5 Mineralogy and Minerals Study Notes

Mineralogy and Minerals

5.1 Introduction

• Minerals serve as the building blocks of our planet.

• Minerals make up most rocks and sediments of the solid Earth.

• Minerals are important resources for mankind.

  • Industrial minerals: raw materials for chemicals, concrete, wallboard.

  • Ore minerals: source of valuable metals (copper, gold) and energy resources (uranium).

  • Gems: beautiful forms of certain minerals used in jewelry.

• Certain minerals pose health and environmental hazards.

• Mineralogy: the study of minerals.

• Mineralogist: A geoscientist specializing in the study of minerals

5.2 What Is a Mineral?

• Geological definition of a mineral:

  • Naturally occurring.

  • Solid.

  • Crystalline material.

  • Formed by geologic processes.

  • Definable chemical composition.

  • Almost always inorganic.

Naturally Occurring

• True minerals grow in nature, not in factories.

• Synthetic minerals are materials manufactured to have characteristics virtually identical to real minerals.

Formed by Geologic Processes

• Traditionally, minerals resulted from solidification of molten rock or direct precipitation from a water solution, without the involvement of living organisms.

• Geologists now consider solid, crystalline materials produced by organisms to be minerals.

• Biogenic mineral: term used when discussing materials produced by organisms.

Solid

• Matter in the solid state can maintain its shape indefinitely, so it will not conform to the shape of its container.

• Minerals cannot be liquids or gases.

Crystalline Material

• IIn a crystalline material, atoms are arranged in a fixed pattern, held together by chemical bonds.

  • Crystal structure: The three-dimensional geometric arrangement of these atoms or ions.

Definable Chemical Composition

• It’s possible to write a chemical formula for a mineral.

• Some minerals contain only one element (e.g., diamond and graphite, both have the formula $C$).

• Most minerals are compounds of two or more elements (e.g., quartz has the formula $SiO_2$).

• Some mineral formulas are complicated (e.g., biotite is $K(Mg,Fe)<em>3(AlSi</em>3O<em>{10})(OH)</em>2$).

Inorganic

• Organic chemicals:

  • Include carbon-carbon and/or carbon-hydrogen bonds.

  • Form in living organisms or have structures similar to those of chemicals that form in living organisms.

  • Examples: sugar ($C<em>{12}H</em>{22}O_{11}$), fat, plastic, propane, and protein.

• Almost all minerals are inorganic, in that they are not organic chemicals.

• Mineralogists now consider a few dozen organic substances formed by “the action of geologic processes on organic materials” to be minerals.

Minerals vs. Glass

• Both are solids but minerals are crystalline, while glass is not.

• Atoms, ions, or molecules in a mineral are ordered into a geometric arrangement.

• Those in a glass are arranged in a semi-chaotic way.

Examples of Minerals

• Motor oil: Not a mineral (organic liquid).

• Table salt (NaCl): Mineral (natural solid crystalline compound).

• Oyster shell: Mineral (biogenic).

• Rock candy: Not a mineral (organic chemical, sugar).

Basic Concepts from Chemistry

• Element: A pure substance that cannot be separated into other materials.

  • There are 92 naturally occurring elements.

  • Each has a name (e.g., hydrogen, carbon, silicon, oxygen, uranium) and a corresponding symbol (e.g., H, C, Si, O, U).

• Atoms and their components: The smallest piece of an element retaining the characteristics of the element is an atom.

• Atom: Consists of a nucleus surrounded by a cloud of orbiting electrons.

  • Nucleus: a compact ball of protons and neutrons (except hydrogen).

  • Electron mass: about $1/1,836$ that of a proton.

• Electron cloud: Consists of distinct orbitals, or electron shells, each of which contains a specific number of electrons.

• Charge: Characterizes the way a particle responds to an electrical current or to a magnet.

  • Electrons: negative charge.

  • Protons: positive charge.

  • Neutrons: neutral charge.

  • Like charges repel, unlike charges attract.

• Atomic number: The number of protons in an atom of an element.

  • Determines the elemental identity of an atom.

• Atomic mass: Approximately equals the number of protons plus neutrons in an atom of an element.

• Isotope: Atoms that have the same atomic number, but a different atomic mass, are called isotopes of an element.

5.3 Beauty in Patterns: Crystals and Their Structure

What Is a Crystal?

• Crystal: A single, continuous (uninterrupted) piece of a crystalline material bounded by flat surfaces, called crystal faces, that form naturally as the crystal grows.

  • Comes from the Greek word krystallos, meaning ice.

