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Jan 15 2014

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Petrology Media Library

The information posted here is not in a particular order. If you want to find something specific on this page, I suggest using page text finder on your computer (Ctrl + F). Important terms and information are in bold. For additional photographs and information, please visit the following pages:
Mineralogy Media Library
Photomicrographs of Minerals
Sedimentary Petrology Media Library

Igneous Petrology

Aphanetic verses Phaneritic Textures

In hand sample, aphanetic describes rocks with very fine grained minerals (hard to see with naked eye). Phaneritic is the opposite of that. In thin section, predominately small crystal size in a sample is described as aphanetic. The image posted under Plutonic versus Volcanic is a good example of Aphanetic (plutonic) verses Phaneritic(volcanic).

Competitive Growth

This is characterized by the wavy boundaries between two or more grains.

Competitive Growth - 10x XPL (~2mm across) Competitive Growth - XPL

Compositional Zoning

As the mineral grows, the chemical composition of the magma in the immediate vicinity evolved. This may cause a variation in chemistry in each layer of addition to the forming mineral. This is most commonly observed in plagioclase feldspars. In plagioclase, concentric bands around the crystal that undergo extinction (XPL) at various angles indicate such zoning. Normal zoning will have albite-rich rim and anorthite-rich core. Reverse zoning will have the opposite. By using an electron microscope or an electron probe, you may determine if it is normal or reverse zoning. Zoning is a type of disequilibrium texture.

Plagioclase Zoning - 4x (xpl) Plagioclase Zoning - XPL

Chloritization

It is a type of Chloritization where mafic minerals are converted to chlorite.

Cumulaus Textures

These textures are caused by accumulation of crystals due to gravitational settling (crystal segregation) in a magma chamber.

Cumulaus are the crystals that formed very early. They are usually well formed and euhedral. In “normal” magmatic conditions, one could expect the cumulaus minerals to be upper minerals in Bowen’s Reaction Series (Olivine, Pyroxenes, etc, etc).

Intercumulaus describes the minerals that formed in between the already formed cumulaus grains. Since the space is limited, these crystals often have subhedral to anhedral shape. SOme grains may even have formed with unusual shapes due to limited availability of components and space.

The following image is a near perfect example of cumulate and intercumulate minerals. The Chromite formed first and then Muscovite formed in between.

Chromite (cumulaus); Muscovite (intercumulaus)-PPL

An example of plagioclase as the cumulaus alongside clinopyroxene as the intercumulus.

Plagioclase (Cumulus); Clinopyroxene (intercumulus)-2X-xpl Plagioclase (Cumulus); Clinopyroxene (intercumulus)-4X-xpl

An example of Enstatite as the cumulaus with clinopyroxene(cpx) and plagioclase as intercumulus minerals.

Enstatite (Cumulus) Cpx & Plagioclase (Intercumulus)-4x-xpl

Heteradcumulus

Info to be updated… Note that the plagioclase (“background”) in the following image is in extinction.

Heteradcumulus - XPL

Also research on Orthocumulate and Adcumulate textures.

Cumulophyric Texture

Please read Glomeroporphyritic Texture

Deformation Twins

Occur on albite twining on plagioclase and on calcite. Deformation twins lack straight lamellar form, which is observed in normal twins. They are characterized by the wedge shaped or bent twins. About 3 mm across the image.

Plagioclase Deformation Twins (4x XPL)

Devitrification

Devitrification characterized by spherulites and perlitic cracks.

Devitrification - 10x PPL (2mm across) Devitrification - 10x XPL (2mm across)

Embayment

The embayed texture is characterized by having “dips” or bay-like sections in the crystal as a result of resorption. This is a type of disequilibrium texture.

Embayment - 4x XPL (~5mm across) Embayment - 10x XPL (~2mm across)

Equigranular Texture

The sample has grains about the same size. Opposite term is ineuigranular. Also check the image for Cumulaus Textures.

Equigranular Texture-XPL  4x XPL (~5mm across)

Glomeroporphyritic Texture

Clusters of phenocrysts crystals involving one type of mineral. The term cumulophyric is used when there are more than one type of mineral in a cluster.

