Rock id quiz

Hardness

Resistance of a mineral to scratching. Measured using the Mohs Hardness Scale

• 1 = Talc

• Human Fingernail: ~ 2.5

• Copper Wire: ~ 3

• Iron / Wire Nail: ~ 4

• 7= quartz

• 10 = Diamond

Cleavage

The tendency of a mineral to break along flat planes of weakness in its crystal structure.

Fracture

Irregular breakage when a mineral does not have cleavage planes.

• concoidal fracture in obsidian and quartz

Twinning

An intergrowth of two or more crystals in a symmetrical arrangement. Common in feldspars. Useful for identifying plagioclase feldspar

Quartz

• Colour: Clear, white, grey

• Hardness: 7

• Cleavage: None

• Fracture: Conchoidal

• Twinning: Rare

• Weathering Resistance: Very high

• felsic

Orthoclase (K-feldspar)

• Colour: Pink, cream, white

• Hardness: 6

• Cleavage: 2 directions at ~90°

• Twinning: Common

• Weathering Resistance: Moderate

• felsic

Plagioclase Feldspar

• Colour: White to grey → can be clear

• Hardness: 6

• Cleavage: 2 directions at ~90°

• Twinning: Distinct parallel striations

• Weathering Resistance: Moderate

• felsic

Biotite

• Colour: Black to dark brown

• Hardness: 2.5-3

• Cleavage: Perfect in one direction

• Twinning: Rare

• Weathering Resistance: Low

• mafic

Muscovite

• Colour: Colourless to light brown

• Hardness: 2-2.5

• Cleavage: Perfect in one direction

• Splits into thin sheets

• felsic

Hornblende (Amphibole)

• Colour: Dark green to black

• Hardness: 5-6

• Cleavage: 2 directions at 60° and 120°

• Weathering Resistance: Moderate

• mafic

Pyroxene

• Colour: Dark green to black

• Hardness: 5-6

• Cleavage: 2 directions at ~90°

• Common in mafic igneous rocks

Olivine

• Colour: Olive green

• Hardness: 6.5-7

• Cleavage: Poor

• Fracture: Irregular

• Weathering Resistance: Low

• mafic

Colour Index

A measure of how dark an igneous rock is. Darker rocks contain more mafic minerals while lighter rocks contain more felsic minerals.

Granite

• Colour Index: Low (light coloured)

• Composition: Felsic (>63% SiO₂)

• Minerals: Quartz, K-feldspar, plagioclase, biotite

• Texture: Coarse-grained

• Cooling History: Intrusive

• Formation Environment: Slow cooling deep underground

Rhyolite

• Colour Index: Low

• Composition: Felsic

• Minerals: Quartz, feldspars

• Texture: Fine-grained

• Cooling History: Extrusive

• Formation Environment: Rapid cooling at surface

• Note: Quartz-rich magmas tend to be explosive

Diorite

• Colour Index: Intermediate

• Composition: Intermediate (52-63% SiO₂)

• Minerals: Plagioclase, amphibole

• Texture: Coarse-grained

• Cooling History: Intrusive

• Formation Environment: Slow cooling underground

note: grano-diorite is more felsic than diorite + has more white

Andesite

• Colour Index: Intermediate

• Composition: Intermediate

• Minerals: Plagioclase, amphibole

• Texture: Fine-grained

• Cooling History: Extrusive

• Formation Environment: Volcanic arcs and volcanoes

Gabbro

• Colour Index: High (dark)

• Composition: Mafic (45-52% SiO₂)

• Minerals: Pyroxene, plagioclase

• Texture: Coarse-grained

• Cooling History: Intrusive

• Formation Environment: Slow cooling beneath surface

Basalt

• Colour Index: High

• Composition: Mafic

• Minerals: Pyroxene, plagioclase, olivine

• Texture: Fine-grained

• Cooling History: Extrusive

• Formation Environment: Lava flows, oceanic crust

Peridotite

• Colour Index: Very High

• Composition: Ultramafic (<45% SiO₂)

• Minerals: Olivine, pyroxene

• Texture: Coarse-grained

• Cooling History: Intrusive

• Formation Environment: Upper mantle

dacite

• Rock Type: Igneous

• Composition: Intermediate to felsic

• Colour Index: Low to Intermediate (~15–40% dark minerals)

• Main Minerals: Plagioclase feldspar, quartz, biotite, hornblende

• Texture: Usually aphanitic or porphyritic

• Colour: Light grey with scattered dark crystals ("salt and pepper" appearance)

