Carbonate Petrography

Carbonate petrography is the study of limestones, dolomites and associated deposits under optical or electron microscopes greatly enhances field studies or core observations and can provide a frame of reference for geochemical studies.

25 strangest Geologic Formations on Earth

The strangest formations on Earth.

What causes Earthquake?

Of these various reasons, faulting related to plate movements is by far the most significant. In other words, most earthquakes are due to slip on faults.

The Geologic Column

As stated earlier, no one locality on Earth provides a complete record of our planet’s history, because stratigraphic columns can contain unconformities. But by correlating rocks from locality to locality at millions of places around the world, geologists have pieced together a composite stratigraphic column, called the geologic column, that represents the entirety of Earth history.

Folds and Foliations

Geometry of Folds Imagine a carpet lying flat on the floor. Push on one end of the carpet, and it will wrinkle or contort into a series of wavelike curves. Stresses developed during mountain building can similarly warp or bend bedding and foliation (or other planar features) in rock. The result a curve in the shape of a rock layer is called a fold.

Showing posts with label crystal. Show all posts
Showing posts with label crystal. Show all posts

30+ Thin Section Photos That Will Develop Your Interest in Petrography

 “thin section” of rock is a sample that is mounted to a microscope slide and cut so thin that you can see light through it. The process of creating a thin section is a blend of artistry, technology and science.  
The art of preparing thin sections has been critical to understanding the core samples that scientists are observing. Thin section samples allow scientists to observe minerals in rocks, their crystal structure and texture at a microscopic level.

Want to revise how do geologists study rock? Follow this link to see our blog on "Studying Rock".

In this blog, we're taking you into the journey of thin section photos that were captured and given by students and young professionals from Finland, Ireland, Denmark, Czech Republic and Plymouth (UK). 


Again our purpose is to encourage students and professionals' research by promoting "learning and scope" of Geology through our blogs. Help us to help others in learning and understanding geology. See this link that how you can contribute to Learning Geology.


Note: We are using following thin section photos by having permission from their owners. If 
you like to use these photos, leave us a message or email us here.

1. A beautiful heart shaped hornblende in XPL (cross polarized light) view.It is a thin section of basalt with some secondary mineralization in the vesicles. Plagioclase is present in the form of black and white matrix and large phenocryst (with some zoning). Alignment of plagioclase grains is indicative of the "flow" of magma.

Photo Credits: Astaley

2. Thin Section of a Biotite and Muscovite, XPL view 

Photo Courtesy: Laura

3. Thin Section of a Plagioclase (orthoclase) and Pyroxene, XPL 

Photo Courtesy: Laura


4. Eclogite in Thin Section, XPL

Photo Courtesy: Laura

 5. Cummulate Rock with Pyroxene and plagioclase, XPL

Photo Courtesy: Laura

6. Blueschist, XPL

Photo Courtesy: Laura

7. Agglomerate in a Thin Section, XPL view

  
     Agglomerates are pyroclastic igneous rocks that consist almost wholly of angular or rounded lava fragments of varying size and shape. Fragments are usually poorly sorted in a tuffaceous matrix, or appear in lithified volcanic ash. (Britannica.com)

Photo Courtesy: Laura
8. Thin Section of a Pigeonite and Olivine, XPL

Photo Courtesy: Laura

9. Olivine phenocryst in Basaltic Lapilli, XPL

Photo Courtesy: Laura

10. Thin Section of a Gabbro, XPL

Showing minerals; Pyroxene and Olivine, plagioclase and others. Learn more about Gabbro here.


Photo Courtesy: Laura

 11. Another beautiful thin section of a Gabbro, XPL

Photo Courtesy: Laura
12. Thin Section of a Greenschist, XPL

Photo Courtesy: Laura

13. Thin Section showing intrusion of rocks from magma chamber into country rocks, XPL

Photo Courtesy: Jack Lewis Donnelly


14. Thin Section of a Sillimanite - a mineral found in rocks formed by the metamorphism of a mudstone. (XPL view)

Photo Courtesy: Jack Lewis Donnelly
15. Microgeode in ultrabasic vulcanite (
a rare copper telluride mineral), 30 µm thin section, PPL and XPL 

Photo Courtesy: Petr Hyks

Photo Courtesy: Petr Hyks
                                                              See original photo here

16. Muscovite & biotite (30 µm thin section, PPL and XPL)


Photo Courtesy: Petr Hyks


Photo Courtesy: Petr Hyks
Same photo in XPL view. See original photo here

17. Quartz and epidote (30 µm thin section, PPL and XPL)

Photo Courtesy: Petr Hyks

Photo Courtesy: Petr Hyks
                                                        See original here.

