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Colour

Most people are most interested in the colour of a mineral. This is especially important for choosing minerals as gemstones for jewellery. After all, jewellery has to match the outfit (maybe that’s why ‘‘diamonds are a girl’s best friend,’’ they go with everything)!

One of the problems geologists find in using colour to identify minerals of a certain group is that some minerals can be very different. Some colours are called idiochromatic. Their chemistry gives them their colour. Malachite which has a lot of copper is always green because copper gives it that colour.

Minerals that are usually colourless and take on the colour of small impurities are called pseudochromatic. Depending on the impurity, they can have a variety of colours. If a mineral contains bits of iron, it will take on a reddish colour.

Allochromatic minerals are generally colourless and transparent. They get their colour from the small changes in their crystalline make up or from structural flaws. In corundum, for example, the substitution of iron and titanium for aluminium gives a blue sapphire, while iron by itself produces a yellow sapphire.

Minerals like quartz come in lots of colours. Some of the colours that quartz can take are listed below:

* Colorless quartz,

* Rose quartz (all shades of pink),

* Milky quartz (white and whitish grey),

* Citrine quartz (yellow, yellowish brown, and orange),

* Smoky quartz (brown, brownish black, and black), and

* Amethyst (light to deep purple).

Many minerals, depending on their chemical content and formation, are found in different colours. Ruby and sapphire are both varieties of corundum with the same chemical composition (AlO3) and hardness (9), but two very different colours. Most people know that rubies are red and sapphires are blue.

However, just to keep you guessing, sapphires can also be colourless, green, yellow, or purple! Tourmaline is thought to have the greatest number of colour variations. A ‘‘chameleon of colour,’’ tourmaline is a prismatic crystal and is found to occur in seven different forms. These include elbaite (multicoloured), school (black), buergerite and dravite (brown), rubellite (pink), chromdravite (green), and uvite (black, brown, and yellowish green). Long, tourmaline crystals can be pink on one end and green on the other! They look like some rare cosmic gem from a science fiction movie.

A few minerals, like ruby, are fluorescent. They absorb blue and ultraviolet light and then release some of the energy back in the red part of the light spectrum.

STREAK

The streak of a mineral is simple to remember. It is just what it says, a powdery streak made when rubbing a sample across an unglazed surface. The streak is a more dependable way to test a mineral than colour since it is nearly always the same for different minerals. Sometimes a hard sample must have a small bit crushed with a geological hammer to get a sample to test. A streak may be colourless, white, golden yellow, yellow, reddish brown, red, grey, brown, or black. The streak of a mineral is often not the same colour as the mineral appears to the eye. For example, the mineral crocoite is orange-red and its streak is yellow. Wulfenite can be orange, yellow, brown, grey, or greenish brown, but its streak is white.

LUSTER

Luster is the word geologists use to describe the way light reflects off the surface of a mineral or crystal. The amount of light absorbed and a mineral’s texture affect luster. The different types of luster consist of dull, metallic, vitreous (glassy), adamantine, pearly, greasy, silky, and waxy. These are pretty straightforward and were used by some of the earliest people in describing different minerals. For example, gold and platinum have metallic lusters, but not microcline, which has a vitreous or pearly luster. Most silicates, sulfates, halides, oxides, hydroxides, carbonates, and phosphates have a vitreous luster.

A diamond’s high luster or that of highly reflective, transparent, or translucent mineral is known as an adamantine luster. Zircon, cuprite, and some forms of sulfur and cinnabar have this type of luster. One thing to remember is that depending on the mineral and the environment in which it was formed, luster can be different in different parts of the same sample, as well as in different samples (from different places) of the same mineral.

TRANSPARENCY

Depending on the way minerals are bonded, the light will pass through a mineral in different amounts. When you can see right through a mineral like glass, it is said to be transparent. If the light is slightly blocked, making the mineral look foggy and unclear, it is said to be translucent. If a mineral sample is solid and lets no light pass through at all, it is called opaque.

A transparent mineral can be seen through, while a translucent mineral is hazy, and an opaque mineral lets no light pass through at all.

A sample’s transparency isn’t always the same all the way through. Crystals are often transparent to translucent across a sample. For example, amethyst and olivine crystals are usually transparent to translucent within the same sample. Opal is transparent to opaque across a sample, while copper and jamesonite are opaque.

