Textures and structures of igneous rocks

Crystal development

euhedral: grains bounded by its own perfect to near-perfect crystal growth faces

subhedral: partly bound by its own growth faces, or growth faces only moderately well developed

anhedral: irregular; little or no evidence for its own growth faces

Euhedral crystals of garnet in a metamorphic rock

Anhedral garnet grain and biotite flakes in augite-biotite granite

Subhedral augite

Crystalinity

Igneous rocks range in crystalinity from entirely crystals to entirely glass. Holocrystalline: Rock completely composed of

crystals (mineral grains). Hypocrystalline: mostly crystals, but some glass in

the rock. Hypohyaline: Mostly glass, but some crystals in

the rock. Holohyaline: made completely of glass (obsidian)

Grain size

• aphanitic: grains too small to see without a microscope, but rock isn’t glassy • phaneritic: grains visible with the unaided eye – fine grained: < 1 mm (average long dimension of

grains) – medium grained: 1 mm: 5 mm – coarse grained: 5 mm: 3 cm – very coarse grained = pegmatitic: > 3 cm

• Granular texture- A term equal to phaneritic and used as follows:

equigranular: most of the grains are approximately equal in size

Inequigranular: grains of significantly different sizes

• seriate: smooth variation in grain size from small to large

• 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– Vitrophyric- Special term used for porphyric rocks with

a glassy matrix.– Poikilitic : Special term for porphyritic rocks containing

phenocrysts with many smaller mineral inclusions.– Ophitic : Special term used for porphyritic rocks

containing pyroxene (augite) phenocrysts with randomly oriented grains of tabular plagioclase feldspar

Exolution of gas

Magma is composed of melted minerals and dissolved gas. Reduced confining pressure allows magma/lava to boil. This boiling causes some kind of vesiculation:• Vesicular texture- Gas caviitites make up less

than 50% of the rock. A vesicle is a single gas cavity.

• Pipe vesicles- Tube-like elongate vesicles that result from rising gases

• Amygdular - Vesicles are infilled with younger minerals. The filling is called an Amygdule and is composed of quartz, opal, chalcedony and or zeolite minerals

• Trachytic- Special flowage structure involving small Alkali feldspar grains in Syenetic rocks.

• Diktyataxitic- Containing vesicles into which microphenocrysts protrude

Intergrowth textures

• Graphic- an intergrowth of quartz and K-feldspar in which the quartz forms a crystallographically-controlled networks;

- the rock shows angular wedge-like forms. Usually occurs with quartz in microcline• Micrographic (granophyric)- graphic but only

visible in microscope

• Symplectite- A replacement texture in which a mineral is replaced by an intergrowth of one or more minerals.

• Myrmekitic- Is defined by wormy (rounded) intergrowths o quartz and K-feldspar in plagioclase which is adjacent to K-feldspar. Probably forms as a result of subsolidus exchange at the contact of K-feldspar and plagioclase

• Ophitic- hypidiomorphic granular with euhedral plagioclase enclosed within anhedral augite

• Subophitic: like ophitic, but augites only partly enclose the plagioclase

• Intergranular:euhedral plagioclase surrounded by finer grains of augite

Reaction textures

• Poikitic-smaller grains of one mineral are completely enclosed in large, optically continuous grains of another mineral.

• Corona- a crystal of one mineral is surrounded by a rim, or “mantle” of one or more crystals of another mineral.

• Kelyphitic- a fine-grained, fibrous intergrowth of multiple phases such as pyroxene, spinel and amphibole, typically developed forming a rim (or a corona) surrounding garnet.

• Sieve texture in corroded, partially resorbed plagioclase corses.

It is thought that this texture may be formed in at least two ways: - If a plagioclase crystal is placed into a magma in which it is

not in equilibrium (by magma mixing), it will become corroded, and melt will penetrate into the crystal structure. The crystal may also become rounded by partial resorption. New plagioclase of a different composition will precipitate from the magma and perhaps form a rim around the corroded core.

-Alternatively, the same effects could possibly be produced by volatile-loss from decompression as a magma rises to shallower regions in the crust.

Sieve texture