18. Olivine (30 µm thin section, PPL and XPL)

Photo Courtesy: Petr Hyks
See this photo here on Petr Hyks' website

19. Zircons in biotite (30 µm thin section, PPL views, showing extinction)
                                                                                         

                                                                                  
                          Photo Courtesy: Petr Hyks
                              See this photo here on Petr's website






20. Zircon in biotite (30 µm thin section, XPL)

Photo Courtesy: Petr Hyks
See this photo here on Petr's website

21. Kyanite surrounded by muscovite (30 µm thin section, PPL and XPL)

Photo Courtesy: Petr Hyks
See these photos on Petr's page here and here

22. Zircon crystal in chloritized biotite (30 µm thin section, PPL and XPL)



     
Photo Courtesy: Petr Hyks
See these photos on Petr's page  here and here
Petr Hyks is 21 year old geology student from Masaryk University in Brno (Czech Republic). He has uploaded 5000+ photos about geology, astronomy and meteorology on his Flickr page. Follow this link to visit his website. Thank you Petr for contributing to Learning Geology and helping others to learn geology through your thin section photos. 🙂 Now following 10 thin section photos are from a geology student of University of Helsinki, Finland.

23. Thin Section of Olivine Diabase in XPL and PPL view.

Photo Courtesy: GeoAmethyst

24. Thin Section of Basalt in XPL view
        Having minerals: Olivine (in center) plagioclase, pyroxene and other accessory minerals

Photo Courtesy: GeoAmethyst

25. Thin Section of a Trachyte, XPL view

    Trachyte is an igneous volcanic rock with aphanitic to porphyritic texture. It is volcanic equivalent of Syenite. Major or essential minerals are alkali feldspar with less amount of plagioclase, quartz or feldspathiod. 

Photo Courtesy: GeoAmethyst

26. Thin Section of a Harzburgite, XPL view
      Harzburgite is an ultramafic igneous rock. It chiefly contains plagioclase (under 10%) , olivine, orthopyroxene (enstatite), clinopyroxene (diopside) and biotite. There could be a small amount of talc, carbonate, tremolite, cummingtonite, chlorite, serpentine and titanite.

Photo Courtesy: GeoAmethyst

27.  Another thin section of Harzburgite, XPL view

Photo Courtesy: GeoAmethyst

28.  Thin Section of Pyroxenite (an ultramafic igneous rock), XPL view

Photo Courtesy: GeoAmethyst

29. Thin Section of Trachyte showing Sandine mineral in center, XPL view

Photo Courtesy: GeoAmethyst
 
30.  Thin Section of Andesite, XPL view
       It is an extrusive igneous, of intermediate composition, with aphanitic to porphyritic texture.              Here this thin section is showing chiefly hornblende and plagioclase.

Photo Courtesy: GeoAmethyst

31. Thin Section of Alkali Basalt (silica undersaturated) in XPL view.

Photo Courtesy: GeoAmethyst


32. Thin Section showing small clinopyroxene grains within orthopyroxene

Photo Courtesy: GeoAmethyst

Like this article? Leave a comment down or send us your valuable suggestion or feedback here  to help us in improving this article.
Useful Websites: 

1. Polarized light Microscopy (Image Gallery)
2. How to make a thin section
3. Petrographic thin section preparation
4. Guide to Thin Section Microscopy
5. Index of Minerals in Thin Section
6.
Optical Petrography website by an Italian Geologist

7. Carbonate Thin Section Images and 
Exercises

Igneous Origin of Diamonds

Diamonds are a rare occurrence on the surface of the planet because it takes extremely hot and high pressure conditions to create them. Physical and chemical conditions where diamonds form only exist in the mantle, nearly 70 miles down or more. In that environment in the upper mantle, diamonds may be a common mineral! It takes incredible events, nothing that has ever been witnessed in historic times, to bring diamonds to the surface.
Kimberlite-Diamond-mantle-rock

Diamond deposits around the world (that have any economic significance) are associated with volcanic features called 
diatremes.  A diatreme is a long, vertical pipe formed when gas-filled magma forces its way through the crust to explosively erupt at the surface. Kimberlite a special kind of intrusive igneous rock associated with some diatremes that sometimes contain diamonds, typical coarse grained an bluish in color.    

