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 mineral. Show all posts
Showing posts with label mineral. 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

Copper

What is Copper?

Copper is a chemical element with symbol Cu (from Latin: cuprum) and atomic number 29. It is a soft, malleable and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a reddish-orange colour. It is used as a conductor of heat and electricity, as a building material and as a constituent of various metal alloys, such as sterling silver used in jewellery, cupronickel used to make marine hardware and coins and constantan used in strain gauges and thermocouples for temperature measurement.
Copper is found as a pure metal in nature, and this was the first source of the metal to be used by humans, c. 8000 BC. It was the first metal to be smelted from its ore, c. 5000 BC, the first metal to be cast into a shape in a mold, c. 4000 BC and the first metal to be purposefully alloyed with another metal, tin, to create bronze, c. 3,500 BC.
In the Roman era, copper was principally mined on Cyprus, the origin of the name of the metal, from aes сyprium (metal of Cyprus), later corrupted to сuprum, from which the words copper (English), cuivre (French), Koper (Dutch) and Kupfer (German) are all derived. The commonly encountered compounds are copper(II) salts, which often impart blue or green colours to such minerals as azurite, malachite, and turquoise, and have been used widely and historically as pigments. Architectural structures built with copper (usually roofing elements) corrode to give green verdigris (or patina). Decorative art prominently features copper, both in the elemental metal and in compounds as pigments. Copper compounds are also used as bacteriostatic agents, fungicides, and wood preservatives.
Copper is essential to all living organisms as a trace dietary mineral because it is a key constituent of the respiratory enzyme complex cytochrome c oxidase. In molluscs and crustaceans copper is a constituent of the blood pigment hemocyanin, replaced by the iron-complex haemoglobin in fish and other vertebrates. In humans, copper is found mainly in the liver, muscle, and bone. The adult body contains between 1.4 and 2.1 mg of copper per kilogram of body weight. Hence a healthy human weighing 60 kilogram contains approximately 0.1 g of copper. However, this small amount is essential to the overall human well-being.

Characteristics

Physical

Copper, silver and gold are in group 11 of the periodic table, and they share certain attributes: they have one s-orbital electron on top of a filled d-electron shell and are characterised by high ductility and electrical and thermal conductivity. The filled d-shells in these elements contribute little to interatomic interactions, which are dominated by the s-electrons through metallic bonds. Unlike metals with incomplete d-shells, metallic bonds in copper are lacking a covalent character and are relatively weak. This observation explains the low hardness and high ductility of single crystals of copper. At the macroscopic scale, introduction of extended defects to the crystal lattice, such as grain boundaries, hinders flow of the material under applied stress, thereby increasing its hardness. For this reason, copper is usually supplied in a fine-grained polycrystalline form, which has greater strength than monocrystalline forms.
The softness of copper partly explains its high electrical conductivity (59.6×106 S/m) and high thermal conductivity, the second highest (second only to silver) among pure metals at room temperature. This is because the resistivity to electron transport in metals at room temperature originates primarily from scattering of electrons on thermal vibrations of the lattice, which are relatively weak in a soft metal. The maximum permissible current density of copper in open air is approximately 3.1×106 A/m2 of cross-sectional area, above which it begins to heat excessively.
Copper is one of four metallic elements with a natural colour other than gray or silver, the others being caesium (yellow), gold (yellow), and osmium (bluish). Pure copper is orange-red and acquires a reddish tarnish when exposed to air. The characteristic colour of copper results from the electronic transitions between the filled 3d and half-empty 4s atomic shells – the energy difference between these shells corresponds to orange light. The same mechanism causes the yellow colour of gold and caesium.
As with other metals, if copper is put in contact with another metal, galvanic corrosion will occur.

Chemical

Copper does not react with water but it does slowly react with atmospheric oxygen to form a layer of brown-black copper oxide which, unlike the rust that forms on iron in moist air, protects the underlying metal from further corrosion (passivation). A green layer of verdigris (copper carbonate) can often be seen on old copper structures, such as the roofing of many older buildings and the Statue of Liberty. Copper tarnishes when exposed to some sulphur compounds, with which it reacts to form various copper sulphides.

Isotopes

There are 29 isotopes of copper. 63Cu and 65Cu are stable, with 63Cu comprising approximately 69% of naturally occurring copper; both have a spin of  3⁄2. The other isotopes are radioactive, with the most stable being 67Cu with a half-life of 61.83 hours.Seven metastable isotopes have been characterised; 68Cu is the longest-lived with a half-life of 3.8 minutes. Isotopes with a mass number above 64 decay by β−, whereas those with a mass number below 64 decay by β+. 64Cu, which has a half-life of 12.7 hours, decays both ways.
62Cu and 64Cu have significant applications. 62Cu is used in 62Cu-PTSM as a radioactive tracer for positron emission tomography.

