Tag Archives: geology

Sedimentary 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
Petrology Media Library (Igneous and Metamorphic)

Sedimentery Petrology

Cements on substrates

Acicular

Prismatic (4x) ~ 3500 um across

Pour-filling Cements

Syntaxial Overgrowth

Cement: Syntaxial Overgrowth (4x) ~ 5000 um F.V. ppl Cement: Syntaxial Overgrowth (10x) ~ 1600 um F.V. ppl

Fusulinid

The image shows morphological structures (as opposed to micro-structures). Scale: ~900 μm across.

Fusulinid at 10x (~900 um across)

Green Algae

Dasycladacean Green Algae

Scale: ~1000 μm across the middle Dasycladacean.

Dasycladacean Green Algae (4x) ~ 1000 um

Ooids

Few different cortices can be observed in the image.

Centered Ooid at 10x (~420 um across)

Pitted Peloids

Can be classify as a lump of pitted peloids as opposed to grapestones. Scale: ~1200 μm across.

Pitted Peloids XPL at 10x (~1200 um)

Pisolite

Pisolite is a rock made of Pisoids

Prismatic Microstructures

Homogeneous Prismatic

Homogeneous Prismatic microstructure identifier; weeping extinction across the shell fragment. Scale: ~1300 μm across.

Molluscs; Bivalve at 4x (~1300 um across)

Scleractinian coral

Scleractinian Coral (4x) ~ 3000 um across

Sponge Spicules

Width the center (*) Sponge Spicule: ~250 μm

Stromatolites

Bulbus Stromatolites

Non-skeletal Constituents-Carbonate Petrology

Coated grains

Ooids
– Size: 0.25 mm to 2 mm diameter
– Identification: clear nucleus or site of the nucleus. Several (not always) concentric cortical layers. Cortical layers are continuous (no overlapping). Generally larger than peloids but much smaller than oncoids. When the stage is rotated under XPL, they will undergo pseudo-uniaxial cross type extinction.
– Microfabric: tangential, radial and/or random crystals in each cortical layer.
– Rocks that predominantly made of ooids known as oolite.

Pisoids
– Size: > 2 mm to < 10 mm diameter - Identification: very similar (or rather same except the size) to ooids. Can the nucleus be observed???? - Rocks that predominantly made of pisoids are known as pisolite.

Note: Oooids and Pisoids both have nucleus. However, often pisoids nucleuses are harder to identify. Each layer of these two types of grains are known as cortical layers the boundaries of cortical layers are known as cortex.

Oncoids
– Size: > 2 mm; the outline of oncoids usually can be observed on thin sections with naked eye. Larger than peloids and ooids.
– Formed as a result of building layers on biological or lithological clastic nucleus.
– Found in all forms of water conditions (fresh/brackish/marine).
– Identification: irregular in shape with cortical layers of variable thickness. Overlapping discontinuous and irregular laminae. Alternating organic (dark) and microspar (light) layers. Overall shape can be described as rather a deformed circular one (asymmetric) as opposed to perfect circular patterns on ooids/pisoids.

Others

Pellets
– Size: 0.03 mm to 0.30 mm
– Identification: lacks internal structures but uniform shape and size.

Peloids
– Size: ~ 30 mm to 100 um; most commonly < 200 um - Modern peloids can be found in low energy environments such as shallow tidal and subtidal platforms. - Identification: lacks morphological structure (not always). In a sample, they are often widely varies in size and shape. - Rocks that predominantly made of peloids are peloidal packstones or wackestones. - Can be described as degraded pellets that have been moved causing mud rip-ups/broken tissues.

Aggregates

There are no universally accepted rules in the Geological community on how to separate grapestones from lumps. In fact, a grains aggregate can be classified as a grapestone by one petrologist and may be successfully disputed by another. However, generally grapestones are tightly packed together or touch each other. They often have the shape of the grains in their outer boundary. Lumps are generally full of “matrix” hence they lacks contact between the grains.

Clasts

Lithoclasts is a general term used to describe sedimentary clasts of pre-existing consolidated carbonate rock types (intra- or extraclasts). Intraclasts are fragmented lithofied or partially lithofied carbonate fragments. Extraclasts are fragmented carbonates particles.

Skeletal Microstructures-Carbonate Petrology

Homogeneous Prismatic

– Size: 1 um wide and 5 – 20 um long.
– Use high power magnification in petrographic microscope.
– Identification: sweeping extinction. Microgranular: Dark coloured in ppl. Opaque in reflected light. Porcelaneous: dark or amber in ppl. Shiny white in xpl. Hyaline: speckled colours in xpl.
– Primary characteristic of foraminifera, molluscs (bivalve).
– Secondary characteristic of trilobites, ostracodes.

Normal Prismatic

– Size: ~ 10s to ~ 100s um long
– Use high power magnification in petrographic microscope, but may also be visible under low power.
– Identification: rod-like crystals that undergo unit extinction.
– Primary characteristic of brachiopods outer layer.
– Secondary characteristic of molluscs.

Complex Prismatic

– Size: < 100 um width. - Identification: rod-like structures with radiating fiber-like crystals. These radiations will make a V-shaped pattern within the rod. When the stage is rotated, the extinction will move across each rod-like structure (move across to one end and move back to the other end). - Primary characteristic of molluscs ONLY.

Composite Prismatic

– Size: ~250 um(?) in width.
– Identification: elongated/rectangular structures with fan-like radiations pointing in the elongation direction.
– Primary characteristic of molluscs ONLY.

Foliated and Nacrerous

– Foliated term is used on calcitic shells while nacreous term is used on aragonitic shells.
– Size: 0.1 – 0.5 um thickness, 4 – 20 um long and 2 – 4 um width
– Foliated identification: long thin lamination-like structures that are near parallel or parallel to the shell surface or short irregular fiber-like inclined structures.
– Primary characteristic of brachiopods.
– Secondary characteristic of molluscs, bryozoans, worm tubes.