Diamond-bearing kimberlite pipes are diatremes that originate in the mantle.
   
Diamonds are xenoliths carried up from deep sources in the mantle, and often occur in association with other gem minerals including garnet, spinel and diopside. Most "economically significant" diamond deposits occur in ancient rocks (Precambrian age), but have been discovered on all continents. Because diamonds are so hard, they survive torturously-long histories, recycled through sedimentary and metamorphic environments without being destroyed. As a result they have been found almost everywhere as very rare, isolated discoveries. Diamonds of microscopic size have been discovered in meteorites and asteroid impact sites, and some metamorphic rocks. They are most extensively mined from kimberlite pipes or from alluvial gravels derived downstream from diamond source areas. It should be noted that most diamonds are not of gem quality, but those that are not are used for industrial purposes.

Credits to Phil Stoffer at geologycafe.com
Text and figures are used with permission.

Crytalline structure

What is a crystal?

Some characteristics of crystals.
The word crystal brings to mind sparkling chandeliers, elegant wine goblets, and shiny jewels. But, as is the case with the word mineral, geologists have a more precise definition. A crystal is a single, continuous (that is, uninterrupted) piece of a crystalline solid, typically bounded by flat surfaces, called crystal faces, that grow naturally as the mineral forms. The word comes from the Greek krystallos, meaning ice. Many crystals have beautiful shapes that look like they belong in the pages of a geometry book. The angle between two adjacent crystal faces of one specimen is identical to the angle between the corresponding faces of another specimen. For example, a perfectly formed quartz crystal looks like an obelisk (a and b in figure above); the angle between the faces of the columnar part of a quartz crystal is always exactly 120°. This rule, discovered by one of the first geologists, Nicolas Steno (1638– 1686) of Denmark, holds regardless of whether the whole crystal is big or small and regardless of whether all of the faces are the same size. Crystals come in a great variety of shapes, including cubes, trapezoids, pyramids, octahedrons, hexagonal columns, blades, needles, columns, and obelisks (c in figure above).
Because crystals have a regular geometric form, people have always considered them to be special, perhaps even a source of magical powers. For example, shamans of some cultures relied on talismans or amulets made of crystals, which supposedly brought power to their wearer or warded off evil spirits. Scientists have concluded, however, that crystals have no effect on health or mood. For millennia, crystals have inspired awe because of the way they sparkle, but such behavior is simply a consequence of how crystal structures interact with light.

Looking inside crystals

Patterns and symmetry in minerals.
What makes crystals have regular geometric forms? This problem was the focus of study for centuries. An answer finally came from the work of a German physicist, Max von Laue, in 1912. He showed that an X-ray beam passing through a crystal breaks up into many tiny beams to create a pattern of dots on a screen (a in figure above). Physicists refer to this phenomenon as diffraction; it occurs when waves interact with regularly spaced objects whose spacing is close to the wavelength of the waves you can see diffraction of ocean waves when they pass through gaps in a seawall. Von Laue concluded that, for a crystal to cause diffraction, atoms within it must be regularly spaced and the spacing must be comparable to the wavelength of X-rays. Eventually, Von Laue and others learned how to use X-ray diffraction patterns as a basis for defining the specific arrangement of atoms in crystals. This arrangement defines the crystal structure of a mineral.
If you’ve ever examined wallpaper, you’ve seen an example of a pattern. Crystal structures contain one of nature’s most spectacular examples of such a pattern (b in figure above). In crystals, the pattern is defined by the regular spacing of atoms and, if the crystal contains more than one element, by the regular alternation of atoms (c in figure above). (Mineralogists refer to a 3-D geometry of points representing this pattern as a lattice.) The pattern of atoms in a crystal may control the shape of a crystal. For example, if atoms in a crystal pack into the shape of a cube, the crystal may have faces that intersect at 90° angles galena  (PbS) and halite (NaCl) have such a cubic shape. Because of the pattern of atoms in a crystal structure, the structure has symmetry, meaning that the shape of one part of the structure is the mirror image of the shape of a neighboring part. For example, if you were to cut a halite crystal or a water crystal (snowflake) in half, and place the half against a mirror, it would look whole again (d in figure above).
The nature of crystalline structure in minerals. The arrangement of atoms can be portrayed by a ball and stick model, or by a packed ball model.
To illustrate crystal structures, we look at a few examples. Halite (rock salt) consists of oppositely charged ions that stick together because opposite charges attract. In halite, six chloride (Cl–) ions surround each sodium (Na+) ion, producing an overall arrangement of atoms that defines the shape of a cube (a and b in figure above). Diamond, by contrast, is a mineral made entirely of carbon. In diamond, each atom bonds to four neighbors arranged in the form of a tetrahedron; some naturally formed diamond crystals have the shape of a double tetrahedron (c in figure above). Graphite, another mineral composed entirely of carbon, behaves very differently from diamond. In contrast to diamond, graphite is so soft that we use it as the “lead” in a pencil; when a pencil moves across paper, tiny flakes of graphite peel off the pencil point and adhere to the paper.  This behaviour occurs because the carbon atoms in graphite  are not arranged in tetrahedra, but rather occur in sheets (d in figure above). The sheets are bonded to each other by weak bonds and thus can separate from each other easily. Of note, two different minerals (such as diamond and graphite) that have the same composition but different crystal structures are polymorphs.

