Mineral Lecture

GEOL 1121 (Written by T. Weiland)

Minerals are a critical part of our everyday life. They are used in the construction of our buildings, homes. roads, and machines. They are used to fertilize our crops, produce energy for our homes, add flavor to our foods, and even make many of our medicines. As civilizations progressed from the Stone Age to the present, humans learned new and different ways to use minerals and earth materials to better their living conditions. Rocks are aggregates of one or more minerals. Therefore, mineral identification is also a crucial part of rock classification.

I. Mineral Definition - naturally-occurring solid with an ordered atomic arrangement and a chemical composition which is fixed or which varies only within well-defined limits.

A. Naturally-occurring - "nonsynthetic" - This eliminates all unnatural man-made substances like plastic and synthetic compounds.

B. Solid - This eliminates gases and liquids.

C. Ordered atomic arrangement - "atoms are arranged in some geometric pattern" - This eliminates inhomogeneous solids such as glass. The ordered atomic arrangement is reflected in the crystal form and cleavage of the mineral (example - the rhombs of calcite).

D. Chemical composition which is fixed or which varies within well-defined limits - "all minerals are characterized by a chemical formula" - Some minerals have chemical substitutions in which ions of similar size and charge freely substitute for each other (such as the plagioclase series).

II. Variations in Composition and Crystalline Structure - Most minerals contain impurities and several also display ionic substitution. There are other minerals which have identical chemical compositions, but different crystalline structures due to the conditions under which they crystallized.

A. Impurities - Most minerals have non-structural ions trapped or included in the atomic structure during growth of the structure.

Example - quartz - SiO2

Impurity

Color

Name

None

Clear

Water Quartz

Water

White

Milky Quartz

Iron

Purple

Amethyst

Titanium

Pink

Rose Quartz

B. Ionic Substitution - variations in composition resulting from a systematic substitution of ions - There are several minerals which display solid solution (solids which act like solutions during crystallization and melting). The olivine group, forsterite (Mg2SiO4) - fayalite (Fe2SiO4) is a good example. Both Mg+2 and Fe+2 have the same charge (+2) and about the same ionic size so that either can fit into the olivine crystalline structure.

Coupled Ion Substitution - the plagioclase series - NaAlSi3O8 to CaAl2Si2O8 is an example of a solid solution with coupled ion substitution - The ions don't have the same charge, but are able to substitute for one another when coupled with another ion. Within the plagioclase crystal structure, Ca+2and Al+3 with a combined charge of +5 substitute for Na+1 and Si+4 which also have a combined charge of +5.

C. Polymorphs - minerals which have the identical chemical composition, but different internal structure - The best examples are the carbon polymorphs, diamond and graphite, which are both composed of pure carbon but have substantial differences in their atomic packing and bonding.

III. Physical Properties

A. Crystal Form - the shape of a mineral when bounded by smooth, planar surfaces which form regular geometric patterns - The formation of crystal faces requires favorable conditions such as cooling rate, pressure, and available space. Crystal form is often used in determining the amount of symmetry present in the crystalline structure.

B. Hardness - measure of the mineral's ability to resist abrasion - Hardness reflects the strength of the bond between atoms within the crystal structure. It is often variable with orientation due to differences in bond strength with changes in crystal orientation.

MOH's Hardness Scale - a relative, not an absolute scale of mineral hardness

1.Talc

2.Gypsum ----------------Fingernail (2.2)

3.Calcite -----------------Copper penny (3.1)

4.Fluorite

5.Apatite -----------------Glass (5.5)

6.Orthoclase ------------Steel (6.5)

7.Quartz

8.Topaz

9.Corundum

10.Diamond

C. Breakage

1. Cleavage - tendency of minerals to break along parallel planes of weaknesses (cleavage planes) within the crystal forming parallel planar surfaces along broken fragments - Cleavage results from weaker bond strengths along the certain planar directions within the mineral. *The number of cleavage planes and the angles between the cleavage planes are important characteristics used in identifying minerals (example - micas, excellent cleavage in 1 direction; halite, good cleavage in three directions, each at 90o to each other; and sphalerite, 6 good directions not at 90o).

*Difference between cleavage and crystal form - crystal form is only an external reflection of atomic structure which is lost when the crystal is broken. In contrast, cleavage is related to planes of weakness which are found throughout the mineral. Cleavage planes will form no matter how finely the crystal is broken.

