Rock Identification: Unearthing Earth's Stories

A Journey Through Minerals and Rocks

Dr. Adam Hogan
NC State Chemistry Department

Dioptase and Mimetite, Tsumeb Namibia

Why Do Rocks… Rock?

• Windows to Earth's History: Rocks tell stories of ancient environments, volcanic eruptions, mountain building, and the evolution of life.
• Essential Resources: Building materials, metals, energy sources, gemstones.
• Understanding Our Planet: Helps us understand natural hazards (earthquakes, volcanoes), landscape formation, and climate change.
• Pure Fascination: The beauty and diversity of the mineral world!

Defining Our Subject: What is a Rock?

Definition

A naturally occurring solid aggregate of one or more minerals.

Components:

• Primarily minerals, but can include mineraloids or organic materials.

Formation:

• Result of dynamic geological processes over vast timescales.

Key Distinctions:

• Minerals: Naturally occurring, inorganic solids with defined chemical composition and crystal structure.
• Soils: Weathered rock material mixed with organic matter.

The Never-Ending Story: The Rock Cycle

The rock cycle illustrates the transformations between the three main rock types.

• Continuous Process: Rocks are constantly transforming from one type to another.
• Driving Forces: Heat, pressure, weathering, erosion, melting, cooling, and cementation.
• Interconnected Journey: All three rock types can transform into any other type.
• Time Scale: These are slow processes, often occurring over millions of years.

Igneous Rocks: Forged in Fire

Definition

Formed from the cooling and solidification (crystallization) of molten rock (magma or lava).

Two Main Categories:

• Volcanic (Extrusive)

  • Formed on Earth's surface from lava (e.g., basalt, rhyolite, obsidian).
  • Cools quickly.

• Plutonic (Intrusive)

  • Formed underground from magma (e.g., granite, gabbro, diorite).
  • Cools slowly.

Igneous Composition: Silica Content Rules

• Felsic (High Silica >65%): Light-colored. Examples: Granite, Rhyolite.
• Intermediate (Silica 55-65%): Medium gray. Examples: Diorite, Andesite.
• Mafic (Silica 45-55%): Dark-colored. Examples: Gabbro, Basalt.
• Ultramafic (Silica <45%): Very dark. Examples: Peridotite, Komatiite.

Igneous Textures: Cooling Rate Matters

• Fine-Grained (Aphanitic)

  • Rapid cooling (extrusive), small crystals, often not visible to naked eye.
  • Examples: Basalt, Rhyolite, Volcanic Glass (Obsidian - no crystals!)

• Coarse-Grained (Phaneritic)

  • Slow cooling (intrusive), large, visible crystals.
  • Examples: Granite, Gabbro, Diorite

• Porphyritic

  • Mixed grain sizes (phenocrysts in a finer groundmass) indicating two-stage cooling.
  • Example: Porphyritic Andesite

Metamorphic Rocks: Changed by Heat & Pressure

Definition

Formed by alteration of existing rocks under high heat and/or pressure, without melting.

Key Conditions:

• Elevated Temperatures (150-850°C) & High Pressures

Solid-State Transformation. Parent Rock can be any type.

Types of Metamorphism

• Contact Metamorphism

  • Heat from igneous intrusion. Examples: Marble, Quartzite, Hornfels.

• Regional Metamorphism

  • Large-scale pressure/heat (mountain building). Examples: Slate, Schist, Gneiss.

• Dynamic Metamorphism

  • Pressure along fault zones.

Metamorphic Textures

Foliated Texture (Layered)

• Slate: Fine-grained sheets.
• Phyllite: Silky sheen.
• Schist: Visible platy minerals.
• Gneiss: Coarse, distinct bands.

Non-Foliated Texture (Not Layered)

• Marble, Quartzite, Hornfels.

Metamorphic Grade: Low → High intensity.

Sedimentary Rocks: Surface Archives

Definition

Formed from accumulation, compaction, and cementation of sediments.

Process: Weathering → Erosion → Deposition → Compaction → Cementation

• Cover ~75% of Earth's land surface.
• Often contain fossils.

Classifying Sedimentary Rocks

Clastic (Fragmental)

• Conglomerate/Breccia, Sandstone, Siltstone, Shale.

Chemical (Precipitated)

• Limestone, Rock Salt, Rock Gypsum, Chert.

Organic (Biological)

• Coal, Chalk, Coquina, Fossiliferous Limestone.

Identifying Minerals: Keys to Rocks

Understanding mineral properties is crucial. Use a combination of properties!

Key Properties:

• Color, Luster, Streak, Hardness, Crystal Habit, Cleavage & Fracture, Special Properties.