• Cooling History: Extrusive (rapid cooling at the surface)

• If Porphyritic: Indicates two-stage cooling:

  • Slow cooling underground forms phenocrysts

  • Rapid cooling at the surface forms fine-grained groundmass

• Formation Environment: Common at volcanic arcs above subduction zones

• Extrusive Equivalent of: Granodiorite

Porphyritic Texture

• Large crystals (phenocrysts) surrounded by fine groundmass

• Indicates two-stage cooling

• Slow cooling underground followed by rapid cooling near the surface

• Phenocrysts formed first at depth

Phenocryst

large crystal that forms during slow cooling deep underground before the remaining magma cools rapidly

Groundmass

Fine-grained material surrounding phenocrysts that forms during rapid cooling

How do sedimentary rocks form?

Form from weathered rock fragments, chemical precipitates, or biological materials through deposition, compaction and cementation (diagenesis).

High-Energy Depositional Environment

Fast-moving water or waves transport and deposit coarse sediments such as conglomerates.
Examples: beaches, alluvial fan

Low-Energy Depositional Environment

Calm environments where fine sediments settle.

clay and silt

Examples: Lakes, deep marine settings, floodplains.

moderate energy depositional environments

medium environment → sand, ooids, sand sized skeletal matter

River channels, coral reefs

Conglomerate

• Grain Size: >2 mm

• Clasts: Rounded

• Energy: High-energy environment

• Depositional Environment: Rivers, beaches

• Texture: Coarse clastic rock

paraconclomerate = gravel in fine matrix

orthoconglomerate = gravel touch each other and interlock

Breccia

• Grain Size: >2 mm

• Clasts: Angular

• Energy: Very high energy, little transport

• Depositional Environment: Fault zones, landslides, debris flows

• Texture: Coarse clastic rock

Sandstone

• Grain Size: 64 μm-2 mm

• Environment: Moderate-energy

• Depositional Environment: Rivers, beaches, deserts

• Texture: Medium-grained clastic rock

Mudstone

• Grain Size: Too fine to see

• Environment: Low-energy

• Depositional Environment: Lakes, floodplains, deep marine basins

• Texture: Fine-grained clastic rock

  • silt, clay

Sorting

• Well Sorted = grains mostly same size

• Poorly Sorted = grains of many sizes

• Indicates transport history and depositional environment

Chemical Maturity

A rock containing mostly resistant minerals (especially quartz) after intense weathering.

Textural Maturity

Well-rounded grains indicating long transport and extensive weathering

angular grains = less transport

Calcirudite (fine to coarse)

• Rock Type: Biochemical/Chemical sedimentary limestone

• Grain Size: > 2 mm

• Equivalent Clastic Rock: Conglomerate/Breccia

• Components: Large shell fragments, fossils, carbonate clasts

• Energy Environment: High-energy

• Depositional Environments:

  • Reef environments

  • Wave-swept shallow marine settings

  • Nearshore marine environments

Identification: Large visible fossil or shell fragments cemented together.

Calcarenite (fine to coarse)

• Rock Type: Limestone

• Grain Size: 0.0625-2 mm (sand-sized carbonate grains)

• Equivalent Clastic Rock: Sandstone

• Components: Sand-sized carbonate particles, shell fragments, ooids

• Energy Environment: Moderate energy

• Depositional Environments:

  • Beaches

  • Shoals

  • Shallow marine shelves

Identification: Feels and looks like a sandstone but reacts with dilute HCl because it is made of calcite.

Calcilutite

• Rock Type: Limestone

• Grain Size: < 0.0625 mm (mud-sized carbonate particles)

• Equivalent Clastic Rock: Mudstone

• Components: Micrite (carbonate mud)

• Energy Environment: Low energy

• Depositional Environments:

  • Lagoons

  • Deep marine settings

  • Quiet shelf environments

Identification: Very fine-grained limestone with no visible grains.

types of biogenic vs clastic sed roc

• conglomerate = calcirudite

• sandstone = calcarenite

• mudstone = caliculite

Contact Metamorphism

• Caused by heat from nearby magma

• High temperature, low pressure

• Localised around intrusions

• Example rock: Hornfels

  • marble → protolith is limestone (calcite)

