Geology 306: Earth Science Laboratory:

Rock Identification Supplement Hydrological cycle and the Rock Cycle

The Rock Cycle

1. The rock cycle and hydrological cyle both involve the recycling of materials, such as water between the surface and the atmosphere or the recycling of the various types of rocks that make up the surface of the Earth.

2. All rocks are classified based upon their mineral content (mineral composition) and visual texture.

a. Texture: The size, shape and arrangement of mineral (crystal) grains or fragments of material.

3. Types of Rocks:

a. Igneous Rocks: are associated with the crystallization and/or solidification of Magma.

i. Intrusive igneous rocks cool slowly underground and form larger crystals, such as granite.

ii. Extrusive igneous rocks cool rapidly on or near the surface and form much smaller crystals (such as basalt) and in some cases no crystals at all, such as obsidian which is an amorphous or glassy material. Extrusive igneous activity can also produce violent volcanic eruptions which can produce more fragmental material.

b. Sedimentary Rocks: are associated with the accumulation of weathering by-products produced by weathering and erosion on the surface, such as sandstone.

c. Metamorphic Rocks: are associated with the alteration and deformation of pre-existing rocks known as parent rocks. These parent rocks are deformed by: Heat, Pressure, and Chemically-active fluids to produce new metamorphic rocks, such as Marble or Gneiss.

4. Igneous Rocks: are classified based on their overall mineral content and visual texture.

a. Mineral composition: is based upon the abundance of Ferromagnesian (minerals rich in iron and/or magnesium) and Non-ferromagnesian minerals (minerals that lack iron and/or magnesium), which is in turn influenced by Bowen’s Reaction Series.

Bowens Reaction Series

i. Bowen’s Reaction Series: describes the sequence of mineral formation in a cooling magma.

• Discontinuous Series or Branch: consists of the ferromagnesian minerals: Olivine, Pyroxene, Amphibole, and Biotite, which are all dark, ferromagnesian minerals.

• Continuous Series or Branch: consists of the non-ferromagnesian minerals, Calcium-rich Plagioclase feldspar and Sodium-rich Plagioclase feldspar, which are lighter colored than the discontinuous branch.

• Muscovite, Potassium Feldspar (Orthoclase), and Quartz: are all light-colored non-ferromagnesian minerals.

ii. Mineral content: will reflect the minerals that are present at the temperature in which the rock solidified. For example, rocks that solidify at higher temperatures may contain more olivine, pyroxene, and calcium-rich plagioclase feldspar. Rocks that solidify at very low temperatures will contain more quartz and potassium feldspar.

b. Compositional classes: can be determined by the amount of light (non-ferromagnesian minerals) and dark (ferromagnesian minerals).

i. Ultramafic: the composition of the mantle (peridotite) and rarely found on the surface. Contains mostly olivine, pyroxene, and a small amount of calcium-rich plagioclase feldspar.

ii. Mafic: Contains mostly dark, ferromagnesian minerals such as: olivine, pyroxene, as well as, calcium-rich plagioclase feldspar, and small amounts of amphibole minerals. These rocks tend to be dark and denser, such as the rock, Basalt.

iii. Intermediate: Contains mostly amphibole, biotite mica, sodium-rich plagioclase feldspar, and small amounts of pyroxene and calcium-rich plagioclase feldspar. Typically seen as half ferromagnesian and half non-ferromagnesian minerals, but can have a range of compositions. Intermediate colors, such as the rock diorite and/or andesite.

iv. Felsic: Contains mostly non-ferromagnesian minerals, such as: quartz, potassium feldspar, and muscovite mica, with small amounts of biotite, amphibole, and sodium-rich plagioclase feldspar. Typically light in color, such as granite

Mineral content of felsic, Intermediate, mafic, and ultra-mafic rock compositions:

Igneous Rock Textural Terms

INTRUSIVE TEXTURES associated with Plutonic environments and slow cooling:

Phaneritic: Uniform, slow-cooling, and coarse-grained, easily seen, e.g. granite (typical intrusive texture)

Pegmatitic: very coarse-grained, larger than your thumb (1-inch) Often forms due to slow cooling of a fluid-rich magma and can produce semi-precious gemstones such as aquamarine, tourmaline, etc. (if this texture is present, the term Pegmatitic is included in the rock name) (E.g. Pegmatitic granite or granite pegmatite)

Porphyritic: Two or more grain sizes are present due to multiple stages or rates of cooling. Phenocrysts (larger crystals) are set in a finer grained matrix due to two or more stages of cooling underground. (If this texture is present, the term Porphyritic is included in the rock name, e.g. porphyritic granite.)