2. Fracture - a lack of cleavage that results in an absence of planar surfaces when the mineral is broken - Minerals that display fracture break like glass (ex. - quartz with conchoidal fracture).

D. Specific Gravity - comparison of a mineral's density with the density of water.

Specific Gravity = density of the mineral / density of equal vol. of water = X / 1 gr./cm3

Example - graphite 2.4 gr./cm3, so Sp.G.=2.4 (unitless);

galena = 7.5, gold = 15 to 19

E. Color - useful for some minerals (ex. olivine is always green), but commonly too variable for most (ex. quartz can be almost any color).

F. Luster - appearance of the mineral in reflected light - Luster is desribed as metallic or non-metallic. Submetallic is further described as vitreous (glassy) or non-vitreous.

G. Streak - color of the mineral when it is powdered - The streak helps eliminate surface effects such as weathering. A white unglazed porcelain plate is used to powder the mineral.

H. Other Properties - magnetism (magnetite), taste (halite), and fluorescence (some fluorite).

IV. Common Rock-forming Minerals - there are over 3,500 known minerals, with several new ones described each year; however, only a small number of these are abundant. Minerals are grouped according to the anion or anion complex that they contain. Example - silicates (SiO4)-4, carbonates (CO3)-2, sulfides (S), oxides (O-2), and halides (Cl-1 or F-2). The minerals in each group often display similar properties and are commonly found together due to their similar chemical composition.

A. Silicates - minerals whose crystalline structure contains the SiO4 tetrahedra - The silica tetrahedra is the basic building block of all silicate minerals. Silica and oxygen make up about 74% of the earth's crust. Silica (+4) bonds with four oxygen (-2) such that there is a residual -4 charge. Other cations (like Ca, Na, Mg, and Fe) either link the tetrahedra together or are incorporated in the tetrahedral structure, determining the mineral form.

**Page 33 of your book shows various linking of the tetrahedra. As these silica tetrahedra are linked together, the ratio of silica to oxygen changes. Example- single tetrahedra (1:4), single chain (1:3), double chain (4:11), sheets (2:5), 3-dimensional frameworks (1:2).

Ferromagnesian Silicates - silicates with iron and/or magnesium in their structure. Most ferromagnesium minerals are dark-colored and more dense than the non-ferromagnesian silicates.

1. Olivine – a solid solution series - MgSiO4-FeSiO4 (single tetrahedra silicates which show a continuous range in ionic substitution). Olivine is a high temperature mineral that lacks cleavage and has a greenish colored, glassy luster, and conchoidal fracture. *It is a major component of the mantle that is common in Fe- and Mg-rich igneous rocks.

2. Pyroxenes - solid solution series with 3 major end members: MgSiO3-FeSiO3 -CaSiO3. These are single chain silicates. Pyroxenes are dark-colored high- temperature minerals with two well-developed cleavage planes at about 90o to each other. The cleavage is along the bonds that join the silica chains. Pyroxenes are abundant in igneous and metamorphic rocks.

3. Amphiboles - complex double-chain silicates which include several different solid solution series - Hornblende is the most common amphibole. Amphiboles are dark-colored minerals that have two well-developed cleavage planes at 56o and 124o to each other. They are common in igneous and metamorphic rocks.

4. Garnet - series of single-tetrahedra silicates which characteristically occur in well-formed near-spherical, twelve-sided crystals - Garnets show extensive variation in color. They are very hard (7-7.5), lack cleavage, translucent to transparent and have a vitreous luster. They are most common in metamorphic rocks.

5. Biotite - iron-rich member of the micas (sheet silicates). Weak Van der Waal's bonds between sheets results in perfect cleavage in one direction. Biotite is a dark-colored mineral with a vitreous luster. It is common in igneous and metamorphic rocks.

Non-ferromagnesian Silicates - silicate minerals without substantial Fe and Mg in their crystalline structure. These are generally lighter-colored than the ferromagnesian silicates.

a. Plagioclase Feldspars - solid solution series between anorthite (CaAl2Si2O8) and albite (NaAlSi3O8). These are light-colored, framework silicates which have two directions of cleavage at about 90o. The Na-rich albite is generally white, whereas the Ca-rich varieties are often blue-gray. All plagioclases are characterized by fine, parallel lines along the cleavage planes (striations). Plagioclase feldspars are one of the most abundant minerals found in igneous and metamorphic rocks.