Property 1: Color

Definition: Visible appearance of the mineral.

Important Notes:

• Often unreliable for identification due to impurities
• Same mineral can have multiple colors (e.g., Quartz varieties)
• Some minerals have characteristic colors (e.g., Sulfur - yellow, Malachite - green)
• Best used in combination with other properties

Property 2: Luster

Definition: How light reflects from a mineral surface.

Types:

• Metallic: Shiny like metal (e.g., Pyrite, Galena)
• Non-metallic:
  • Vitreous: Glassy (e.g., Quartz)
  • Pearly: Pearl-like (e.g., Talc)
  • Earthy: Dull, clay-like (e.g., Kaolinite)
  • Silky: Fibrous appearance (e.g., Chrysotile)

Property 3: Streak

Definition: Color of mineral in powdered form.

Test: Rub on unglazed porcelain. More reliable than surface color.

• Hematite: Reddish-brown streak.
• Pyrite: Greenish-black streak.
• Sulfur: Yellow streak.

Property 4: Hardness

Definition: Resistance to scratching. (Mohs Scale 1-10)

Key Testing Tips:

• Always test on a fresh, unweathered surface
• Test both directions (does A scratch B? does B scratch A?)
• Use common tools: fingernail (~2.5), penny (~3.5), knife/glass (~5.5)
• Hardness is often the most reliable property for identification

Property 5: Crystal Habit

Definition: Characteristic external crystal shape.

Forms: Cubic (Pyrite), Hexagonal (Quartz), Prismatic (Tourmaline), Tabular (Mica), Massive, etc.

Observation Tips:

• Microscope helpful for tiny crystals
• Many minerals have characteristic angles between faces
• Look for repeated geometric patterns

Property 6: Cleavage & Fracture

Cleavage: Breaks on smooth, flat planes.

Examples: Mica (1 dir.), Halite (3 dir. @90°), Calcite (3 dir. not @90°).

Fracture: Irregular breakage.

Conchoidal: Curved (Quartz). Uneven: Rough.

Property 7: Special Properties

• Magnetism: (Magnetite).
• Acid Reaction (Vinegar): (Calcite fizzes).
• Fluorescence (UV): (Some Fluorite, Calcite).
• Taste (Caution!): (Halite - salty).
• Feel: (Talc - soapy).
• Specific Gravity: (Galena - heavy).

Rock Identification Flowchart

Rock Identification Flowchart - Igneous

If you think it's IGNEOUS:

Step 2: Where did it cool?

• Large crystals (>1mm): Cooled slowly underground → Plutonic
• Small crystals (<1mm): Cooled quickly on surface → Volcanic

Step 3: What's the color?

• Light colors (white/pink/gray): High silica → Felsic (Granite/Rhyolite)
• Medium colors (gray): Medium silica → Intermediate (Diorite/Andesite)
• Dark colors (black/green): Low silica → Mafic (Gabbro/Basalt)

Rock Identification Flowchart - Metamorphic

If you think it's METAMORPHIC:

Step 2: Is it foliated (layered)?

YES - Foliated:

• Fine layers, splits easily: Slate
• Shiny, wavy layers: Phyllite
• Visible crystals, platy minerals: Schist
• Coarse bands of light/dark: Gneiss

NO - Non-foliated:

• Fizzes with vinegar: Marble (from limestone)
• Very hard, glassy: Quartzite (from sandstone)
• Dark, fine-grained: Hornfels (contact metamorphism)

Rock Identification Flowchart - Sedimentary

If you think it's SEDIMENTARY:

Step 2: How was it formed?

Clastic (fragments):

• Large pieces (>2mm): Conglomerate/Breccia
• Sand-sized grains: Sandstone
• Fine particles: Siltstone/Shale

Chemical (precipitated):

• Fizzes with vinegar: Limestone
• Tastes salty: Rock Salt
• Very hard, waxy luster: Chert

Organic (biological):

• Black, burns: Coal
• White, soft, chalky: Chalk

ID Challenge: Quartz vs. Calcite

Two common minerals that can be confused:

Quartz:

• Hardness: 7 (scratches glass)
• Cleavage: None (conchoidal fracture)
• Acid Test: No reaction with vinegar
• Luster: Vitreous (glassy)
• Crystal Form: Hexagonal prisms

Calcite:

• Hardness: 3 (scratched by penny)
• Cleavage: Perfect in 3 directions (rhombs)
• Acid Test: Fizzes with vinegar
• Luster: Vitreous to pearly
• Crystal Form: Rhombohedrons, scalenohedral

