enough positively charged ions or share oxygen atoms with adjacent tetrahedrons, reduce need for extra, positively charged ions.

Structures silicate minerals range from isolated silicate structure to framework silicate structures, all oxygen atoms shared adjacent tetrahedrons (Figure 2.12).

Isolated Silicate Structure

Individual silicon-oxygen tetrahedrons bonded together positively charged ions. Example: Olivine (Mg, Fe)₂ SiO₄, common mineral contains two ions, either magnesium (Mg⁺²) or iron (Fe ⁺²) each silicon-oxygen tetrahedron .

Chain Silicates

Forms when two tetrahedron’s oxygen atoms shared adjacent tetrahedrons form chain (slide 143). Each chain, net excess of negative charges. Minerals have single or double chain structure. Single-chain silicate structures, ratio silicon to oxygen 1:3, each mineral group (pyroxene group) has SiO₃ ^-2 in formula, electrically balanced positive ions, hold parallel chains together (slide 146). Double chain silicate, two adjacent single chains sharing oxygen atoms. Amphibole group characterized two parallel chains, every other tetrahedron shares oxygen atom adjacent chains tetrahedron. Chain silicates shaped like columns, needles or fibers (slide 147).

Sheet Silicates

Each tetrahedron shares three oxygen atoms form sheet. Mica group and clay group minerals sheet silicates (slide 148).

Framework Silicates

All four ions shared adjacent tetrahedrons, framework silicate structure. Example: Quartz framework silicate mineral, feldspar also framework silicate mineral. Feldspar, aluminum substitutes some silicon atoms some tetrahedrons, Al⁺³ substitutes for Si⁺⁴ some tetrahedons. Additional positive ions incorporated into crystal structure compensate aluminum’s lower charge. Feldspars, Na⁺,K⁺ or Ca⁺². Feldspars, collectively most abundant mineral group Earth’s crust formulas NaAlSi₃O₈, KAlSi₃O₈ and CaAl₂O₈ (slide 149, 159,160).

Nonsilicate Minerals

Do not contain silica, example: carbonates Co₃ their formulas. Calcite, CaCO₃ member of group, one most abundant minerals Earth’s surface, occurring mainly in limestone.

Gypsum, sulfate mineral (containing SO₄). Sulfides, sulphur not oxygen in formulas, example pyrite (FeS₂). Halite, NaCl member chloride group.

Native elements, one element in formulas, examples: gold (Au), copper (Cu), diamond & graphite (C).

Variations In Mineral Structures & Compositions

Some minerals same chemical composition, different crystalline structures (polymorphism). Example: calcite & aragonite same formula CaCO₃, atomic crystal structures very different. Two distinctive mineral types result separate conditions and processes formation. Example: Aragonite normally indicator high-pressure crystallization.

The Physical Properties of Minerals

Identify uknown mineral, first determine physical properties, match properties with appropriate mineral using mineral identification key.

Hardness – ability mineral withstand scratching, use fingernail (hardness: 2.5), glass plate (hardness 5.5) get hardness sample.

Cleavage – ability mineral break in preferred direction. Minerals may have one, two, three or more directions of cleavage or none, mineral said fracture.

Luster – manner mineral reflects light. Metallic if mineral looks like metal or non-metallic mineral not look like metal.

Streak – color mineral powdered form using streak plate.

Color – Not good way identify minerals, small amounts impurities will change color. Be careful using this property identify minerals.

Specific gravity – ratio of mass of substance to mass equal volume water. Example: Gold specific gravity 19.3 compared to galena specific gravity 7.5.

Striations, exsolution bands – striations fine parallel lines or grooves found surface cleavage planes. Exsolution bands color changes within sample