– Nacrerous identification: almost similar to foliated but more polygonal and aragonites may be separated by a films of conchiolin
– Primary characteristic of molluscs ONLY.

Single-crystal

– Size: relatively larger than other microstructures.
– Identification: since these fragments are made of a single (usually calcite) crystal, they will undergo unit extinction. Typical calcite twining may observe (~ 60/120 type crosses) on the surface. In some samples it may look like a calcite crystal. In others you may observe some morphological features such as an outline of an echinoderm.
– Primary characteristic of echinoderms.
– Secondary characteristic of molluscs, fenestrate bryozoans, sponge spicules.

Crossed-lamellar

– Size: few mm or 100 um
– These are wedges made of 1st and 2nd order lamellae
– Identification: ridge-like elongated structures. Not to be confused with complex prismatic because they could look very very similar.
– Primary characteristic of molluscs (bivalve) ONLY.

Fascicular fibrous structure

– Size: fibers are very thin, but often found in large quantities (may be observed under low power).
– Identification: elongated fiber-like crystals that stacked around coalesced (side-by-side). They may have a bit of circular radiation pattern. Transverse sections will have fan-like growth lines.
– Primary characteristic of corals; Tabulata corals (may also be normal to tabulae). Rugose corals. Scleractinia: (trabecular structures in septal walls, crystals radiating out and upwards)

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)

Effective Study Habits

When one group of students receive their degrees, a new group is always there to take their place. Each one of us have our our way of successfully demonstrating the acquired skills and knowledge on exams. But in some disciplines such as Engineering and Geology, just a one method will not always work. While I am not the modeled student you should look up to, here are few things that worked for me from first to fourth year at the University of Calgary.

Time management

You do not have to make a timetable to use your time effectively. I personally use arbitrary deadlines as opposed to having a timetable. Some of my friends in Geology have allocated time slots for each subject to make sure no subject is left behind till the end of the semester. For me it will only result in rushing through important concepts just to stick to the time slot. This is not very effective for subjects like Geology. Geology is a mixture of Science, Arts and History. It also involves practical knowledge; lab exams. I recommend all first year students to spend more time on your weak points rather than dividing time equally for each subject. For example, if you are struggling with Mineralogy, you should put more time and effect to that subject over others.

Type of subject matters

Do not expect to study Hydrology or Physics the same way you would study for Paleobiology or Geologic History. The amount of memorization without much of a logical process in subjects like Paleobiology is much higher than that of Hydrology. Hydrology involves understanding of mathematical equations derived from empirical Geologic data. Paleobiology involves studying the history of life derived from the empirical Stratigraphic data. Very few students will be able to study both subject using a similar method. I found it is important to understand logical concepts in any subject that involves Mathematics and Physics than just to memorize formulas. While Professors/Instructors may disagree with my view on how each subject is differ from each other, let’s face it, we all know this is one of the reasons why some students are good at memorizing and others at mathematics!

If you are student in Geology or similar multidisciplinary program, I highly recommended improving both memorizing skills and logical skills.

Tips for studying

If the subject require little of no logical thinking then,

  • Learn the concepts right at the first time early on.
  • Repeat what you have learned by transforming your knowledge into others forms; writings down, anticipating questions for exam, etc.
  • Try to teach someone (your friend or even your dog…) by acting like a teacher. Ask your friends to ask you questions to see if you remember.
  • Test what you have memorized few week earlier by trying to “pull-out” your memory often.
  • If you found that you have learned something wrong, correct immediately. Sometimes I have incorrectly answered questions because I memorized the incorrect information.
  • It is scientifically proven that studying in the early morning improves memory. In fact, this technique has been used by Buddhist monks in India for thousands of years. May be you should try weaking up at 4:00 AM even on Saturday instead of studying all night.

If the subject require logical thinking and memorizing (about equally) then,

  • Addition to what is posted above, relate theoretical concepts to real world issues. For example, hydrological conductivity is not only can be summed up with few equations, but also can be relate to Calgary’s groundwater supply.
  • Never memorize a logical concept like Darcy’s Law. Instead find practice problems and use the formula over and over until you understand how to use it. Memorizing a formula will not only hurt you on exams, but also will effect the quality of work you will do as a Professional Geologist or Engineer.
  • If something makes no sense (logically), probably something is wrong. In other words, if you find things you studies makes no sense, may be it is time to stop studying and time to figure out what went wrong. In multi-steps questions, spend more time getting the first part right before you move on to the next step.
  • During studying and exams, make sure your tools good. What I mean is that if you need your calculator in Degree Mode, make sure it is in degree mode before attempting a question.
  • In subjects like Geophysics and Hydrology, there are often connections between the mathematics side and observational science. Try to find relationships between them to understand the concepts better.
  • Some problems may not have right or wrong answers but rather a logical answer. So make sure you can manipulate what your instructor taught you in any form in any way. Do not just go for questions similar to what you have learned in class. I can take a simple question and without changing any values, I can make it into a complicated one. If I can do that, so can the Instructors.
  • Never assume that all questions have to “make sense”. Do not be confused between the “logics” and “making sense”. They are not always synonymous to each other.

In addition… engineering and Geoscience students should always try to relate their lecture materials to lab materials. It will help bridge the gap between the theoretical knowledge and practical applications.

Some students have found other ways to improving their skills like, mediation/yoga, “puppy theory”, sports, pron, etc. Whatever you do I recommended starting early and not so close to the exams. Changing your habits close to exams could actually hurt you rather than helping in improving your grades.