  • no foliation

Regional Metamorphism

• Caused by mountain building and compression

• High temperature and pressure

• Large-scale process

• Produces foliated rocks

  • foliation = repetitive layering, planar structures, or parallel alignment of mineral grains and bands within a rock

  • due to stress

  • squishes grains in one direction

Slate

• Parent Rock: Mudstone/Shale

• Metamorphism: Regional

• Grade: Low

• Texture: Foliated → WEAK, slaty cleavage

Phyllite

• Parent Rock: Slate

• Metamorphism: Regional

• Grade: Low-Medium

• Texture: Wavy foliation → phyllitic foliation

Schist

• Parent Rock: Phyllite / mudstone

• Metamorphism: Regional

• Grade: Medium-High

• Texture: Visible mica crystals → schistocity → developed foliation

Gneiss

• Parent Rock: Schist/mudstone or Granite

• Metamorphism: Regional

• Grade: High

• Texture: Light (Felsic) and dark (mafic) bands → gneissic banding

Marble

• Parent Rock: Limestone

• Metamorphism: Contact or Regional

• Major Mineral: Calcite

• Texture: Non-foliated

Quartzite

• Parent Rock: Quartz Sandstone

• Metamorphism: Contact or Regional

• Major Mineral: Quartz

• Texture: Non-foliated

Quick Identification Checklist:

Igneous

• Colour index?

• Visible crystals?

• Intrusive or extrusive?

• What minerals are present?

• Any porphyritic texture?

Quick Identification Checklist:

Sed

• Grain size?

• Sorting?

• Rounding?

• Composition?

• Depositional environment?

Quick Identification Checklist:

metamorphic

1. Foliated or non-foliated?

2. Metamorphic grade?

3. Parent rock?

4. Contact or regional metamorphism?

Porphyritic Texture

Large crystals (phenocrysts) surrounded by fine groundmass.

Indicates:

• Two-stage cooling

• Slow cooling underground first

• Rapid cooling at surface second

Phaneritic Texture

Coarse-grained, visible crystals.

Indicates:

• Slow cooling

• Intrusive igneous rock

Aphanitic Texture

Crystals too small to see.

Indicates:

• Rapid cooling

• Extrusive igneous rock

intrusive igneous rock textures

porphyritic

phaneritic

aphanitic

Fossiliferous Limestone

• Contains visible fossils

• Biogenic sedimentary rock

• Marine depositional environment

Chalk vs limestone

• Biogenic limestone

• Made from microscopic marine organisms

• Deposited in low-energy marine environments

Coal

• Lithified plant material

• Organic sedimentary rock

Environment:

• Swamps

• Wetlands

Red Mudrocks

Red colour indicates iron oxides.

more chemically mature due to weathering

Dark Brown or Black Mudrocks → why that colour

Contain abundant organic material.

Foliation

Alignment of minerals due to directed pressure.

Common minerals:

• Micas

• Kyanite (blueish grey)

• silliminate (fibrous, greasy, white)

• Actinolite

sedimetnary rock texture

clast = phenocrysts but for sed rocks

matrix = groundmass

metamorphic rock textures

• foliation

• porphyroblasts = phenoctrysts for metamorphic rocks → usually garnet

Garnet

• Mineral Type: Silicate mineral

• Colour: Red to reddish-brown (can also be green, orange, yellow, or black)

• Hardness: 6.5-7.5

• Cleavage: None

• Fracture: Irregular to conchoidal

• Crystal Habit: Often forms dodecahedral (12-sided) crystals

• Twinning: Rare

Formation Environment

• Commonly forms during regional metamorphism

• Indicates medium to high metamorphic grade

• Often found in:

  • Schist

  • Gneiss

  • Garnet-mica schist

laminations

• Very thin layers of sediment or rock, typically less than 1 cm thick.

  • parallel lines of sediment

• Rock Type: Most common in sedimentary rocks, but can also occur in some metamorphic rocks.

• Formation: Produced by successive deposition of fine sediment over time.

• Environment: Usually forms in low-energy depositional environments where fine particles can settle out of suspension.

Examples of Depositional Environments:

• Lakes

• Lagoons

• Floodplains

• Deep marine environments

holocrystalline, hypocrystalline, holohyalline

holocrystalline = all crystals

hypocrystalline = some crystals some glass

holohyalline = all glass

metamorphic vs igneous

Foliation = metamorphic

Sugary crystals / grains not interlocking but more suspended / baked appearance = metamorphic

non foliated and grains are less sugary = igneous