Chapter 3: Minerals

Slide 4: What Is a Mineral?

• Minerals must meet several criteria:

  • 1. Naturally occurring

  • 2. Formed by geologic processes

  • 3. Inorganic

  • 4. Crystalline solid

  • 5. Definite chemical composition

• 4,000 different types of minerals on this planet.

• Theres a little more that 4,400 naturally formed minerals.

• Well over 5,000 known mineral types.

• We can synthesize minerals in a lab, so they are not naturally formed.

• Only 30 mineral species are quite common on this planet.

• Only about 30 mineral species that we see make up the crust.

  • A lot of silicates

  • Olivine

  • Amphibole

  • Horneblende

  • Plagioclase

  • Mica

  • Potassium feldspar

  • Quartz

Slide 5: Naturally Occurring

• You can synthesize diamonds in a lab.

• Synthesizing minerals in a lab create mineral simulates.

Slide 6: Formed by Geological Processes

• Being formed out in nature.

Slide 7: Inorganic

• An inorganic mineral is a naturally occurring substance that has never been alive and is not derived from living organisms.

• Aragonite is inorganic.

• Biogenic minerals - made of biological things, but they themselves aren’t alive (ex: teeth, seashells)

Slide 8: Crystalline Solid

• Atoms, ions, or molecules that make up the solid exist in a regular, well-defined arrangement.

Slide 9: Definable Chemical Composition

• With a definite chemical composition(every time we see the same mineral it has the same chemical composition that can be expressed by a chemical formula).

• Example: Salt = NaCl

Slide 12: Mineral Crystals

• Minerals are composed of 1 or more elements, the atoms are bound together by chemical bonds.

• A crystal is a single continuous piece of crystalline solid typically bounded by flat crystal faces.

  • Minerals are not always found as perfect, full crystals.

• Crystal faces grow naturally as the mineral forms and reflect atomic structure, growing in an orderly arrangement of atoms.

  • Example: prismatic

  • Its touching silicon and oxygen ions in a scaffolding way.

• The geometry of the atomic arrangement defines the crystal structure and the nature of chemical bonding determine the mineral properties.

• What is providing these ions to this crystal to continue it to grow?

  • Can be several different settings:

    • Example: Hydrothermal type setting.

      • Mineral-rich water (i.e. chunks of ions), crystals are able to grow by putting those out of the solution and attaching them in an orderly manner as its growing.

• Lattice structures give us sets of properties for us to test minerals (clues on how to identify mineral).

Slide 13: Chemically Bonding

• Covalent bonds in a mineral can determine strong the mineral is.

• Molecules help form a pattern in the structure of a mineral.

• The two polymorphs of carbon (diamond and graphite) are the hardest and softest minerals, a result of different types of chemical bonds.

• Color is not a good way to identify minerals, do not look at color first. Many minerals vary is color. Quartz can be any color, for example.

• Different minerals grow different crystal shapes.

• Crystal habit - is the ideal shape of the crystal.

Slide 19: Perfect Minerals Don’t Always Happen

• Mineral growth restricted by space prevents development of good crystal faces.

• Where crystals can grow into open space, good crystal faces form euhedral crystals.

• Where growth in inhibited, no faces result and form anhedral crystals (interlocking crystals).

Slides 20-24: How Does Earth Make Its Crystals

• Solidification - occurs when molten rock, such as lava or magma, cools and different minerals grow in succession.

  • Can happen below the earth - intrusive.

  • Can happen above the Earth’s surface - extrusive.

  • Certain minerals crystallize at certain temperatures as the magma is cooling down.

• Precipitation - occurs from volcanic gas, deep sea, hydrothermal vents, or element-rich gas. Occurs when water in a salty desert undergoes evaporation.

• Biomineralization - Refers to the production of minerals by organisms. Reef organisms, extra ions from water to make shells.

• Diffusion - Metamorphic process, atoms migrate through the crystal and new minerals grow inside the rock, happens slowly.

• 3 ways in which minerals can be destroyed:

  • Water

  • Erosion

  • Heat/melting

Slides 26-61

• We did not go through these slides, the rest are notes from the slides.

Mineral Identification - X-ray Diffraction

• X-ray diffraction (XRD) is still used today to identify minerals.

• XRD determines the spacing of lattice planes inside the mineral.

Mineral Identification - Physical Properties Testing

• Mineral identification is a skill. It requires learning mineral physical properties and ways test for them.

• Some properties are readily apparent; others require handling. As with any skill, practice is key to learning how to identify minerals by their properties.

• The properties of minerals (color, streak, luster) are an indication of the crystal structure and chemical composition.

  • The geometry of the atomic arrangement defines the crystal structure and the nature of chemical bonding determine the mineral properties.

• What to look at when identifying minerals:

  • Color

  • Hardness

  • Streak

  • Luster

  • Cleavage

  • Fracture tendency

  • Reaction to acid

  • Crystal habit

  • specific gravity

• Color is a diagnostic of some minerals (turquoise) but a poor indicator for others (quartz).

• Hardness

  • The scratching resistance of a mineral, which is directly linked to chemical bond strength.

  • Whatever does the scratching is harder than whatever is being scratched.

• Streak

  • The color of a mineral when it is powered (rubbing it on an unglazed porcelain plate).

  • Some minerals streak the same as their look, others leave a very diagnostic streak.

  • Some minerals won’t streak.