Glomeropheric Texture - PPL (~11mm across) Glomeropheric Texture - XPL (~11mm across)

Granophyric Texture

This texture is produced as a result of rapid undercooling of quartz and feldspars. The two minerals grow at the same rate at the same time (simultaneous growth). The process is usually triggered by loss of water/dehydration in a granitic melt. They do not form as single crystals but rather penetrate each other in an irregular branching like intergrowths. Under microscope (XPL), the quartz intergrowths will undergo extinct at the same time.

Granophyric texture - XPL Granophyric texture (also CPX in green) - XPL Granophyric texture close-up 40x - XPL

Interstitial Texture

The background of the phenocrysts are one large feldspar. Also related to Cumulaus Textures because “interstitial” is a descriptor/modifier.

Olivine, K-feldspar, Chrom-PPL Olivine, K-feldspar, Chrom-XPL

Inverted Pigeonite

Characteristic of magmas which underwent rapid cooling. Commonly formed in rocks with plutonic origin. Also known as herringbone exsolution. Note that the first two pictures may or may not be inverted pigeonite. However, they also have the exsolution features and could be a partial representation of it. The last image shows what we ideally called inverted pigenoite.

Inverted Pigeonite - XPL Inverted Pigeonite (1) - XPL Inverted Pigeonite (perfect) - XPL Inverted Pigeonite - 10x XPL (~2.0mm across) Inverted Pigeonite - XPL Inverted Pigeonite 2x - XPL

K-feldspar type Exsolution

Caused by limited chemical mixing in the magma. Most commonly seen in K-feldspar with respect to Na-rich verses K-rich chemical environments (also read prethite verses antiperthite in notes or textbook). It can also occur on plagioclase with Si-Al chemical environments. Also check images under Poikilitic Texture

Exsolution - XPL

Ophitic, Subophitic and Nesophitic textures

The term ophitic is used when large pyroxene grains enclose small, random plagioclase laths. The proportion of pyroxenes is much larger than that of plagioclase.

The term subophitic is used when larger plagioclase laths are partially enclosed by pyroxene grains. The volumetric proportion of plagioclase is higher than that of pyroxenes. The two images are examples of nesophitic texture. Images shows subophitic texture.

The term nesophitic is used when large plagioclase has smaller interstitial pyroxenes. The volumetric proportion of the plagioclase is higher.

Subophitic Texture - PPL Subophitic Texture - XPL Subophitic Texture - XPL

Plutonic versus Volcanic

On the following picture, the left one is a thin section from a plutonic rock and the right one is from a volcanic rock. The difference is in the groundmass and the crystal growth. Finer grained groundmass is indicative of volcanic rocks. Volcanic rocks usually have more ground massvolume (relative to the larger crystal volume). Well formed euhedral crystals are indicative of plutonic rocks and they usually have less groundmass volume. Both images have exact same resolution with about 2.8 mm across each Field of View (width).
Difference between plutonic and volcanic rocks.-XPL

Pilotaxitic Texture

Groundmass crystals are randomly oriented.

Pilotaxitic Texture - 4x (~5mm across)

Poikilitic Texture

This is a type of porphyritic texture in which a host phenocryst (known as oikocryst) contains one or more other mineral(s) inclusions. This is caused by magmas that underwent slow cooling (undercooling) first stage and later underwent rapid cooling. During the first stage, smaller crystals were formed. During the last stage, these smaller crystals will be included within the larger faster cooled crystal. The easiest poikilitic grains to identify under microscope have usually only one grain inclusion (picture below). You can also see the faint perthitic exsolution on both PPL and XPL images.

Poikilitic Texture and Perthitic Exsolution - PPL Poikilitic Texture and Perthitic Exsolution - XPL

Pseudomorph Texture

It is a type of replacement texture in which one or several minerals replaces another already formed mineral. The new replacement mineral(s) will maintain the size and shape of the original one.

Rapakivi Texture

It is an overgrowth of plagioclase minerals on alkali feldspar mineral. This texture is caused by epitaxial (heterogeneous) nucleation where new nuclei is formed on a per-existing crystal. In most cases, this can be easily recognizable in hand samples.