SPECIFIC GRAVITY

When geologists are trying to figure out the identity of an unknown sample, they use the above-mentioned characteristics as well as specific gravity (SG). The density of a sample is measured in terms of its specific gravity.

Specific gravity is the ratio of the mass of a substance compared to the mass of an equal volume of water at a specific temperature.

To find the specific gravity of a mineral, compare its weight to the weight of an equal volume of water. For example, a specific gravity of 4 tells geologists that an unknown sample is four times heavier than water. Size doesn’t matter. A larger sample can have a lower specific gravity. This is the case with talc and mercury. A large amount of talc would have a lower specific gravity than a small amount of mercury. The specific gravity of talc is 2.8, while mercury’s specific gravity is 13.6. 

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Crystalline Structure

Most minerals can be found in crystalline form. Whether they are rounded, have shear faces, or have no set form, most minerals have specific internal geometric structures. Sometimes the structures are the same between different minerals, but their chemical compositions are different.

As individual as people, what goes on inside a mineral determines its physical and optical properties, shape, hardness, cleavage, fracture lines, specific gravity, refractive index, and optical axes. The regularly occurring arrangement of minerals, atoms, and molecules in space determines its form. The lattice structure of a mineral is based on its arrangement of atoms, ions, and molecules within an individual sample. There are four different types of bonding that occur in crystalline solids. These determine what type of solid it is. The four types of crystalline solids are molecular, metallic, ionic, and covalent.

Bonding

These types of crystalline solids have molecules at the corners of the lattice instead of individual ions. They are softer, less reactive, have weaker nonpolar ion attractions, and lower melting points. A molecular solid is held together by intermolecular forces. The bonding of hydrogen and oxygen in frozen water shows how hydrogen forms bonds between different water molecules.

Another type of crystalline solid is made up of metals. All metals, except mercury, are solid at room temperature. The temperature needed to break the bonds between positive metal ions in specific lattice positions, like iron disulfide (FeS2), and the electrons around them is fairly high. This strong bonding gives stable molecules flexibility. It allows metals to be formed into sheets (malleable) and be pulled into strands (ductile) without breaking.

A metallic solid like silver is held together by a positively charged ‘‘central core’’ of atoms surrounded by a general pool of negatively charged electrons. This is known as metallic bonding. This arrangement of (+) ions and electrons (-) make metals good conductors of electricity.

Ionic solids form a lattice with the outside positions filled by ions instead of larger molecules. These are the ‘‘opposites attract’’ solids. The contrasting forces give these hard, ionic solids (like magnetite and malachite) highmelting points and cause them to be brittle. Hardness is not the same as brittleness. Brittleness, a measure of mineral strength, is dependent on a mineral’s overall structure. Think of it like building a house without the proper internal supports. Brittle minerals fracture easily

Ionic bonding in a solid occurs when anions (-) and cations (+) are held together by the electrical pull of opposite charges. This electrical magnetism is found in a lot of salts like potassium chloride (KCl), calcium chloride (CaCl), and zinc sulfide (ZnS). Ionic crystals, which contain ions of two or more elements, form three-dimensional crystal structures held together by the strong ionic bonds.

Covalent bonding holds hard solids together. Assembled together in large nets or chains, covalent multilayered solids are extremely hard and stable in this type of configuration. Diamond atoms use this type of structure when arranged into three-dimensional solids. One carbon atom is covalently bonded to four other carbons. This strong crystalline structure makes diamond the hardest known organic solid.

Covalent crystals are all held together by single covalent bonds. This type of stable bonding produces high melting and boiling points. Allotropes are different structural forms of the same element. Graphite, diamond, and buckminsterfullerene are all allotropes of carbon.

The different bonding and forms of carbon in a diamond (pyramid shaped), graphite (flat-layered sheets), or buckminsterfullerene (C60 and C70, shaped like a soccer ball) illustrate the variety and stability of covalent molecules. Nets, chains, and balls of carbon bonded into stable molecules make these solids hard and stable.

Minerals also have well-studied properties, such as color, hardness, crystalline structure, specific gravity, luster (shine or luminescence), cleavage, and tensile strength (resistance to being pulled apart). Many of these properties can vary slightly within a single mineral. Some minerals have very specialized properties like fluorescence and radioactivity.