  • Minerals with a hardness <6 will streak, the porcelain plate has a hardness of 6.

• Luster

  • Luster refers to the way that a mineral surface scatters light.

  • The two main subdivisions of luster are metallic and nonmetallic.

  • Metallic - looks like a metal.

  • Nonmetallic:

    • Silky

    • Vitreous (glassy)

    • Satiny

    • Resinous

    • Pearly

    • Earthy (dull)

    • Adamantine (brilliant)

• Cleavage

  • The tendency for a mineral to break along lattice planes with weaker atomic bonds.

  • Cleavage creates flat, shiny (reflective) surfaces that may occur in steps.

  • Cleavage can be distinguished from crystal faces because it is through-going; faces are external crystal surfaces only.

  • The number of cleavage planes and their angle to one another are diagnostic.

• Fracture

  • Minerals fracture when they break through the lattice planes instead of along them.

• Crystal habit

  • Crystal habit is a useful property of euhedral crystals.

  • Identifying the shape of crystal faces.

• Other less common physical properties that are useful for identifying minerals:

  • Effervescence–reactivity with acid

  • Magnetism–magnetic attraction

  • Taste & Smell –proceed with caution!

  • Feel–tactile response

  • Elasticity–response to bending

  • Diaphaneity–relative transparency

  • Piezoelectricity–electric charge when squeezed

  • Pyroelectricity–electric charge when heated

  • Refractive Index–degree of bending light

  • Malleability–ability to be pounded into thin sheets

  • Ductility–ability to be drawn into thin wires

  • Sectility–ability to be shaved with a knife

Class question: What is the name for a plane where a mineral tends to split due to weaker bonds?

• Cleavage plane

Earth’s Common Elements

• 8 elements make up over 98% of Earth’s crust by weight. (The crust is only about 1% of the entire volume of the Earth).

• Notice that oxygen and silicon are the bulk of those at 74%.

Major Mineral Classes

• Silicates

• Oxides

• Sulfides

• Sulfates

• Halides

• Carbonates

• Native elements

Class question: Which class represents most minerals?

• Silicates

Silicates

• Silicate minerals are by far the most dominant substances comprising Earth’s crust (90%) and mantle (>99%).

• Silicon and oxygen account for more than 74% of crustal mineral mass.

Silica Tetrahedron

• The SiO₄ unit is also known as the silica tetrahedron, a fundamental building block of silicate minerals.

• In silicate minerals, these tetrahedra are arranged and linked together in a variety of ways, from single units to chains, rings, and more complex frameworks.

Silicates - Isolated Tetrahedra

• Silicate minerals with isolated tetrahedra do not share any oxygen atoms. Instead, they are bonded together by cations.

• Examples: Olivine and Garnet

Silicates - Single Chain

• In single-chain silicates, two of the three basal oxygens are bonded together. The Si:O ratio is 1:3.

• Example: Pyroxenes

Silicates - Double Chain

• Double-chain silicates are like two single chains that share oxygens where tetrahedra touch, yielding an Si:O ratio of 2:7.

• Example: Amphiboles

Silicates - Sheet

• Sheet silicates share oxygen along the base of the tetrahedra, but not the oxygen at the top of the tetrahedra. The Si:O ratio is 2:5.

• Examples: Micas and clay minerals.

Silicates - Framework

• In framework silicates, all of the oxygens are shared between adjacent tetrahedra yielding an Si:O ration of 1:2.

• Example: Feldspars

Gems and Gemstones

• Gemstones are valuable minerals b/c of a combination of characteristics: rarity, beauty, color, mystique, etc.

• Raw gemstones are cut and polished on a faceting machine, the facets are not natural crystal faces, nor are they cleavage planes, but they add to the brilliance and refraction of light.

Mining for Minerals to Make Gems

• Many valuable minerals, like diamonds, originate under extremely high pressures, ~150 km below the Earth’s surface.

Extra Notes

• A mineral is naturally occurring, solid, crystalline, with definable chemical composition, and is inorganic.

• Quartz crystals are naturally occurring, usually formed by geological processes, solid, have a crystalline structure and defined chemical composition, and are in organic. Glass is solid, and has a defined chemical composition, but it does not have a crystalline structure, nor was it formed by geological processes.

• Ionic bonds form when cations and anions attract.

• Covalent bonds occur when atoms share electrons.

• Gypsum, corundum, aragonite, and graphite are minerals because they have all six characteristics of a mineral.

• Hematine, cubic zirconia, and menthol look like minerals, but are created in a lab, and do not occur naturally.

• Glass is not a mineral as it does not have a crystalline structure.

• Sugar, menthol, and DNA are not minerals because they are organic compounds.

• If a minerals growth is inhibited so that it displays well formed crystal faces, then it is a euhedral crystal.

• Diamond is the hardest mineral and talc is the softest mineral.

• The unidentified mineral can scratch fluorite, which means it is harder than fluorite and so has a hardness greater than 4. However, it cannot scratch apatite, which means it is softer than apatite and so has a hardness less than 5. Therefore, the hardness of the unidentified mineral is between 4 and 5.

• Cooling of melt initiates crystallization.

• Supersaturated water precipitates minerals like salt as it evaporates

• The presence of a precipitated mineral on a distinct planet would be significant as it indicates there is or was liquid water on the planet.

• Quartz has a hardness of 7, no cleavage, is nonmetallic, and does not react with acid.