Occurrence

Copper is produced in massive stars and is present in the Earth's crust in a proportion of about 50 parts per million (ppm). It occurs as native copper, in the copper sulphides chalcopyrite and chalcocite, in the copper carbonates azurite and malachite, and in the copper(I) oxide mineral cuprite. The largest mass of elemental copper discovered weighed 420 tonnes and was found in 1857 on the Keweenaw Peninsula in Michigan, US. Native copper is a polycrystal, with the largest single crystal ever described measuring 4.4×3.2×3.2 cm.

Physical Properties

Chemical FormulaCu
ColourMetallic, Red, Orange, Brown
Hardness2.5 - 3
Crystal SystemIsometric
SG8.9
TransparencyOpaque
Double RefractionNone
LusterMetallic
CleavageNone
Mineral ClassCopper

Mineral Classification

Mineral Classification 

The 4,000 known minerals can be separated into a small number of groups, or mineral classes. You may think, “Why bother?” Classification schemes are useful because they help organize information and streamline discussion. Biologists, for example, classify animals into groups based on how they feed their young and on the architecture of their skeletons, and botanists classify plants according to the way they reproduce and by the shape of their leaves. In the case of minerals, a good means of classification eluded researchers until it became possible to determine the chemical makeup of minerals. A Swedish chemist, Baron Jöns Jacob Berzelius (1779–1848), analyzed minerals and noted chemical similarities among many of them. Berzelius, along with his students, established that most minerals can be classified by specifying the principal anion (negative ion) or anionic group (negative molecule) within the mineral. We now take a look at principal mineral classes, focusing especially on silicates, the class that constitutes most of the rock in the Earth.

How Can You Tell One Mineral From Another?

How Can You Tell One Mineral From Another? 

Amateur and professional mineralogists get a kick out of recognizing minerals. They might hover around a display case in a museum and name specimens without bothering to look at the labels. How do they do it? The trick lies in learning to recognize the basic physical properties (visual and material characteristics) that distinguish one mineral from another. Some physical properties, such as shape and colour, can be seen from a distance. Others, such as hardness and magnetization, can be determined only by handling the specimen or by performing an identification test on it. Identification tests include scratching the mineral by another object, placing it near a magnet, weighing it, tasting it, or placing a drop of acid on it. Let’s examine some of the physical properties most commonly used in basic mineral identification.

Crystals and Their Structure

Crystals and Their 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  (figure above a, b); 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 (figure above c).

What Is a Mineral?

What Is a Mineral? 

To a geologist, a mineral is a naturally occurring solid, formed by geologic processes, that has a crystalline structure and a definable chemical composition. Almost all minerals are inorganic. Let’s pull apart this mouthful of a definition and examine its meaning in detail.
  • Naturally occurring: True minerals are formed in nature, not in factories. We need to emphasize this point because in recent decades, industrial chemists have learned how to synthesize materials that have characteristics virtually identical to those of real minerals. These materials are not minerals in a geologic sense, though they are referred to in the  commercial world as synthetic minerals. 
  • Formed by geologic processes: Traditionally, this phrase implied processes, such as solidification of molten rock or direct precipitation from a water solution, that did not involve living organisms. Increasingly, however, geologists recognize that life is an integral part of the Earth System. So, some  geologists consider solid, crystalline materials produced by organisms to be minerals too. To avoid confusion, the term “biogenic mineral” may be used when discussing such  materials. 
  • Solid: A solid is a state of matter that can maintain its shape indefinitely, and thus will not conform to the shape of its container. Liquids (such as oil or water) and gases (such as air) are not minerals (Some Basic Concepts from Chemistry). 
  • Crystalline structure: The atoms that make up a mineral are not distributed randomly and cannot move around easily. Rather, they are fixed in a specific, orderly pattern. A material in which atoms are fixed in an orderly pattern is called a crystalline solid. 
  • Definable chemical composition: This simply means that it is possible to write a chemical formula for a mineral (Some Basic Concepts from Chemistry). Some minerals contain only one element, but most are compounds of two or more elements. For example, diamond and graphite have the formula C, because they consist entirely of carbon. Quartz has the formula SiO2 it contains the elements silicon and oxygen in the proportion of one silicon atom for every two oxygen atoms. Calcite has the formula CaCO3, meaning it consists of a calcium (Ca ) ion and a carbonate (CO3 ) ion. Some formulas are more complicated: for example, the formula for biotite is K(Mg,Fe)3(AlSi3O10)(OH)2. 
  • Inorganic: Organic chemicals are molecules containing some carbon-hydrogen bonds. Sugar (C12H22O11), for example, 
  • is an organic chemical. Almost all minerals are inorganic. Thus, sugar and protein are not minerals. But, we have to add the qualifier “almost all” because mineralogists do consider about 30 organic substances formed by “the action of geologic processes on organic materials” to be minerals. Examples include the crystals that grow in ancient deposits of bat guano.