Habit

Minerals come in many different sizes, shapes, and colors. The diversity and combination of colors within the same chemical formula keeps mineralogists guessing when they collect a new sample that doesn’t seem to fit the system.

A mineral or aggregate’s physical size and shape are called its habit.

There are several basic mineral habits mostly used to identify mineral specimens. They include the following:

* Acicular (thin, needle-like masses),

* Bladed (sharp-edged, like a knife),

* Dendritic (plant-like shape),

* Fibrous (furry),

* Granular (grainy),

* Lamellar (thin layers, plates, or scales),

* Massive (no specific shape),

* Reniform (rounded, globular masses),

* Rosette or radiating,

* Prismatic (flat or pointed ends with long, parallel sides), and

* Tabular (overlying flat squares).

Depending on the conditions present at the time crystals are formed, broad differences in a mineral or aggregate’s habit are possible.

Twinning

When a mineral sample has two or more nonparallel crystals that intersect and grow together, it is known as twinning. Twinning is often found in twin sets. A rare chrysoberyl specimen, measuring 8 cm across and containing three twinned crystal sets, was found in Espirito Santo, Brazil. This is an example of a chrysoberyl trilling.

When the crystals push against each other and form a mass, it is called contact twinning. However, if one penetrates and cuts through the structure of another at an angle, it is known as penetration twinning.

Cleavage

In geology, cleavage is determined by the way a mineral breaks when struck with a rock hammer. Depending on the crystalline structure, it cleaves between flat, well-defined planes. These planes are separated between layers of atoms or other places, where bonding between atoms is weakest. Cleavage faces are not as smooth as crystalline faces, but tend to cleave the same way each time the sample is broken. Depending on the structure of the mineral, cleavage breaks are described as perfect (breaks along the base or between crystals in the sample), distinct, indistinct, or none. Most minerals with basal, rhombic, prismatic, or cubic cleavage break along or between parallel planes. Those mineral types are commonly large and easy to spot. Galena, dioptase, and hematite are all examples of minerals with crystalline structures that break along cleavage planes.

Fracture

When you hit a sample with a rock hammer and it breaks without any real rhyme or reason, this is called a fracture. The sample has surfaces that are rough and uneven (compared to the easily seen shapes of cleaved samples). Most minerals fracture and cleave depending on their habit, but some only fracture. Fractures are described as uneven, conchoidal (shell-like), jagged, and splintery. A rough opal, for example, splits into a curved, shell-like fracture. The different parts of the split can have a wide spectrum of colors, from light blue to the rainbow of color found in ‘‘fire’’ opal.

Hardness

A physical characteristic of mineral identification that doesn’t change from one sample to another is hardness. Hardness is constant because a mineral’s chemistry is usually constant. Samples of the same mineral content can change a bit from one to the next, but in general they are about the same. Variations are only found when a mineral is poorly crystallized or is really an aggregate of different minerals.

Minerals with tightly packed atoms and strong covalent bonds are the hardest minerals. Minerals with metallic bonds or weak interconnected forces are the softest minerals. Talc, rated at the bottom of the hardness scale, is an example of an extremely soft mineral.

A mineral’s hardness, established by its physical structure and chemical bonding, is its resistance to being scratched.

Hardness is tested through scratching. A scratch on a mineral is actually a mark produced by surface microfractures of the mineral. Fractures take place when bonds are broken or atoms are pushed aside (metals). A mineral can only be scratched by a harder mineral. In 1812, French mineralogist, Friedrich Mohs, proposed a scale using set values as standards to test an unknown sample’s hardness against. Before Mohs set the standard, hardness was mostly done through guesswork. It was tough to describe hardness to other geologists unless they were right there in the field or lab holding the sample themselves.

The Mohs’ Scale of Hardness starts with talc at 1 and ends with diamond at 10, the higher the number, the harder the mineral.

This scale is not precise, but it gives geologists a common frame of reference to use when testing a sample’s hardness. The Mohs’ Hardness scale is one tool used by geologists and mineralogists around the world to tell different minerals apart. To use this scale, you have to have some of the minerals found in the scale on hand.

Some geologists begin hardness testing of an unknown mineral against orthoclase to see if the unknown mineral can scratch it. If the unknown mineral scratches the orthoclase, then it must be of hardness greater than 6. If the apatite scratches the unknown, then the unknown mineral must be of a hardness less than 6. If they scratch each other, then the unknown sample has a hardness of 6.