• The angle between two adjacent crystal faces of any mineral specimen is identical to the angle between the corresponding faces of any other specimen of the same mineral.

• The angle between crystal faces in one mineral isn’t necessarily the same as that in another.

• Crystals come in a great variety of shapes, including cubes, trapezoids, pyramids, octahedrons, blades, needles, columns, and obelisks.

Looking Inside a Mineral

• Max von Laue (1912) showed that an X-ray beam passing through a crystal breaks up into many tiny beams that produce a pattern of dots on a screen.

  • Diffraction: occurs when waves interact with regularly spaced objects whose spacing approaches the wavelength of the waves.

• Atoms within a crystal must be regularly spaced, and the spacing must be comparable to the wavelength of X-rays.

• Arrangement of atoms defines the crystal structure of a mineral.

• Crystal structures contain one of nature’s most spectacular examples of a pattern, the regular spacing of atoms and alternation of atoms if more than one element is present.

Crystal Lattice

• Mineralogists refer to a three-dimensional arrangement of points representing this pattern as a crystal lattice.

• The pattern of atoms in a crystal may control the shape of the crystal.

• Symmetry: the shape of one part of the structure represents the mirror image of the shape of a neighboring part.

Models of Crystal Structure

• Can be constructed using a cluster of balls packed together in an orderly way.

• Different colors or sizes represent different atoms, ions, or ionic molecules.

• Sticks represent chemical bonds.

• Packing of atoms or ions differs from mineral to mineral.

Anions and Cations

• Anions (extra electrons) tend to be bigger than cations. In minerals with ionic bonding, cations tend to nestle snugly in the spaces between anions.

Polymorphs

• Two or more different minerals that have the same chemical composition but different crystal structures are called polymorphs of a material.

• Diamond: each carbon atom packs together with four neighbors to form a tetrahedron.

• Graphite: carbon atoms lie in sheets with weak bonds holding the sheets together.

The Formation and Destruction of Minerals

• A new mineral crystal can form in one of five ways:

  1. Solidification of a melt (freezing of a liquid).

  2. Precipitation from a water solution.

  3. Solid-state diffusion.

5.4 How Can You Tell One Mineral from Another?

• Physical properties are used to distinguish minerals.

  • Color

  • Streak

  • Luster

  • Hardness

  • Specific Gravity

  • Crystal Habit

  • Special Properties

Color

• Results from the way a mineral interacts with light.

• A mineral absorbs certain wavelengths, so the color you see represents the wavelengths the mineral does not absorb.

• Color variations commonly reflect the presence of impurities.

  • Example: Trace amounts of iron may give quartz a reddish color.

Streak

• Color of a powder produced by pulverizing the mineral.

• Obtained by scraping the mineral against an unglazed ceramic plate.

• More reliable clue to a mineral’s identity than the color of a whole crystal.

Luster

• The way a mineral surface scatters light.

  • Metallic luster: looks like metal.

  • Nonmetallic luster: does not look like metal (silky, glassy, satiny, resinous, pearly, and earthy).

Hardness

• Resistance of a mineral to scratching.

• Represents the ability of bonds in the crystal structure to resist being broken.

• Mohs hardness scale: lists minerals in sequence of relative hardness.

  • A mineral with a Mohs hardness of 5 can scratch all minerals with a hardness of 5 or less.

Specific Gravity

• Represents the density of a mineral.

• Defined by the ratio between the weight of a volume of the mineral and the weight of an equal volume of water at 4°C.

• Ex: $quartz = 2.65$, $water = 1.00$. Specific gravity of quartz is 2.65

Crystal Habit

• Shape of a single crystal with well-formed crystal faces or the character of an aggregate of many well-formed crystals that grew together as a group.

• Habit depends on the internal arrangement of atoms in the crystal.

• Mineralogists refer to common geometric shapes using adjectives such as cubic, prismatic, bladed, platy, or fibrous.
  *

Special Properties

• Calcite ($CaCO<em>3$): reacts with dilute hydrochloric acid ($HCl$) to produce carbon dioxide ($CO</em>2$) gas.

• Dolomite ($CaMg[CO<em>3]</em>2$): also reacts with acid but not as strongly.

• Graphite: makes a gray mark on paper.

• Magnetite: attracts a magnet.

• Halite: tastes salty.

• Plagioclase: has striations (thin parallel corrugations or stripes) on its crystal faces.