b. Potassium Feldspars - solid solution series between albite (NaAlSi3O8) and orthoclase (KAlSi3O8). The K-feldspars are also 3-dimensional framework silicates which display 2 directions of cleavage at about 90o. The pink color of orthoclase is diagnostic. Potassium feldspars are common in igneous and metamorphic rock types.

c. Quartz - a three-dimensional silicate (SiO2) of almost pure silica and oxygen - It is one of the most common minerals in the earth's crust. There is no residual charge in the silica tetrahedra because all of the oxygen are shared by two silica atoms. This results in a very resistant mineral which often survives after all the other components a the rock break down forming river and beach sand. Quartz displays conchoidal fracture, hardness of 7, and a glassy luster. Color is highly variable.

d. Muscovite - white mica (sheet silicate) with the same characteristics as biotite, but with a white to silver color and transparent to translucent nature. Muscovite is common in igneous and metamorphic rocks.

B. Carbonates - minerals which contain the carbonate (CO3)-2 anion complex.

Calcite - CaCO3 - calcium carbonate which occurs as thick masses of limestone, chalk and marble . It is relatively soft (3), has perfect rhombohedral cleavage (75o), and reacts with HCL.

2HCl + CaCO3 CO2 + H2O + Cl -1+ Ca+2

Calcite is commonly precipitated from concentrated solutions or extracted from sea water by marine organisms. (coral reefs and muds of the Keys and south Florida).

C. Sulfides - minerals which contain the sulfur anion (S).

1. Galena - PbS - lead sulfide which has a metallic luster, perfect cubic cleavage, and a high specific gravity (7.5). It is the most important source of lead.

2. Pyrite - Fe2S - iron sulfide which is a yellow, metallic mineral which has a hardness of 6-6.5 and lacks cleavage. It has a greenish-black streak. It is an important source of sulfur.

D. Oxides - minerals which contain oxygen anions (O).

1.Hematite - Fe2O3 - iron oxide which is commonly dark red to steel blue-black - It gives a deep red streak, lacks cleavage and has a moderately high specific gravity (5-6.5). Hematite is mined for its iron content.

2. Limonite - Fe2O3.H2O - a yellowish-brown, hydrous iron oxide which usually forms by the weathering of iron minerals - It is characterized by a yellow streak, absence of cleavage and a dull rusted metallic texture. It is also mined for iron.

E. Halides - minerals which contain Cl, F or any of the other halogen elements as anions.

1. Halite - NaCl - sodium chloride characterized by cubic cleavage, clear or transparent nature, salty taste, and a resinous luster. It forms by the precipitation from sea water.

2. Fluorite - CaF2 - calcium fluoride which has good cleavage in four directions, variable color, hardness of 4 and a specific gravity of 3.

F. Others - Native elements ((Au, Ag, Cu, C), sulfates (CaSO4) anhydrite and CaSO4.2H2O qypsum), and clay minerals (kaolinite)

V. Uses Of Minerals - See Appendix C in your book (pages 669-672).

A. Economic Minerals - minerals with a commercial value

Ore - a mineral or aggregate of minerals that is economically minable. (varies according to demand)

1. Precious Metals - These include native metals (gold, silver, copper) that occur in pure deposits and also metal-bearing minerals such as galena (lead) and hematite, limonite, magnetite (iron). Many of these are Strategic Minerals, minerals that are critical to our national defense. We currently import >50% of the top 13 strategic minerals such as Cobaltite and chromite.

2. Nonmetallic Ores - These include all nonmetallic minerals that are used most commonly for industrial or constructional purposes. Examples include halite (food preparation), kaolinite (ceramics), calcite (concrete, fertilizer), gypsum (wall board), quartz (glass, concrete), and fluorite (iron smelting).

B. Gemstones - precious or semi-precious stones that are cut and polished for ornamental purposes. They are characterized by high hardness, unusuall color, brillant luster, and rare occurrence.

1. Value - based on quality and size - The quality is determined by the number and type of impurities and umperfections (fractures, discoloration). The size is measured in the following units:

Carat - a fifth of a gram

Point - a hundreth of a carat

(note karat is a measure of the proportion of gold mixed in a gold/alloy mixture)

2. Common Examples

Diamond (hardness = 10)

Corundum (hardness = 9) includes ruby (red) and sapphire (blue)

Garnet (hardness = 7)

Topaz (hardness = 8)

Quartz (hardness = 7) includes amethyst, citrine, tiger's eyer