Rock Identification Laboratory
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►►►Igneous Rocks are aggregates of one or more minerals formed through the crystallization of magma
or by the quenching from a silica rich liquid. Magma is a complex hot mobile solution of silicate
composition, which consists of liquids, gases and crystals. Crystallization from magma occurs between
1200-600 degrees Celsius. Minerals with higher crystallization temperatures form first, followed by lower
temperature minerals.
The 8 most common elements constitute over 95% of the total volume of most igneous rocks. These 8
elements combine to form 6 dominant minerals: quartz, potassium, feldspar (K-spar), plagioclase feldspar,
amphibole, pyroxene and olivine. Crystallization sequence is generally from high temperature (~1200) to
low temperature (~600): Olivine, Ca-plagioclase, pyroxene, amphibole, Na-plagioclae, biotite, K-feldspar,
muscovite and quartz. Igneous rocks are classified on the basis of percent silica, mineralogy and texture.
Percent Silica
1. Felsic Rocks – generally light colored with over 65% silica. Primary minerals include quartz,
potassium feldspar and biotite and/or muscovite.
2. Intermediate Rocks – generally grayish or salt and pepper in color with 52-65% silica.
Primary minerals include Na plagioclase, quartz, amphibole, K-feldspara and biotite
3. Mafic Rocks – generally dark colored with 45-52% silica. Primary minerals include Ca
plagioclase, pyroxene, olivine and amphibole.
4. Ultramafic Rocks – generally light greenish to dark green with less then 45% silica. Primary
minerals include olivine, pyroxene and plagioclase.
Mineralogic – criteria for the identification of igneous rocks
1. Presence of absence of quartz
2. Composition of the feldspars (K-spar, Na of Ca plagioclase)
3. Type of proportion of ferromagnesium minerals (amphibole, pyroxene, olivine)
Texture – refers to the size, shape, variability, boundary relations and abundance of crystals. Crystal size
is influenced by depth of cooling, rate of cooling, temperature and pressure conditions (T-P), magma
viscosity, amount of magma, volatile material and space.
1. Phaneritic – individual crystals are visible with the naked eye. Phaneritic textures develop in
deep magma chambers, with stable T-P conditions and slow rates of cooling.
2. Aphanitic – individual crystals are visible with the aid of a microscope. Aphanitic textures
develop due to fairly rapid cooling associated with volcanic eruptions.
3. Porphyritic – two sizes of crystals, inferring two distinct cooling periods.
4. Glassy – develops when instantaneous quenching of magma occurs. As a result of the quick
solidification, crystals do not form. Instead, the magma forms a volcanic glass.
5. Fragmental – volcano ejected material consisting of broken angular fragments ranging in size
from fine ashs welded together to from a variety of fragmental rocks.
a. Vesicular – pumice (light colored) and scoria (dark)
b. Tuffaceous – tuff fragments are smaller than 4 mm in diameter
c. Breccia – angular fragments are greater than 4 mm in diameter
Common Igneous Rock Classification
Texture
Phaneritic
Aphanitic
Felsic
Granite
Rhyolite
Porphyritic
Granite or
Diorite or
Gabbro or
Rhyolite
Andesite
Basalt
porphyry
porphyry
porphyry
Granite
pegmatite
Pumice
Scoria
Obsidian
Volcanic Tuff
Pegmatitic
Vesicular
Glassy
Pyroclastic
(<2mm)
Pyroclastic
(>2mm)
Intermediate Mafic
Diorite
Gabbro
Andesite
Basalt
Ultramafic
Dunite
Komatite
(rare)
Volcanic Breccia
Dominant Minerals in Igneous Rocks
Felsic
Quartz
K-Feldspar
Na-Plagioclase
Amphibole
Biotite
Muscovite
Intermediate
Na-Plagioclase
Amphibole
Pyroxene
Quartz
K-Feldspar
Biotite
Mafic
Ca-Plagioclase
Pyroxene
Olivine
Amphibole
Ultramafic
Olivine
Pyroxene
Ca-Plagioclase
►►►Sedimentary Rocks: Rocks formed through the compaction, cementation and lithification of
sediment derived from pre-existing rocks, or by the direct chemical precipitation or through the
accumulation of organic debris.
Sedimentary rocks are classified on the basis of their composition and texture. The composition of
sedimentary rocks is dependent upon source rock, environment of deposition, and the effects of weathering
and erosion.
Texture – Size and shape of individual grains with in sedimentary rocks, amount of matrix, and cementing
agents. Matrix refers to the clastic material that fills the void spaces between grains. Cements are
precipitated minerals that bind grains (and matrix) together.
Common Cements – include silica (quartz, chert), calcite and hematite
Common sedimentary minerals – quartz, clays, calcite, hematite, k-feldspar, plagioclase, dolomite,
gypsum and halite.
I.
Clastic Texture – Contains fragments of pre-existing rocks
Conglomerate – Rounded particles with grains larger then 2mm in diameter.