Fracture and Cleavage

• Fracture: the way a mineral breaks which depends on the internal arrangement of atoms in the minerals

• Cleavage: If a mineral breaks to form distinct planar surfaces that have a specific orientation in relation to the crystal structure

5.5 Organizing Knowledge: Mineral Classification

Classification schemes help organize information and streamline discussion.

Minerals are classified based on their chemical makeup.

Baron Jöns Jacob Berzelius (1779–1848) analyzed minerals and established that most minerals can be classified by specifying the principal anion or anionic group within the mineral.

This approach divides the 4,000 known minerals into mineral classes.

Mineral Classes

Mineralogists distinguish several principal classes of minerals:

Silicates:

• Definition: Minerals containing the SiO44−SiO44−​ anionic group.

• Examples: Quartz (SiO2SiO2) and feldspar (KAlSi3O8KAlSi3O8​).

Sulfides:

• Definition: Minerals consisting of a metal cation bonded to a sulfide anion (S2−S2−-).Examples:Galena().Examples:Galena(PbS)andpyrite()andpyrite(FeS_2).Oxides:Definition:Mineralsconsistingofmetalcationsbondedtooxygenanions.Examples:Hematite().Oxides:Definition:Mineralsconsistingofmetalcationsbondedtooxygenanions.Examples:Hematite(Fe2O3)andmagnetite()andmagnetite(Fe3O4).Halides:Definition:Mineralswheretheanionisahalogenorsalt−producingion(suchaschloride,).Halides:Definition:Mineralswheretheanionisahalogenorsalt−producingion(suchaschloride,Cl^{-},orfluoride,,orfluoride,F^{-}$).

• Examples: Halite, or rock salt (NaClNaCl), and fluorite (CaF2CaF2​).

Carbonates:

• Definition: Minerals containing the CO32−CO32−​ molecule as the anionic group.

• Examples: Calcite (CaCO3CaCO3) and dolomite (CaMg[CO3]2CaMg[CO3]2​).

Native metals:

• Definition: Minerals consisting of pure masses of a single metal.

• Notes: The metal atoms are bonded by metallic bonds.

• Examples: Copper and gold.

Sulfates:

• Definition: Minerals consisting of metal cations bonded to SO42−SO42−​ anionic groups.

• Notes: Many sulfates form by precipitation out of water at or near the Earth’s surface.

• Example: Gypsum (CaSO4⋅2H2OCaSO4⋅2H2O).

BOX 5.2 PUTTING GEOLOGY TO USE

Hazardous Minerals

Some minerals contain chemicals that can poison you.

• Arsenopyrite (FeAsSFeAsS) contains arsenic which, when exposed to oxygen-bearing groundwater or air, undergoes a chemical reaction with oxygen to become a chemical that can dissolve in water, causing arsenic poisoning.

Asbestos

• Asbestos refers to a group of several silicate minerals that grow in clumps of fine, flexible needles or fibers.

• Minerals in this group have a very high melting temperature and are very strong.

• Some kinds of asbestos, if inhaled, become embedded in human lungs and can cause cancer.

• Asbestos has been banned for most applications.

• Remodeling or demolition projects for pre-1980s buildings require contractors to follow strict rules for asbestos abatement, including:

  • Enclosing the areas where asbestos is being removed.

  • Workers must wear protective clothing and minimize the production of asbestos dust.

Silicates

Quartz, feldspar, and many other silicate minerals can pose a danger if pulverized.

• The dust of pulverized silicate minerals can, if inhaled, become embedded in lungs and cause silicosis.

• The irritation caused by the dust triggers the growth of fibrous masses that eventually block access to air and make breathing difficult.

Silicates: The Major Rock-Forming Minerals

• Silicate minerals make up over 95% of the continental crust.

• Rocks of the oceanic crust and of the Earth’s mantle consist almost entirely of silicates, so silicates are the most common minerals on Earth.

• Silicates contain the SiO44−SiO44−​ anionic group.

• Four oxygen atoms surround a single silicon atom, defining the corners of a tetrahedron, called the silicon-oxygen tetrahedron (or silica tetrahedron).

Classes of silicate minerals:

• Independent tetrahedra:

  • Definition: Tetrahedra are independent and do not share any oxygen atoms.

  • Examples: Olivine and garnet.

• Single chains:

  • Definition: Tetrahedra link to form a chain by sharing two oxygen atoms.

  • Example: Pyroxenes.

• Double chains:

  • Definition: Tetrahedra link to form a double chain by sharing two or three oxygen atoms.

  • Example: Amphiboles.

• Sheet silicates:

  • Definition: Tetrahedra share three oxygen atoms and link to form two-dimensional sheets.

