Download Texture and Structure Lab Lab #2

Texture and Structure Lab
Lab #2
I. Introduction
A. General Statement: All igneous rocks have texture and many have a structure.
Together they define the FABRIC of a rock.
B. Fabrics formed during liquid phase of Magma: mostly liquid but some solids.
1. Incipient Crystals (just beginning to form):
a. Crystallites: crystals are too small to identify. Look like dust particles on
thin edges of the translucent rock. Occur in glassy, volcanic rocks like
obsidian.
b. Microlites: Incipient crystals that have some identifiable properties. Occur
in rocks like rhyolite.
c. Skeletal Crystals: Incompletely formed brystals that are best seen in thin
section. Snow flakes are a good example in nature.
2. Phenocrysts: visible crystals that stand our from the matrix or groundmass of the
rock. Often some of the earliest minerals to form during crystallization.
a. Shapes of phenocrysts:
i. Euhedral Crystals: All phenocrysts (crystals) that are
bounded by their own crystal faces. (Same as idiomorphic).
ii. Broken crystals: Early formed phenocrysts that have been broken by
movement of magma, etc.
iii. Corroded and Embayed Crystals: Early formed phenocrysts are
chemically unstable in melt. Sharp edges become rounded and other
edges are eaten into by younger minerals.
b. Orientation of Phenocrysts:
i. Flow Fabric (structure): Elongated crystals become aligned by flowage
of magma.
ii. Lineation: alignment of needle-like minerals in rock.
iii. Foliation: alignment of platy minerals.
C. Effect of Volatiles in Magma (liquid phase): Magma is composed of melted minerals
and dissolved gas. Reduced confining pressure allows magma/lava to boil. This
boiling causes some kind of vesiculation:
1. Types of Vesiculation:
a. Vesicular structure: Gas caviitites make up less than 50% of the rock. A
vesicle is a single gas cavity.
b. Scoriaceous Structure (Scoria): Dark colored rock with > 50% vesicles.
Vesicles are large and nearly equidimensional because of low viscosity of
magma/lava.
c. Pumacious Structure (Pumice): light colored rock with > 50% vesicles.
Vesicles are small and often drawn out into long tubes because of flowage
of high viscosity lava.
d. Amygdaloidal Structure: Vesicles are infilled with younger minerals. The
filling is called an Amygdule and is composed of quartz, opal, chalcedony
and or zeolite minerals.
D. Other Features Associated with the liquid phase:
1. Xenoliths: Foreign rock fragments found in igneous rocks.
2. Pillow Structure: Indicate lava flowed into water and then crystallized.
E. Textures formed during main solidification stage
1. Degree of Crystallinity:
a. Holocrystalline: Rock completely composed of crystals (mineral grains).
b. Hypocrystalline: mostly crystals, but some glass in the rock.
c. Hypohyaline: Mostly glass, but some crystals in the rock.
d. Holohyaline: made completely of glass (obsidian)
2. Shape of individual crystals/minerals:
a. Euhedral (idiomorphic): bounded by its own crystal faces and usually early
formed.
b. Subhedral: crystals bounded in part by own faces.
c. Anhedral: minerals are not bounded by own crystal faces. Usually lateforming minerals that fill remaining open spaces in the rock.
3. Rock Textures Defined by Grain Size:
a. Grain size terms:
i. Pegmatitic -------------‡>2cm (20mm)
ii. Coarse Grained-------‡5mm-20mm
iii. Medium Grained------‡2mm-5mm
iv. Fine Grained ----------‡2mm but visible to eye.
b. Aphanitic Texture: Grains are fine grained, with some more visible than
others.
c. Phaneritic Texture: most grains are in the 2-20 mm range. Exp. The rock
has a phaneritic texture, with most grains being medium grained.
i. Granular Texture: A term equal to phaneritic and used as follows:
a. Equigranular: Rock has a granular or phaneritic texture, but all
grains are about the same size.
b. Inequigranular: Rock is granular or phaneritic, but the mineral
grains are not the same size.
d. Porphyritic Texture: Rock has two distinct grain sizes. Large grains are
phenocrysts and small grains make up the matrix or groundmass. Matrix
may be phaneritic, aphanitic or glassy.
i. Vitrophyric Texture: Special term used for porphyric rocks with a glassy
matrix.
ii. Poikilitic Texture: Special term for porphyritic rocks containing
phenocrysts with many smaller mineral inclusions.
iii. Ophitic Texture: Special term used for porphyritic rocks containing
pyroxene (augite) phenocrysts with randomly oriented grains of tabular
plagioclase feldspar.
4. Textures in Granular/Phaneritic Rocks defined by single crystal shapes.
a. Panidiomorphic-Granular Texture: Any granular/phaneritic rock composed
entirely of euhedral grains. (very rare)
b. Hypidiomorphic-Granular Texture: Any granular/phaneritic rock where most
grains are subhedral, but some are euhedral and anhedral. (very common)
c. Allotriomorphic-Granular Texture: Any rock composed mostly of anhedral
grains.
i. Aplite: A name given to hypabyssal (subvolcanic) rocks with this texture.
(rather rare)
5. Other Textures and Structures:
a. Reaction Rims: magma reacts with surface of earlier formed minerals.
b. Trachytic Structure: Special flowage structure involving small Alkali feldspar
grains in Syenetic rocks. This linear structure is typical in the rock Trachyte.
c. Miarolitic Structure: Incompletely filled gas cavity in phanaritic rock. Filling
minerals project inward. Cavity may be completely filled.
d. Orbicular Structure: Ovoid masses composed mostly of layers of biotite
around a nucleus made of parent igneous material.
e. Graphic Structure: Regular intergrowth of quartz in feldspar common in
Pegmatite.
f. Comb Structure: Crystals in a vein grow perpendicular to vein wall.
Indicates infilling of an open fracture.
F. Textures and Structures Formed After Crystallization:
1. Devitrification: The slow change of glass (a super cooled liquid) into tiny crystals
with time. (not phenocrysts)
2. Perlitic Structure: Concentric shelly cracks formed in some water-rich volcanic
glasses, producing spherical balls having a pearl-gray luster. Each ball may
have a nucleeus of unaltered, black glass known as an Apach Tear. (not
phenocrysts)
3. Spherulitic Structure: Spherical bodies from microscopic size to many feet in
diameter formed in volcanic glass. Each body is composed of radiating fibers
composed of feldspar and quartz. Not Phenocrysts.
4. Cataclastic Structure: Results from the bending or crushing of solid mineral
and/or rock material.
a. Bent Grains:
b. Broken Grains:
c. Mylonitization: Rock is completely granulated and sheared out to look like
the metamorphic rock called schist. Usually formed in fault zones.