The formation and destruction of minerals

New mineral crystals can form in five ways. First, they can form by the solidification of a melt, meaning the freezing of a liquid to form a solid. For example, ice crystals, a type of mineral, are made by solidifying water, and many different minerals form by solidifying molten rock. Second, they can form by precipitation from a solution, meaning that atoms, molecules, or ions dissolved in water bond together and separate out of the water. Salt crystals, for example, precipitate when you evaporate salt water. Third, they can form by solid-state diffusion, the movement of atoms or ions through a solid to arrange into a new crystal structure, a process that takes place very slowly. For example, garnets  grow by diffusion in solid rock. Fourth, minerals can form at interfaces between the physical and biological components of the Earth System by a process called biomineralization. This occurs when living organisms cause minerals to precipitate either within or on their bodies, or immediately adjacent to their bodies. For example, clams and other shelled organisms extract ions from water to produce mineral shells. Fifth, minerals can precipitate directly from a gas. This process typically occurs around volcanic vents or around geysers, for at such locations volcanic gases or steam enter the atmosphere and cool abruptly. Some of the bright yellow sulphur deposits found in volcanic regions form in this way.
The growth of crystals.
The first step in forming a crystal is the chance formation of a seed, or an extremely small crystal (a in figure above). Once the seed exists, other atoms in the surrounding material attach themselves to the face of the seed. As the crystal grows, crystal faces move outward but maintain the same orientation (b in figure above). The youngest part of the crystal is at its outer edge.
In the case of crystals formed by the solidification of a melt, atoms begin to attach to the seed when the melt becomes so cool that thermal vibrations can no longer break apart the attraction between the seed and the atoms in the melt. Crystals formed by precipitation from a solution develop when the solution becomes saturated, meaning the number of dissolved ions per unit volume of solution becomes so great that they can get close enough to each other to bond together.
As crystals grow, they develop their particular crystal shape, based on the geometry of their internal structure. The shape is defined by the relative dimensions of the crystal (needle- like, sheet-like, etc.) and the angles between crystal faces. Typically, the growth of minerals is restricted in one or more directions, because existing crystals act as obstacles. In such cases, minerals grow to fill the space that is available, and their shape is controlled by the shape of their surroundings. Minerals without well-formed crystal faces are anhedral grains (c in figure above). If a mineral’s growth is unimpeded so that it displays well-formed crystal faces, then it is a euhedral crystal. The surface crystals of a geode, a mineral-lined cavity in rock, may be euhedral (d in figure above).
A mineral can be destroyed by melting, dissolving, or some other chemical reaction. Melting involves heating a mineral to a temperature at which thermal vibration of the atoms or ions in the lattice break the chemical bonds holding them to the lattice. The atoms or ions then separate, either individually or in small groups, to move around again freely. Dissolution occurs when you immerse a mineral in a solvent, such as water. Atoms or ions then separate from the crystal face and are surrounded by solvent molecules. Chemical reactions can destroy a mineral when it comes in contact with reactive materials. For example, iron-bearing minerals react with air and water to form rust. The action of microbes in the environment can also destroy minerals. In effect, some microbes can “eat” certain minerals; the microbes use the energy stored in the chemical bonds that hold the atoms of the mineral together as their source of energy for metabolism.
Credits: Stephen Marshak (Essentials of Geology)