EXTRUSIVE TEXTURES associated with volcanic environments and rapid cooling or violent volcanic eruptions:

Aphanitic: Uniform, rapid cooling, fine-grained, not visible except under a microscope, e.g. basalt (typical extrusive texture).

Porphyritic: phenocrysts in a finer grained matrix due to two or more stages of cooling with the last stage of cooling occurring on the surface. (If this texture is present, the term Porphyritic is included in the rock name, e.g. porphyritic basalt.)

Glassy: similar to glass, due to very rapid cooling and the lack of crystal formation, e.g. obsidian.

Vesicular or Cellular: many visible vesicles formed as magma cooled around gas bubbles that were escaping, thus preserving the spherical shape of the gas bubbles. (When large very large vesicles are present the term vesicular is added to the rock name, e.g. vesicular basalt, otherwise specific rock names such as scoria may be used.)

Amygdaloidal: Vesicles are filled in with secondary minerals, e.g. amygdaloidal basalt. (If this texture is present, the term Amygdaloidal is included in the rock name, e.g. Amygdaloidal Basalt.)

Frothy: glassy & vesicular, many small vesicles, foamy-looking, e.g. pumice.

Fragmental/Pyroclastic: particles, ash, fragments (which are typically angular) of rock, etc. fused together, e.g. welded tuff, volcanic breccia.

Some Igneous Rock Classifications

Emplacement of Plutonic Structures / Formation of Plutons

Sedimentary Rocks

The following diagram depicts the chemical equations important to chemical weathering and the formation of chemical cements.

1. Sedimentary rocks: form through the accumulation of weathering by-products, of which there are many types. These by-products will produce different types of sediments based upon their origin. Diagenesis describes the sequence of events or processes that transform a sediment into a sedimentary rock and often includes: deposition, burial, and Lithification (compaction and cementation) of sediments.

Diagenesis and the transformation of Sediment into Sedimentary Rock

2. Types of Sediments and sedimentary rocks and their textures:

a. Clastic Detrital Sediments: are particles or fragments of other materials, such as sand grains. These particles are broken bits of other rocks and/or minerals and can be transported by wind, ice, and water which can influence the shape and characteristics of the particles. These particles get deposited and undergo Diagenesis, which includes the cementing of the particles together to form a rock, such as sandstone.

b. Non-Clastic Sediments:

i. Chemical Sediments: form through the precipitation of formerly dissolved materials, such as the formation of salt crystals from an evaporating salty lake, to produce rock salt. These rocks often appear crystalline.

ii. Biological, Organic Sediments: sediments associated with the accumulation of biological material, such as leaf litter, shells, fossils, coral reefs, etc. These rocks can be fossiliferous and contain many fossils, such as fossiliferous limestone.

c. You can sometimes find combinations sediments accumulating in some environments. For example, an accumulation of mud will often form mudstone and shale, but if it contains abundant fossils too, it would be a fossiliferous mudstone or fossiliferous shale. Such a rock would have both clastic and biological material and could have a Bio-clastic texture.

3. As particles are transported, they may be reduced in size and could also undergo rounding and sorting of the particles. Typically, when clastic sediments first form, they may have a jagged, and fragmental appearance, but as they travel, the sharp corners are often knocked off and the particle becomes smaller with smoother, rounded shapes. For example, in a stream, particles will tumble along and become more spherical and uniform in shape. Sorting describes the uniformity of grain sizes in a sedimentary rock.