ID Challenge: Pyrite vs. Gold

"Fool's Gold" vs. the real thing:

Pyrite ("Fool's Gold"):

• Hardness: 6.5 (harder than steel)
• Streak: Greenish-black
• Behavior: Brittle, breaks when struck
• Crystal Form: Cubic crystals
• Specific Gravity: 5.0

Gold:

• Hardness: 2.5-3 (soft, scratched by penny)
• Streak: Golden yellow
• Behavior: Malleable, bends when struck
• Crystal Form: Usually nuggets or flakes
• Specific Gravity: 19.3 (very heavy)

ID Challenge: Fluorite vs. Amethyst

Purple minerals can be tricky:

Fluorite:

• Hardness: 4 (scratched by knife)
• Cleavage: Perfect octahedral (4 directions)
• Crystal Form: Cubic or octahedral
• Fluorescence: Often fluoresces under UV light
• Specific Gravity: 3.2

Amethyst (Purple Quartz):

• Hardness: 7 (scratches glass)
• Cleavage: None (conchoidal fracture)
• Crystal Form: Hexagonal prisms
• Fluorescence: Usually none
• Specific Gravity: 2.65

ID Challenge: Fluorite vs. Calcite

When both are clear or white:

Fluorite:

• Hardness: 4 (scratched by knife)
• Acid Test: No reaction with HCl
• Cleavage: Perfect octahedral
• Specific Gravity: 3.2
• Fluorescence: Often fluoresces

Calcite:

• Hardness: 3 (scratched by penny)
• Acid Test: Fizzes vigorously with vinegar
• Cleavage: Perfect rhombohedral
• Specific Gravity: 2.7
• Double Refraction: Shows double images

ID Challenge: Hornblende vs. Black Tourmaline (Schorl)

Two dark minerals that can be confused:

Hornblende:

• Hardness: 5-6 (scratched by steel)
• Cleavage: Perfect in 2 directions at 56° and 124°
• Crystal Form: Prismatic, often stubby
• Luster: Vitreous to silky
• Habit: Often in metamorphic rocks

Black Tourmaline (Schorl):

• Hardness: 7-7.5 (scratches glass)
• Cleavage: None (conchoidal fracture)
• Crystal Form: Long prismatic, triangular cross-section
• Luster: Vitreous to resinous
• Habit: Often in pegmatites and granites

ID Challenge: Serpentine vs. Jade

Two green minerals often confused:

Serpentine:

• Hardness: 2.5-4 (soft, scratched by knife)
• Luster: Waxy to greasy, sometimes silky
• Texture: Often fibrous or massive
• Feel: Soapy or greasy to touch
• Occurrence: Metamorphic rocks, often altered ultramafic rocks

Jade (Jadeite/Nephrite):

• Hardness: 6-7 (scratches glass)
• Luster: Vitreous to greasy
• Texture: Extremely tough and compact
• Feel: Smooth, cool to touch
• Occurrence: High-pressure metamorphic rocks

ID Challenge: Hematite vs. Magnetite

Two iron oxides with different properties:

Hematite:

• Hardness: 5-6 (scratched by steel)
• Streak: Red-brown to cherry red
• Magnetism: Not magnetic (or weakly magnetic)
• Luster: Metallic to earthy
• Color: Reddish-brown, black, or steel gray

Magnetite:

• Hardness: 5.5-6.5 (scratched by steel)
• Streak: Black
• Magnetism: Strongly magnetic (attracts iron)
• Luster: Metallic
• Color: Black to dark gray

ID Challenge: Shale vs. Slate

Distinguishing sedimentary from metamorphic:

Shale:

• Origin: Sedimentary (compressed mud/clay)
• Texture: Soft, earthy, breaks along bedding planes
• Hardness: 1-2 (very soft, scratched by fingernail)
• Smell: Often has earthy/muddy smell when wet
• Layers: Breaks along original sedimentary layers

Slate:

• Origin: Metamorphic (from shale under heat/pressure)
• Texture: Hard, dense, perfect cleavage
• Hardness: 3-4 (scratched by knife)
• Sound: Rings when struck with hammer
• Layers: Breaks along metamorphic cleavage planes

ID Challenge: Olivine vs. Epidote

Two green minerals in different rock types:

Olivine:

• Hardness: 6.5-7 (scratches glass)
• Color: Olive green to yellow-green
• Cleavage: None (conchoidal fracture)
• Occurrence: Mafic/ultramafic igneous rocks (basalt, peridotite)
• Crystal Form: Usually granular masses

Epidote:

• Hardness: 6-7 (scratches glass)
• Color: Yellow-green to dark green
• Cleavage: Perfect in one direction
• Occurrence: Metamorphic rocks, altered igneous rocks
• Crystal Form: Prismatic crystals, often striated

NC Geology: A Rich Tapestry

North Carolina contains three distinct geological regions, each with unique rock types and formation histories.