Mineral Identification

How to identify minerals?


Amateur and professional mineralogists get a kick out of identify minerals. They might hover around a display case in a museum and name specimens without bothering to look at the labels. How do they do it? The trick lies in learning to recognize the basic physical properties (visual and material characteristics) that distinguish one mineral from another. Some physical properties, such as shape and colour, can be seen from a distance. Others, such as hardness and magnetization, can be determined only by handling the specimen or by performing an identification test on it. Identification tests include scratching the mineral by another object, placing it near a magnet, weighing it, tasting it, or placing a drop of acid on it. Let’s examine some of the physical properties most commonly used in basic mineral identification. 
  1. Colour: Colour results from the way a mineral interacts with light. Sunlight contains the whole spectrum of colours; each colour has a different wavelength. A mineral absorbs certain wavelengths, so the colour you see when looking at a specimen represents the wavelengths the mineral does not absorb.  Certain minerals always have the same colour, but many display a range of colours. Colour variations in a mineral are due to the presence of impurities. For example, trace amounts of iron may give quartz a reddish colour. 
  2. Streak: The streak of a mineral refers to the colour of a powder produced by pulverizing the mineral. You can obtain a streak by scraping the mineral against an unglazed ceramic plate. The colour of a mineral powder tends to be less variable than the colour of a whole crystal, and thus  provides a fairly reliable clue to a mineral’s identity. Calcite, for example, always yields a white streak even though pieces of calcite may be white, pink, or clear. 
  3. Luster: Luster refers to the way a mineral surface scatters light. Geoscientists describe luster by comparing the appearance of the mineral with the appearance of a familiar substance. For example, minerals that look like metal have a metallic luster, whereas those that do not have a non-metallic luster. Terms used for types of non-metallic luster include silky, glassy, satiny, resinous, pearly, or earthy. 
  4. Hardness: Hardness is a measure of the relative ability of a mineral to resist scratching, and it therefore represents the resistance of bonds in the crystal structure to being broken. The atoms or ions in crystals of a hard mineral are more strongly bonded than those in a soft mineral. Hard minerals can scratch soft minerals, but soft minerals cannot scratch hard ones. Diamond, the hardest mineral known, can scratch most anything, which is why it is used to cut glass. In the early 1800s, a mineralogist named Friedrich Mohs listed some minerals in sequence of relative hardness; a mineral with a hardness of 5 can scratch all minerals with a hardness of 5 or less. This list, the Mohs hardness scale, helps in mineral identification. To make the scale easy to use, common items such as your fingernail, a penny, or a glass plate have been added. 
  5. Specific gravity: Specific gravity represents the density of a mineral, as represented by the ratio between the weight of a volume of the mineral and the weight of an equal volume of water at 4°C. For example, one cubic centimetre of quartz has a weight of 2.65 grams, whereas one cubic centimetre of water has a weight of 1.00 gram. Thus, the specific gravity of quartz is 2.65. In practice, you can develop a “feel” for specific gravity by hefting minerals in your hands. A piece of galena (lead ore) feels heavier than a similar-sized piece of quartz. 
  6. Crystal habit: The crystal habit of a mineral refers to the shape of a single crystal with well-formed crystal faces, or to the character of an aggregate of many well-formed crystals that grew together as a group. The habit depends on the internal arrangement of atoms in the crystal.  A description of habit generally includes adjectives that highlight the shape of the crystal. For example, crystals that are roughly the same length in all directions are called equant or blocky, those that are much longer in one dimension than in others are columnar or needle-like, those shaped like  sheets of paper are platy, and those shaped like knives are bladed. 
  7. Special properties: Some minerals have distinctive properties that readily distinguish them from other minerals. For example, calcite (CaCO3) reacts with dilute hydrochloric acid (HCl) to produce carbon dioxide (CO2) gas. Dolo mite (CaMg[CO3]2) also reacts with acid, but not as strongly. Graphite makes a gray mark on paper, magnetite attracts a magnet, halite tastes salty, and plagioclase has striations (thin parallel corrugations or stripes) on its surface.
  8. Fracture and cleavage: Different minerals fracture (break) in different ways, depending on the internal arrangement of atoms. If a mineral breaks to form distinct planar  surfaces that have a specific orientation in relation to the crystal structure, then we say that the mineral  has cleavage and we refer to each surface as a cleavage plane. Cleavage forms in directions where the bonds holding atoms together in the crystal are the weakest. Some minerals have one direction of cleavage. For example, mica has very weak bonds in one direction but strong bonds in the other two directions. Thus, it easily splits into parallel sheets; the surface of each sheet is a cleavage plane. Other minerals have two or three directions of cleavage that intersect at a specific angle. For example, halite has three sets of cleavage planes that intersect at right angles, so halite crystals break into little cubes. Materials that have no cleavage at all (because bonding is equally strong in all directions) break either by forming irregular fractures or by forming conchoidal fractures. Conchoidal fractures are smoothly curving, clamshell-shaped surfaces; they typically form in glass. Cleavage planes are sometimes hard to distinguish from crystal faces.
Credits: Stephen Marshak (Essentials of Geology)