4. Cementing agents: are typically materials that have formed through chemical weathering reactions to produce dissolved materials that can infiltrate the deposit of sediments and then precipitate in the pore spaces between the sediments and thus bind the particles together, such as dissolved calcite, quartz, or iron oxides.

CLASSIFICATION OF SEDIMENTARY ROCKS

Clastic / Detrital Sedimentary Rocks

Texture / particle size

Composition Comments Rock Name

Coarse grained

( >2 mm)

Quartz, quartzite, and chert are

dominant

Poorly sorted rounded rock fragments of any rock type.

CONGLOMERATE

Poorly sorted angular rock fragments of any rock type.

BRECCIA

Coarse to fine grained

Fragments of any rock type

(associated with glaciers)

Poorly sorted, nonstratified, and angular rock fragments.

Sometimes the larger particles are elongate with striations on the flat

surfaces.

TILLITE

Medium grained

(1/16 - 2 mm)

Primarily Quartz grains

Usually moderately to well sorted and rounded.

QUARTZ SANDSTONE

SANDSTONE

Quartz and at least 25% K-Feldspar

grains

Usually forms from rapid erosion, with visible K-feldspar grains that

are often medium to poorly sorted.

ARKOSE SANDSTONE

Quartz grains, small rock fragments and

clay minerals.

Often gray in appearance due to the increased presence of clay

particles. Usually less well sorted than quartz Sandstone

GRAYWACKE SANDSTONE

Fine grained (1/256 - 1/16

mm)

Fine grained quartz and clay minerals.

Silt-sized particles with a gritty feel. SILTSTONE

Very fine grained

(< 1/256 mm)

Very fine grained quartz and clay

minerals. Clay-sized particles with a soft feel SHALE (Mud- or Clay-stone)

Sedimentary Rocks continued…

Chemical or Biochemical / Organic Rocks

Texture / Particle size

Composition Comments Rock Name

Medium to coarse crystalline grains

Calcite ( CaCO3 )

Crystalline texture is visable. Forms from a fine-grained lime mud.

CRYSTALLINE LIMESTONE

Microcrystalline Exhibits Conchoidal fracture MICRITE LIMESTONE

Aggregates of Oolites

Oolites (round spherical fossil features) OOLITIC LIMESTONE

Fossiliferous Loosely cemented fossils and fossil

fragments with little matrix. COQUINA

Fossiliferous, with finer grained

matrix

Abundant fossils in a calcareous (calcite) matrix

FOSSILIFEROUS LIMESTONE

Microscopic Shells of microscopic organisms, clay, very

soft. Can be powdery. CHALK

Banded Calcite Usually forms as dripstones - stalactites or

stalagmites. TRAVERTINE

Similar to Limestone

Dolomite (CaMg(CO3 )2)

Also known as dolomite, reacts with acid only if powdered. Chemical replacement!

DOLOSTONE

Crypto-crystalline

Chalcedony (SiO2)

Hard, dense masses or beds and exhibits conchoidal fracture. Variety of colors

possible. CHERT

Micro-crystalline to

Crypto-crystalline

Chalcedony, Quartz (SiO2)

Hard, dense masses or beds and exhibits conchoidal fracture. Tends to be gray to

black and can exhibit a chalky rind. FLINT

Fine to Coarse crystalline grains

Gypsum (CaSO4 *2H2O)

An evaporite, inorganic precipitate. ROCK GYPSUM

Fine to Coarse crystalline grains

Halite (NaCl) An evaporite, inorganic precipitate. Salty

taste. ROCK SALT

Microscopic (SiO2) Silica Shells of microscopic organisms

(Diatoms), very soft and similar to chalk, but will not react with acid.

DIATOMACEOUS EARTH

Fibrous Brownish plant

material Soft, Porous. Will burn. Plant material may

still be visible PEAT

Dense and uniform in

appearance, but its density is low.