Coastal Plain Region

Characteristics:

• Age: Youngest geological region (Cretaceous to Recent)
• Elevation: Sea level to ~300 feet
• Rock Types: Primarily sedimentary rocks
• Common Formations: Sand, clay, limestone, marl

Notable Features:

• Rich in fossils (marine shells, shark teeth)
• Unconsolidated sediments from ancient sea floors
• Phosphate deposits in some areas
• Evidence of changing sea levels over time

Piedmont Region

Characteristics:

• Age: Ancient Paleozoic roots (300-500 million years)
• Elevation: 300-1,500 feet
• Rock Types: Metamorphic and igneous rocks
• Common Rocks: Gneiss, schist, slate, granite

Notable Features:

• Evidence of ancient mountain building (Appalachian orogeny)
• Granite plutons and metamorphic core complexes
• Gold-bearing quartz veins (historic mining areas)
• Diabase dikes cutting through older rocks

Blue Ridge Region

Characteristics:

• Age: Oldest rocks in NC (up to 1.8 billion years)
• Elevation: 1,500-6,684 feet (Mt. Mitchell)
• Rock Types: High-grade metamorphic and igneous
• Common Rocks: Gneiss, amphibolite, quartzite, granite

Notable Features:

• Highest elevations in eastern North America
• Ancient basement rocks exposed through erosion
• Spectacular mineral diversity (garnet, mica, feldspar)
• Thrust faults from ancient continental collisions

Exploring NC Geology: Resources

NC Geological Survey (DEQ):

• Primary source: NC Geological Survey
• Interactive Maps: Geologic Maps

Books:

• Smithsonian Handbooks: Rocks & Minerals
• Audubon Field Guide to Rocks and Minerals
• Exploring Geology of the Carolinas

Websites:

• Minerals.net, Mindat.org, USGS.gov

Apps:

• Rockd - Geology field app for finding and sharing locations

State Parks & Museums also offer info.

Common NC Rock Formations

Understanding typical rock formations helps with field identification:

Quartz Float

What is it?

Loose pieces of quartz veins weathered from host rock and scattered across the landscape.

Where to find:

• Piedmont region creek beds and hillsides
• Often indicates gold-bearing areas (historical mining)
• Common in areas with metamorphic host rocks

Identification:

• White to milky appearance
• Hardness 7 (scratches glass)
• No cleavage (conchoidal fracture)
• Often angular pieces from vein breakage

Pegmatite Dikes

What is it?

Very coarse-grained igneous intrusions with crystals >1 inch.

Where to find:

• Blue Ridge and Piedmont regions
• Cut through gneiss and schist
• Often in road cuts and quarries

Identification:

• Giant crystals of quartz, feldspar, mica
• Light-colored (felsic composition)
• Sharp contact with host rock
• May contain gemstones (beryl, tourmaline)

Diabase Dikes

What is it?

Dark, fine-grained igneous intrusions that cut through older rocks.

Where to find:

• Throughout NC, especially Piedmont
• Linear features in landscape
• Often form ridges due to erosion resistance

Identification:

• Dark gray to black color
• Fine-grained texture
• Mafic composition (plagioclase + pyroxene)
• Magnetic properties (contains magnetite)

Hydrogeology Note:

• In Triassic basins, water tends to flow along these dikes due to fracturing and weathering

Granite Outcrops

What is it?

Massive plutonic igneous rock exposures.

Where to find:

• Piedmont region prominently
• Form domes and tors (Stone Mountain)
• Quarries and natural exposures

Identification:

• Coarse-grained (phaneritic) texture
• Light-colored (quartz, feldspar, mica)
• Granitic composition
• Often shows jointing patterns

Gneiss Exposures

What is it?

High-grade metamorphic rock with distinctive banding.

Where to find:

• Blue Ridge and Piedmont regions
• Road cuts and stream beds
• Forms the basement of much of western NC

Identification:

• Alternating light and dark bands
• Coarse-grained texture
• Foliation (layered structure)
• Common minerals: quartz, feldspar, biotite

Key Takeaways

• Rocks: Aggregates of minerals, 3 types (Igneous, Sedimentary, Metamorphic) via Rock Cycle.
• Mineral ID: Combination of physical properties.
• NC has diverse geology. Practice is key!

Questions?

Thank you! Go explore rocks and minerals!