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)

Minerals

What is a mineral?


To a geologist, a mineral is a naturally occurring solid, formed by geologic processes, that has a crystalline structure and a definable chemical composition. Almost all minerals are inorganic. Let’s pull apart this mouthful of a definition and examine its meaning in detail.
  • Naturally occurring: True minerals are formed in nature, not in factories. We need to emphasize this point because in recent decades, industrial chemists have learned how to synthesize materials that have characteristics virtually identical to those of real minerals. These materials are not minerals in a geologic sense, though they are referred to in the  commercial world as synthetic minerals.  
  • Formed by geologic processes: Traditionally, this phrase implied processes, such as solidification of molten rock or direct precipitation from a water solution, that did not involve living organisms. Increasingly, however, geologists recognize that life is an integral part of the Earth System. So, some  geologists consider solid, crystalline materials produced by organisms to be minerals too. To avoid confusion, the term “biogenic mineral” may be used when discussing such  materials. 
  • Solid: A solid is a state of matter that can maintain its shape indefinitely, and thus will not conform to the shape of its container. Liquids (such as oil or water) and gases (such as air) are not minerals.  
  • Crystalline structure: The atoms that make up a mineral are not distributed randomly and cannot move around easily. Rather, they are fixed in a specific, orderly pattern. A material in which atoms are fixed in an orderly pattern is called a crystalline solid. 
  • Definable chemical composition: This simply means that it is possible to write a chemical formula for a mineral. Some minerals contain only one element, but most are compounds of two or more elements. For example, diamond and graphite have the formula C, because they consist entirely of carbon. Quartz has the formula SiO2 it contains the elements silicon and oxygen in the proportion of one silicon atom for every two oxygen atoms. Calcite has the formula CaCO3, meaning it consists of a calcium (Ca ) ion and a carbonate (CO3 ) ion. Some formulas are more complicated: for example, the formula for biotite is K(Mg,Fe)3(AlSi3O10)(OH)2. 
  • Inorganic: Organic chemicals are molecules containing some carbon-hydrogen bonds. Sugar (C12H22O11), for example, is an organic chemical. Almost all minerals are inorganic. Thus, sugar and protein are not minerals. But, we have to add the qualifier “almost all” because mineralogists do consider about 30 organic substances formed by “the action of geologic processes on organic materials” to be minerals. Examples include the crystals that grow in ancient deposits of bat guano.
With these definitions in mind, we can make an important distinction between minerals and glass. Both minerals and glass are solids, in that they can retain their shape indefinitely. But a mineral is crystalline, and glass is not. Whereas atoms, ions, or molecules in a mineral are ordered into a crystal lattice, like soldiers standing in formation, those in a glass are arranged in a semi-chaotic way, like people at a party, in small clusters or chains that are neither oriented in the same way nor spaced at regular intervals. If you ever need to figure out whether a substance is a mineral or not, just check it against the criteria listed above. Is motor oil a mineral? No it’s an organic liquid. Is table salt a mineral? Yes it’s a solid crystalline compound with the formula NaCl. Is the hard material making up the shell of an oyster considered to be a mineral? Microscopic examination of  an oyster shell reveals that  it consists of calcite, so it can be called a biogenic mineral. Is rock candy a mineral? No. Even though it is solid and crystalline, it’s made by people and it consists of sugar (an organic chemical).
Credits: Stephen Marshak (Essentials of Geology)