Rounded grains indicate that the sediment has been eroded a long distance from
its source rock
Breccia – Angular particles with grains larger than 2mm in diameter.
Angularity of grains indicates that the sediment has not traveled far from its
source rock.
Sandstone – Grain diameters range from 1/16 – 2 mm. Described as fine,
medium or coarse grained sands. Grains are visible.
Arkose – Sandstone rich in feldspar (>25%), indicating continental
source rock (granite)
Graywacke – Sand sized fragments in a muddy matrix, indicating
incomplete weathering and rapid deposition.
Lithic Sandstone – Rock fragments within a sandy matrix, indicating
possible local source, incomplete weathering and rapid deposition.
Quartz Sandstone – Sandstone in which quartz dominates, indicating
stable depositional setting near continental shelf.
Banded Sandstone – Sandstone with alternating color bands,
commonly quartz
Siltstone – Grain diameters range from 1/256 – 1/16 mm. Grains are
microscopic. The sediment texture feels gritty to the hand or teeth. Fine quartz
fragments, micas, clay minerals, feldspar fragments and other minerals.
Shale – Grain diameters are less than 1/256 mm in diameter. Grains are
microscopic. Minerals commonly include clay minerals and micas.
II.
Bioclastic Texture – Forms by the accumulation of organic material
Fossiliferous (Skeletal) Limestone – Calcite rock containing fossils.
Coquina Limestone – Calcite rich rock composed entirely of shell fragments.
Bituminous Coal – Derived from the compaction and lithification of plant
material, commonly forms in equatorial swamps.
III.
Chemical Terxture – Forms by the direct precipitation from water
Limestone – Rock containing calcium carbonate, commonly forms in the
marine environment.
Chemical Limestone – Rock consisting of calcite crystals.
Oolitic Limestone – Rock consisting of “ooid” grains.
Micrite Limestone – Muddy limestone
Chalk – White, soft, “chalky” limestone consisting of microscopic
foraminifera shells.
Travertine – Banded limestone from caves or hot springs.
Tufa – Porous limestone associated with hot springs.
Dolostone – Carbonate rock containing dolomite
Chert – Rock composed of microcrystalline quartz, which forms in the deep
marine environments through the accumulation of siliceous shells of diatoms
and radiolarians.
Banded Chert – Chert with alternating color bands.
Jasper – Red Chert
Flint – Black Chert
Rock Salt – Evaportie rock containing halite.
Rock Gypsum – Rock composed of gypsum, which forms in restricted
environments where water loss through evaporation exceed that which enters the
system.
Ironstone – Hematite, magnetite or limonite (rusty) rich rock.
►►►Metamorphic rocks form by the alteration of pre-existing rocks as a result of heat and/or pressure.
The process of metamorphism may involve the rotation of preexisting grains, the alteration of existing
minerals or the growth and recrystallization of new minerals. Metamorphic rocks are classified on the basis
of their composition and texture.
I.
Foliated Texture – Planar fabric develops as a result of regional metamorphism
resulting from deviatric (directed) stress. Commonly, the metamorphic planar
fabric develops perpendicular to the greatest compressive force, usually
associated with tectonic shortening. With increasing heat and pressure, the
sequence from low grade to high grade is from: L – Slate, Phyllite, Schist,
Gneiss, Migmatite – H.
Slate – Flat planar slaty cleavage with clay sized grains. Common minerals:
clays, micas, talc and chlorite. Parent material: shale
Phyllite – Waxy, wavy foliated texture characterized by clay sized grains..
Common minerals: clays, mica, talce and chlorite. Parent material: shale
Schist - Medium to coarse grained wavy schistose cleavage. Common minerals:
Mica, garnet, amphibole, kyanite and staurolite. Parent material: shale, slate,
phyllite and volcanic rocks.
Gneiss – Coarse, recrystallized rock characterized by gneissic (color banded)
layering. Common minerals: feldspars, quartz, amphibole, mica and garnet.
Parent material: sandstone, granite, schist and mafic rocks
Migmatite – High grade gneiss that approaches igneous rock (interlocking
crystals) exture characteristics due to extensive melting. Parent material: gneiss
II.
Non-Foliated Texture – Occur in metamorphic rocks which lack planar fabric.
Non-foliated textures commonly develop in rocks that experience equal stress in
all directions, producing roughly equigranular crystals.
Marble – metamorphosed limestone
Metaquartzite – metamorphosed quartz sandstone
Metaconglomerate – metamorphosed conglomerate, with chlorite and epidote
Metabreccia – metamorphosed breccia, common minerals: chlorite, epidote
Anthracite Coal – high grade coal: metamorphosed bituminous coal
Greenstone – chlorite, epidote, talc, serpentine rich rock, altered
mafic/ultramafic
Serpentinite – foliated serpentine rich greenstone rock
III.
Non-Foliated Contact Metamorphic Rocks – Developed locally around
igneous intrusions.
Hornsfels – hydrothermally altered country rock
Skarns – Hydrothemally altered, multicollerd carbonate rock
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