Dr. Adam Hogan
NC State Chemistry Department
aehogan2@ncsu.edu

Image Credits

All images used under Creative Commons or public domain licenses:

• Dioptase crystal - Rob Lavinsky, iRocks.com (CC BY-SA 3.0)
• Stone Mountain State Park granite outcrop - Wikimedia Commons (CC BY-SA 3.0)
• Stone Mountain aerial view - Wikimedia Commons (CC BY-SA 3.0)
• Sauratown Mountain, Piedmont NC - Wikimedia Commons (CC BY-SA 3.0)
• Quartz crystal - Parent Géry (CC BY-SA 3.0)
• Potassium feldspar - Rob Lavinsky, iRocks.com (CC BY-SA 3.0)
• Orthoclase-Biotite - Rob Lavinsky, iRocks.com (CC BY-SA 3.0)
• Rock cycle diagram - Wikimedia Commons (Public Domain)
• Igneous rocks chart - Woudloper (CC BY-SA 4.0)
• Metamorphic facies - Woudloper (CC BY-SA 4.0)
• Metamorphic rock - P199 (CC BY-SA 2.0)
• Sedimentary rocks - USGS (Public Domain)
• Sandstone - USDA (Public Domain)
• Hematite streak - Amcyrus2012 (CC BY-SA 4.0)
• NC Geology map - NC Department of Environmental Quality
• Mohs scale - National Park Service (Public Domain)
• Bowen's series - Wikimedia Commons (Public Domain)
• Volcano eruption - Giles Laurent (CC BY-SA 4.0)
• Migmatite - Geomartin (CC BY-SA 4.0)
• Amphibole schist - James St. John (CC BY 2.0)
• Almandin garnet - Géry Parent (CC BY-SA 3.0)
• Amethyst - Rob Lavinsky, iRocks.com (CC BY-SA 3.0)
• Chrysotile - Stan Celestian (CC BY-SA 3.0)
• Malachite - Rob Lavinsky, iRocks.com (CC BY-SA 3.0)
• Pyrite cubic crystals - Rob Lavinsky, iRocks.com (CC BY-SA 3.0)
• Obsidian - Vassil (CC BY-SA 3.0)
• Autunite UV fluorescence - Alchemist-hp (CC BY-SA 3.0)
• Igneous rock identification chart - Wikimedia Commons (Public Domain)
• Metamorphic rock identification chart - Wikimedia Commons (Public Domain)
• Sedimentary rock identification chart - Wikimedia Commons (Public Domain)
• Pegmatite body - Wikimedia Commons (CC BY-SA 3.0)
• Quartz float pieces - Google Images (Educational Use)
• Quartz outcrop - Wikimedia Commons (CC BY-SA 3.0)
• Diabase dike - Wikimedia Commons (CC BY-SA 3.0)
• Rock identification flowchart - Custom diagram created for presentation
• Rock collection - Wikimedia Commons (CC BY-SA 3.0)
• Coastal plain - Wikimedia Commons (Public Domain)
• Blue Ridge exfoliation dome - Wikimedia Commons (CC BY-SA 3.0)
• Olivine in basalt - Wikimedia Commons (CC BY-SA 3.0)
• Peridot in basalt - Wikimedia Commons (CC BY-SA 3.0)
• Chattanooga Shale - Wikimedia Commons (CC BY-SA 3.0)
• Slate sample - Wikimedia Commons (CC BY-SA 3.0)
• Magnetite specimen - Wikimedia Commons (CC BY-SA 3.0)
• Hematite specimen - Wikimedia Commons (CC BY-SA 3.0)
• Serpentine specimen - Wikimedia Commons (CC BY-SA 3.0)
• Jade specimen - Wikimedia Commons (CC BY-SA 3.0)
• Hornblende specimen - Wikimedia Commons (CC BY-SA 3.0)
• Tourmaline specimen - USGS (Public Domain)
• Calcite specimen - Estonian Museum of Natural History (CC BY-SA 3.0)
• Fluorite specimen - Wikimedia Commons (CC BY-SA 3.0)
• Amethyst geode - Wikimedia Commons (CC BY-SA 3.0)
• Pyrite specimen - Wikimedia Commons (CC BY-SA 3.0)
• Gold nuggets - Wikimedia Commons (CC BY-SA 3.0)
• NC Geological Map 2024 - NC Department of Environmental Quality
• Gneiss outcrop - Wikimedia Commons (CC BY-SA 3.0)
• NC Rock Formations Collage - Custom collage created for presentation
• Emerald in quartz pegmatite - Wikimedia Commons (CC BY-SA 3.0)