  • Examples: Micas and clays.

  • Notes: Sheet silicates have a single strong cleavage direction, and they occur in “books” of very thin sheets.

• Framework silicates:

  • Definition: Each tetrahedron shares all four oxygen atoms with its neighbors, forming a three-dimensional structure.

  • Examples: Feldspar and quartz.

  • Plagioclase tends to be white.

  • Orthoclase (K-feldspar) tends to be pink.

5.6 Something Precious: Gems!

• Gemstone: A mineral that has special value because it is rare and people consider it beautiful.

• Gem: A cut and finished gemstone ready to be set in jewelry.

• Precious stones: diamond, ruby, sapphire, and emerald.

• Semiprecious stones: topaz, tourmaline, aquamarine, and garnet.

• Pearls form in living oysters when the oyster extracts calcium and carbonate ions from water and precipitates them around an impurity, such as a sand grain, embedded in its body.

• Amber: fossilized tree sap which consists of organic compounds that are not arranged in a crystal structure, so it does not meet the definition of a mineral.

• Gem-quality garnets: clean, clear, large, unfractured crystals are unusual.

Cutting Gemstones

• Most gems used in jewelry have been “cut,” meaning that workers have made the smooth facets on the gem by using a faceting machine.

• Facets are not the natural crystal faces of the mineral, nor are they cleavage planes.

• A faceting machine consists of a doping arm and a lap.

• Different cuts have names, such as “brilliant,” “French,” “star,” and “pear.”

• Grinding facets takes a lot of work—a typical engagement-ring diamond with a brilliant cut has 57 facets!

PP NOTES

Chapter 5: Patterns in Nature – Minerals

Chapter 5 Goals

• Define a Mineral

  • Understand the criteria needed to classify a substance as a mineral.

• Atomic Bonding

  • Learn how the bonding between atoms affects a mineral.

• Creation of Minerals

  • Explore the processes through which minerals are formed.

• Mineral Identification

  • Learn methods and physical tests to identify minerals.

• Mineral Classes

  • Understand the various classes of minerals.

What Is a Mineral? Why Do We Care?

• Visual Example:

  • Crystals of gypsum precipitated from the hot water that once filled a cave.

• Observation Prompt:

  • What observations can be made from these natural crystals?

How is a Mineral Defined?

• Natural Occurrence:

  • Must form naturally (not synthetic) through geologic processes.

• Solid State:

  • Maintains its shape indefinitely.

• Crystalline Structure:

  • Atoms are fixed in place; the substance is organized.

• Definite Chemical Composition:

  • Has a specific chemical formula.

• Inorganic:

  • Does not originate from biological processes.

• Activity: Think-Pair-Share

  • Decide if each of the following is a mineral and justify why: • Ice • Water • Plastic • Bricks • Copper wire • Salt • Sugar • Glass • Lava • Coal

Mineral Extraction

• Example:

  • Using copper extracted from malachite or other copper ore minerals to make copper pots.

• Prompt:

  • Consider other examples of mineral extraction.

Mineral Properties

• Crystalline Requirement:

  • Minerals must have a crystalline (ordered atomic) structure.

  • Note: Solids without atomic order are classified as glasses.

• Crystals:

  • Definition:

    • A crystal is a continuous piece of a crystalline solid, usually bounded by flat faces.

  • Symmetry:

    • The ordered atomic arrangement provides symmetry to the crystal.

    • Equivalent faces on different samples of the same mineral maintain the same angular relationship.

  • Crystal Variety Observation:

    • Consider how different factors affect crystal appearance.

• Identification Challenge:

  • When using only physical properties to identify an unknown gemstone, what challenges might arise?

Atomic Bonding

• Fundamentals:

  • The geometry of atomic packing and type of chemical bonds determine physical properties.

  • Regular 3-D patterns of atoms influence crystal shapes and other properties.

• Bonding Specifics:

  • How elements pack into the crystal lattice depends on ion size and charge.

  • Key Point:

    • Anions are usually larger than cations, affecting the packing arrangement.

X-ray Diffraction: Inside of a Mineral

• Function:

  • Allows us to “see” how light passes through minerals.

• Outcome:

  • Analysis of diffraction patterns reveals the arrangement of atoms.

  • Patterns (spacing and alternation of elements) can influence crystal face angles (e.g., tighter spacing may yield 90° intersections).

Formation of Minerals

• Solidification of a Melt:

  • Liquid turns to solid (example to be determined).

• Precipitation from a Water Solution:

  • Dissolved ions settle out and bond; example: salt formation.