Rapakivi Texture (epitaxial nucleation)

Seritization

It is a replacement process in which feldspars and/or feldspathoids by fine white micas. This is observed as a fined dust like (dotted) appearance on feldspars. This texture is caused by hydrothermal alteration.

Seritization of Feldspar - 4x (3mm-grain) - PPL Seritization of Feldspar - 4x (3mm-grain) - XPL

Skeletal Texture

TO BE UPDATED…

Spinifex Texture

Usually easy to recognize in hand samples. They are sub-parallel to dendritic growth of olivine or clinopyroxene crystals in some quenched ultramafic rocks generated from lava flows.

Spinifex Texture on a Rock Spinifex Texture - 4x PPL (~5mm across) Spinifex Texture - 4x XPL (~5mm across)

Sieve Texture

The following image shows a mineral (olivine) that has been formed in two different chemical conditions. The larger euhedral crystal is the initial crystal. After most of the initial crystal has formed, the mineral has started to remelt producing a slightly different composition of olivine at the corners (top right and bottom left). This process is known as resorption. An electron microprobe may be used to analyze the the chemical differences between them. This is a type of disequilibrium texture.

Olivine - XPL (magma mix variation)

Trachyte Texture

Caused by alignment of minerals (usually the elongated minerals such as feldpars) due to flow. Found in aphanetic rocks and easy to observer in hand samples than thin sections.

Trachytic (Trachytordal) texture Trachytic (Trachytordal) mineral alignment - XPL Trachytic (Trachytordal) min. alignment indicated with arrow - XPL

Xenolith verses Xenocryst

Xenolith is an inclusion of country rock. Xenocryst is an inclusion of a foreign crystal that may or may not be from the country rock (eg. Olivine).


Metamorphic Petrology

Andalusite

First two pictures: a beautiful example of Andalusite “cross” in PPL and XPL.

Andalusite Cross - PPL Andalusite Cross -XPL

Andalusite - PPL Andalusite - XPL (extinct) Andalusite - XPL

Coronas

Corona is a rim (or several successive rims=coronas) of one or several minerals surrounding another unstable mineral. These reaction rims are formed as a result of partial replacement of minerals at the grain boundary of the unstable mineral (middle). This is a solid state process.

Order of minerals for this particular example is; Olivine (center) – Orthopyroxene – Clinopyroxene – Hornblende- Spinel – Garnet – Plagioclase (outer most). This is the most common order of coronas(??).

Coronas - 2x PPL (~11 mm across) Coronas - 2x XPL (~11 mm across) Coronas - 4x PPL (~5mm across) Coronas - 4x XPL (~5mm across) Coronas - 2x PPL (~11 mm across) Coronas - 2x XPL (~11 mm across) Several coronas on a rock

Fabric Types

To be updated…

S and L type fabrics S and L type fabrics labeled

Grossular Garnet

Garnet is isotropic (remain extinct under XPL). However, grossular garnet will have zoning like layers within it.

Grossular Garnet - PPL Grossular Garnet - XPL

Jadeite

Fiber like appearance with blue-brown interference colours.

Jadeite blue colour - XPL Jadeite brown colour - XPL

Kyanite

Kyanite - PPL Kyanite - XPL

Kyanite in rock hand sample

Porphyroblasts

The following two images shows Garnet as a porphyroblast. By definition, porphyroblast is a larger crystal grown within a finer grained groundmass.

Garnet porphyroblast with Muscovite, Plage, Quartz and Biotite - 4x PPL (~5mm across) Garnet porphyroblast with Muscovite, Plage, Quartz and Biotite - 4x XPL (~5mm across)

Tremolite

Fibrous like Wollastonite but bright interference colours than Wollastonite. Not to be confused with Wollastonite.

Tremolite - 2x XPL (~11 mm across)

Wollastonite

Fibrous but lower interference colours than Tremolite. Not to be confused with Tremolite.

Wollastonite - 2x XPL (~11 mm across)

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