Chapter 3-Igneous Minerals and Rocks
Chapter 3
copyright 2007-Roger Weller
last edited: 10/3/07
Igneous
Rock Table
(shows relationships)
albite
-is a sodium-rich plagioclase feldspar;
often white,
a property from which the name is derived.
amphibole
-is a group of rock-forming silicate
minerals, rich in iron, magnesium, and calcium.
Crystals are often elongated.
amygdule
-is a gas pocket in an igneous rock that has
been filled in with a secondary mineral such as calcite
or quartz.
-basalt, amygdaloidal with thomsonite
andesite
-is a fine grained igneous rock intermediate
in composition between rhyolite and basalt.
It is named after its common
occurrence in the Andes mountains of South America.
-andesite, porphyritic
-assorted views of andesite
Apache tears
-are rounded, dark, translucent obsidian
nodules enclosed in perlite.
These glassy nodules are often tumble polished for
jewelry..
-obsidian, Apache tears
-tumble polished Apache tears
aphanitic texture
-is also known as fine grained. This
igneous texture refers to very fine grained igneous rocks such
as
basalt and
rhyolite.
aplite
-is a fine-grained equivalent of granite
often in the form of a vein.
As a granite body cools, fractures open up and the
remaining magma fills these fractures, cooling
relatively
quickly to form the aplite veins.
-aplite vein exposed on Mt.Whitney-1
-aplite vein exposed on Mt.Whitney-2
-aplite vein in decomposed granite
assimilation
-occurs as an upward moving magma melts the rocks above it and thus adds the
elements of these
rocks to its composition.
-assimilation-Opal's Pals
augite
-is the most common member of the pyroxene
group of silicate minerals. It is usually black or
blackish green in color.
-augite cleavage
-augite cleavage-2
basic igneous rock
-is an igneous rock rich in dark-colored silicate minerals.
basalt
is a dark, fine-grained (aphanitic) volcanic
rock.
a quick
guide to basalt (a photo tour)
biotite
-is a black mica.
Bowen’s reaction series-continuous series
-describes how the composition of developing
feldspar crystals changes as the magma cools.
The first plagioclase feldspar
crystals to form at high temperatures are calcium-rich (anorthite)
and the last
crystals to form are sodium-rich (albite). During the entire crystallization
process the
already formed feldspar crystals chemically react with the magma.
-labradorite
-andesine
-oligoclase
Bowen's reaction series-discontinuous series
-is part of Bowen's Reaction series that
describes the sequence in which the ferromagnesian
crystallize as magma cools.
The first member of the discontinuous series to for is olivine,
followed by
pyroxene, then amphibole, and finally biotite.
-biotite
Bowen's reaction series-residual, low temperature silicates
-These are the last silicate minerals to crystallize out of a cooling magma.
-muscovite
(white mica)
-quartz
-orthoclase
feldspar
chill zone
-occurs at the outer surface of an igneous pluton where it is in contact with
cold country rocks.
The magma cools faster along its outer surface,
producing a finer grain texture than the rest of the
intrusion
which cools more
slowly.
-chill
zone-Opal's Pals
coarse grained
-is a descriptive term for an igneous rock
texture in which the mineral grains in the rock are visible
to the naked eye.
-diorite, polished
country rock
-is the pre-existing rock into which a magma body intrudes.
crystallization
-is the process of disorganized atoms and
molecules in a solution or melt coming together to form
a solid with
an
organized, repeating geometrical pattern of atoms.
crystal settling
-As a magma body cools and starts to crystallize, the heavier (denser)
crystals that are forming sink
to the
bottom of the magma chamber, thereby
altering the composition of the remaining magma.
A good example is where olivine
crystals settle within a basaltic magma.
-crystal
settling-Opal's Pals
diopside
-is a green pyroxene mineral.
diorite
-is a coarse grained igneous rock containing mostly albite (white) feldspar and hornblende.
ferromagnesian silicate
-is a silicate minerals rich in iron and magnesium, such as olivine, augite, hornblende, and biotite.
fine grained
-is an igneous rock texture in which the
mineral grains are smaller than what the naked eye can
distinguish;
also called aphanitic.
gabbro
-is a dark, coarse grained igneous rock.
glass
-is a solid in which the atoms are all in a chaotic distribution and not crystallized.
gneiss
-is a highly metamorphosed rock that is just
one step away from becoming a granite.
This metamorphic rock is often described
as looking like a layered or lineated granite.
granite
-is a coarse grained igneous rock made up of feldspar, mica, and quartz.
-polished granite
-polished granite-2
granodiorite
-is a coarse grained igneous rock midway in composition between granite and diorite.
-close up view of granodiorite on Mt.
Whitney
gravitational magmatic segregation
-Under the influence of gravity, the denser components of a magma sink downwards
and the
lighter components rise.
-gravitational magmatic segregation-Opal's Pals
groundmass
-is the mass of small crystals that surround
the phenocrysts in a porphyry.
-groundmass
with phenocrysts
hornblende
-is the most common member of the amphibole
family of silicate minerals.
Usually hornblende forms dark brown, elongated
crystals in igneous rocks such as diorite.
-hornblende
igneous petrologist
-is a geologist who studies igneous rocks.
-petrologist-Opal's Pals
igneous rocks
-are rocks formed as magma cools and solidifies.
-rough granite
-igneous
rocks-Opal's Pals
labradorite
-is a calcium-rich plagioclase feldspar, often identified by its iridescent schiller.
mafic rock
-is an igneous rock rich in iron and magnesium silicates.