• Solid State Diffusion:

  • Atoms or ions migrate within a solid to form a new crystal structure; example: garnets forming under heat and pressure.

• Biomineralization:

  • Mineral formation at the interface between physical and biological substances.

  • Example: Shells formed by clams.

• Precipitation Directly from a Gas:

  • Process observed at volcanic vents or geysers where cooling gas molecules bond together.

  • Example: Formation of sulfur minerals.

• Crystal Growth ⇒

  • Crystals begin as small “seed” crystals and continue to grow, developing characteristic crystal shapes.

Mineral Destruction

• Mechanisms:

  • Minerals can be destroyed by:

    • Melting

    • Dissolution in aqueous solution

    • Chemical reactions with other minerals (especially at elevated temperature and pressure)

Crystal Growth Process

• Stage 1:

  • Early crystals act as seeds for further growth.

• Stage 2:

  • As crystals grow, they may encounter other crystals:

    • Open spaces allow for well-defined (euhedral) crystal faces.

    • Restricted spaces lead to inhibited growth and anhedral crystals (lack of defined faces).

• Stage 3:

  • Growth can be limited by available space, affecting the development of crystal faces.

Partner Review (Interactive Activity)

• Question:

  • What are the six characteristics that define a mineral?

Mineral Identification

• Overview:

  • Identification is a skill involving the recognition and testing of physical properties.

• Key Identification Properties:

  • Color:

    • Can be diagnostic (e.g., malachite) but is a poor indicator for minerals like quartz.

  • Streak:

    • The color of the crushed mineral (revealed on an unglazed porcelain plate).

  • Luster:

    • Describes how the mineral’s surface scatters light.

    • Two primary types:

      • Metallic

      • Nonmetallic (with several subdivisions)

  • Hardness:

    • Reflects scratching resistance and relates directly to atomic bond strength.

• Hardness Scale (Mohs Scale) & Values:

• Additional Identification Characteristics:

  • Crystal Habit:

    • Useful in identifying minerals with well-formed (euhedral) crystal faces.

  • Cleavage:

    • Tendency of a mineral to break along planes of weak atomic bonding.

    • Minerals lacking lattice planes of weakness tend to fracture rather than cleave.

  • Other Properties:

    • Effervescence (reactivity with acid)

    • Magnetism

    • Taste

    • Smell

    • Feel (tactile response)

    • Elasticity (response to bending)

    • Diaphaneity (degree of transparency)

    • Piezoelectricity (electric charge when squeezed)

    • Pyroelectricity (electric charge when heated)

    • Refractive Index (degree of light bending)

    • Malleability (ability to be pounded into thin sheets)

    • Ductility (ability to be drawn into thin wires)

    • Sectility (ability to be shaved with a knife)

• Cleavage Examples:

  • Examples of Cleavage – 1

  • Examples of Cleavage – 2

• Partner Review (Interactive Activity):

  • List and describe the characteristics that help identify minerals.

The Mineral Classes

• Major Classes Include:

  • Silicates*

  • Sulfides

  • Oxides

  • Halides

  • Carbonates

  • Native Metals

  • Sulfates

• Silicates Specifics:

  • Dominance:

    • Make up over 95% of the continental crust and nearly 100% of the oceanic crust; also predominantly compose the mantle.

  • Building Block:

    • The silicon-oxygen tetrahedron is the fundamental unit.

  • Abundance:

    • Only about 50 minerals are truly abundant; eight elements account for 98% of crustal mineral mass.

  • Groups Based on Tetrahedra Linkage:

    • Isolated Tetrahedra:

      • Do not share any oxygens; bonded together by cations.

    • Single-Chain Silicates:

      • Two of three basal oxygens are bonded together; Si:O ratio = 1:3 (e.g., pyroxenes).

    • Double-Chain Silicates:

      • Two single chains share oxygen atoms; Si:O ratio = 2:7 (e.g., amphiboles).

    • Sheet Silicates:

      • Tetrahedra share oxygens along the base but not at the top; Si:O ratio = 2:5 (e.g., micas, clay minerals).

    • Framework Silicates:

      • All oxygens are shared between tetrahedra; Si:O ratio = 1:2 (e.g., quartz, feldspars).

• Silica Tetrahedron – Elemental Percentages:

  • Oxygen: 46.6%

  • Silicon: 27.7%

  • Aluminum: 8.1%

  • Iron: 5.0%

  • Calcium: 3.6%

  • Sodium: 2.8%

  • Potassium: 2.6%

  • Magnesium: 2.1%

  • All others: 1.5%