Carbon

Highly altered, compacted, carbonized plant remains (plant matter has been converted to

carbon.) Will burn. COAL

Metamorphic Rocks

1. Metamorphic rocks: have been altered and/or deformed but have not yet melted. The rock prior to deformation is referred to as the Parent rock. Metamorphism is caused by increases in Heat, Pressure and Chemically active fluids.

a. Pressures: stresses applied to the rock which can cause the rock to change shape and is often associated with folding and faulting as well.

i. Confining pressures: are pressures that are associated with burial of rocks at depth and are typically equal in all directions.

ii. Directed or differential Pressures: are pressures or stresses associated with plate interactions and are not equal but are stronger in a particular orientation. Can cause foliation

• Compression or compressive stresses: → Plates collide and the crust gets thicker.

• Tension or tensile stresses: → Plates pull apart and the crust is thinned.

• Shear stresses: Plates slide past each other.

b. Foliation: is a preferred alignment in the rock caused by directed pressures and can include the alignment of mineral crystals, folds, rock cleavage, color banding, or color streaking, etc.

c. Metamorphic Textures: is based upon the type and presence or absence of foliation.

i. Foliated: Rock contains a preferred alignment in the form of foliation.

ii. Non-Foliated: Rock does NOT contain a preferred alignment in the form of foliation. The rock appears more uniform with crystals in random orientations.

iii. Weakly-foliated: Rock may exhibit foliation, but it is very faint and harder to see.

d. Mineral Composition: is based upon the dominant minerals in the rock. Metamorphic rocks often contain index minerals, which are minerals that form at specific temperature and/or pressure conditions. These index minerals can be used to determine the degree or intensity of metamorphism, as well as, to differentiate between different rocks. For example, a muscovite schist would have a different composition than a biotite-garnet schist, but they would both have a scaly foliation.

i. Index minerals often form Porphyroblasts, which typically grow large crystals because the mineral is at its optimum heat and pressure conditions.

CLASSIFICATION OF METAMORPHIC ROCKS

FOLIATED ROCKS

Crystal size Rock name Comments

Very fine, not visible SLATE Smooth, Flat Rock Cleavage crosses sedimentary layers

Fine grains, not visible PHYLLITE Foliation well developed, but may be wavy; rock has a "silky" luster

Coarse Muscovite schist

Visible with unaided eye S Chlorite schist Types of schist, recognized on mineral content,

Mostly micaceous minerals C Biotite schist reflect increasing intensity of metamorphism

Often with porphyroblasts H Garnet schist from top downward and often have a scaly appearance

I Staurolite schist

S Kyanite schist

T Sillimanite schist

Coarse, mostly GNEISS Well-developed color BANDING or color streaking

Non-micaceous minerals

NONFOLIATED ROCKS

Precursor rock Rock name Comments

QUARTZ SANDSTONE QUARTZITE Composed of interlocking quartz grains

LIMESTONE MARBLE Composed of interlocking calcite grains

CONGLOMERATE STRETCHED-PEBBLE Original pebbles distinguishable, but strongly

CONGLOMERATE Deformed and elliptical in shape

GREENSTONE Composed of epidote and chlorite

BASALT OR GABBRO AMPHIBOLITE Composed of amphibole and plagioclase and may contain garnet

SERPENTINITE Composed of serpentine

MINERALS COMMONLY PRODUCED IN METAMORPHISM

No. of

cleavage

NAME COMPOSITION HABIT H directions Color

Chlorite hydrous Fe, Mg, Al silicate sheets 2 1 dark green

Epidote Ca, Fe, Al silicate massive 5 2 green

Talc hydrous Mg silicate sheets and foliated masses 1 1 white, black, green

Staurolite Fe, Al silicate long crystals 7 2 lt.-dk. brown

Garnet Fe, Mg, Ca, Al silicate stubby crystals 7.5 0 red, brown, black

Kyanite Al silicate bladed 5 & 7 2 gray-blue

Sillimanite Al silicate needles 5 2 white

Graphite carbon massive or sheets 2 1 metal gray

Serpentine hydrous Mg, Fe silicate massive or lamellar, appears polished 3 - 5 0 or 1 green - black

Additional Metamorphic Rock Classification from Tarbuck and Lutgens Textbook

* Migmatites can exhibit partial melting and therefore can have characteristics of both metamorphic and igneous rocks!