-basalt with fresh broken surface
magma mixing
-occurs when two or more magma bodies underground come into contact with each
and mix
together
creating a magma with a new composition.
magmatic differentiation
-occurs as a magma body cools and crystallizes. The minerals that
crystallize first remove certain
elements
from the magma, thus changing the
composition of the remaining magma.
muscovite
-is also known as white mica; it is valuable
because of its transparency and high electrical
insulating property.
-muscovite crystal
-muscovite in pegmatite
-mica
book-Opal's Pals
obsidian
-is volcanic glass.
olivine
-is an iron magnesium silicate that is
usually yellow-green in color.
This mineral is one of the first minerals to
form as a basaltic magma starts to cool and crystallize.
orthoclase feldspar
-is a potassium feldspar.
partial melting
-occurs as a subducted slab of crust moves
down a subduction zone.
Those minerals with the lowest melting temperatures
melt first.
-partial
melting-Opal's Pals
pegmatite
-is a very coarse grained igneous rock.
The
crystals within a pegmatite are larger than one half inch.
perlite
-is a volcanic glass with high water content that is riddled with concentric cracks.
perthitic structure
-is a wood-like pattern of small, short
bands of a translucent feldspar embedded in a slightly
more transparent
feldspar.
This type of structure is often used to identify orthoclase feldspar.
This structure is produced by an exsolution process as an orthoclase crystal
crystallizes and
then slowly cools.
-perthitic structure-1
-perthitic
structure-2
phaneritic texture
-is coarse grained, a term applied to
igneous rocks whose constituent grains are large enough to
see with the naked
eye.
phenocrysts
-are the large crystals within a porphyry.
-porphyry
-phenocryst-Opal's Pals
pitchstone
-is a form of volcanic glass that is in the process of breaking down (devitrifying).
plagioclase feldspars
-are the calcium and sodium rich feldspars.
plagioclase twinning
-On a cleavage surface of plagioclase
feldspar, light is reflected off of a series of straight-edged,
narrow strips
on
cleavage surface.
The cleavage surface actually consists of two slightly
different
crystal orientations which
formed as the plagioclase crystal slowly cooled.
Light is being reflected
off one set of micro-cleavage surfaces
from
one of the crystalline orientations.
If a plagioclase
specimen is tipped again
slightly with respect to the
light source, the other set of micro-cleavage
surfaces will then reflect light.
-plagioclase-polysynthetic
twinning
-albite-plagioclase
twinning
plutonic (intrusive) rocks
-are coarse-grained igneous rocks that have cooled slowly underground.
-gabbro
-diorite
-granite
polysynthetic twinning
-See the definition for polysynthetic twinning.
-plagioclase-polysynthetic twinning
porphyritic texture
-is an igneous rock texture that is a
combination of aphanitic (fine grained) and phaneritic (coarse
grained)
textures.
porphyry
-is an igneous rock containing two different sizes of crystals.
-porphyry, cut
-porphyry-Opal's
Pals
pumice
-is a very light-weight silicic volcanic
rock consisting of a glass foam.
Pumice is so light weight that it can float on
water.
-pumice, various colors
-pumice-Opal's
Pals
-pumice as an abrasive tool-1
-pumice as an abrasive tool-2
pyroclastics
-are the hot, fiery (molten) fragments tossed out of a volcano.
-volcanic bombs of various sizes
pyroxene
-is a group of single chain silicate
minerals.
Augite, a member of this silicate family of minerals, is a major
constituent of basalt.
quartz
-is one of the most common minerals on the
Earth, consisting of silicon and oxygen.
This mineral is number 7 on Mohs scale
of hardness.
Quartz occurs in many different forms and may be crystalline,
cryptocrystalline, or massive.
rhyolite
-is an aphanitic (fine grained) pastel-colored volcanic rock.
-rhyolite, tan colored
schlieren
-are bands of melted xenoliths that have not been completely mixed into the
magma.
-schlieren-Opal's Pals
scoria
-is a very vesicular form of basalt; dark-colored with many small gas bubble holes.
-basalt, variety scoria
texture
-refers to the size and shape of the constituents of the rock: coarse-grained,
fine-grained, glassy,
vesicular, fibrous, etc.
thunderegg
-is an igneous concretion, usually round,
formed in conjunction with obsidian.
These concretions start hollow within a
obsidian flow and may either stay hollow and become
geodes,
or they may fill
with agate or opal. The scientific term for type of structures is lithophysa
(plural-lithophysae).
ultramafic igneous rock
-is made almost entirely of dark-colored
iron and magnesium silicates.
Peridotite is an ultramafic rock
made of
olivine (peridot).
-peridotite
vesicular basalt
-is a fine grained, dark igneous rock with gas bubble holes.
-basalt with fresh broken surface
-vesicular basalt
-vesicular
basalt-Opal's Pals **
vesicle
-is a gas bubble hole in a volcanic rock.
-vesicles in basalt
volcanic (extrusive) rocks
-are fine grained or glassy igneous rocks that have been erupted from
volcanoes.
-basalt
-andesite
-rhyolite
-obsidian
-pumice
Weller's spaghetti theory of magma
-explains how the constituents of magma (silica, cations, and
dissolved gases)
affect the viscosity of a magma.
-Weller's
spaghetti theory of magma-Opal's Pals *
xenolith
-is a baked inclusion of country rock surrounded by an intrusive igneous rock.
-xenolith from Mt. Whitney-2
-xenolith-Opal's
Pals