To get closer to an unknown mineral’s hardness, it can be tested against other less hard standards like apatite or fluorite. If it is softer than apatite and fluorite, try gypsum until you find the approximate hardness. Since the Mohs’ scale is a relative scale, one mineral sample may be scratched by another and given a certain hardness. It might be slightly more or less depending on other factors like shape or size.

It is important to remember to perform a hardness test on the backside or not easily seen part of a mineral. Some inexperienced collectors and students, in their excitement to discover more about a mineral, scratch right across a perfect crystal face. This ruins the specimen for display or jewelry! A fractured, cleaved, or unnoticeable part of the mineral still gives an accurate hardness test and doesn’t damage a beautiful specimen’s best face.

If they don’t have a Mohs’ Hardness Scale, some amateur geologists and students add a ‘‘hardness kit’’ to their rock hunting gear. The Mohs’ scale is useful for wide comparisons between minerals, so testing a sample with a fingernail, copper penny, or knife blade often gives a rough idea as to its hardness.

One way to remember the minerals on the Mohs’ scale is to make up a memory aid using the first letter of each of the Mohs’ minerals (talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, corundum, and diamond). It can be anything. Mine is, ‘‘The Geologist’s Cat Found An Old Queen’s Toffee Colored Diamond.’’

Remember that the Mohs’ Scale of Hardness is comparative and not absolute. Fluorite, with a hardness of 4, is not twice as hard as gypsum with a hardness of 2. Although talc is a 1 and diamond a 10 on the Mohs’ scale, the hardness difference between them is really about one hundred fold. The hardness differences between calcite and fluorite (3 and 4) are not the same as the differences between corundum (9, like ruby and sapphire) and diamond (10). Hardness is especially important when choosing gemstones. Except for apatite (5), turquoise (5–6), and opal (51 2 – 61 2), very few soft minerals can be cut as gems. People with jewelry made from these minerals are usually warned against cleaning them in vibrating cleaning machines since they can easily break.

Soft minerals are usually best for viewing and not for wearable jewelry.

People who buy malachite (31 2 – 4) earrings and drop one on a hard surface are surprised when it shatters. After all, their amethyst (7) earring hadn’t broken when it was dropped. Common gemstones like topaz (8), jasper (7), and aquamarine (7–8) have a hardness of 7 or more. Hardness also plays a big part in the selection of industrial minerals used for grinding, polishing, and other abrasive tasks. Soft minerals like talc and graphite are used as high-temperature lubricants, pencil lead, talcum powder, and to give shine to paper.

An absolute hardness scale has different values than the relative Mohs’ scale. Using precise instrumentation, mineralogists are able to measure the absolute hardness of minerals with much more precision. Most minerals are fairly close in hardness, but as hardness increases, the hardness differences increase by greater and greater amounts.

Absolute hardness is a precise measurement of a mineral’s hardness and not dependent on a comparison with other samples. For example, the absolute hardness of talc is 1. Diamond is 1600 times harder! When most people talk about diamonds, rubies, and sapphires, they consider them to be the same hardness and lump them together. However, geologists know better. Rubies and sapphires are different varieties of corundum which has an absolute value of 400. Diamonds are four times harder with an absolute value of 1600.

It’s easy to see why diamond gets a lot of respect as the Earth’s hardest natural mineral. Although there are a lot of compounds being formed and studied with the idea of creating something harder than diamond, the super-compressed, tightly bonded structure of carbon (diamond) is pretty amazing.

Most minerals have small differences in hardness according to the direction of the scratch and the orientation of the scratch. The environment in which a mineral formed within a rock can affect its hardness. For example, cyanide has a range (51 2 – 7) of hardness levels depending on these factors.

Impurities and ion substitutions can also affect the hardness of a sample. A huge specimen (several hundred pounds) is often softer than a single crystal because of its crystal structure, so hardness is most accurate when tested on individual crystals.

Sometimes a dust trail appears on a mineral after it has been ‘‘scratched ’’ by a softer mineral. It looks as if the softer mineral has scratched the harder mineral, but the ‘‘scratch’’ is really just a dust trail across the unyielding surface of the harder mineral.

The most beautiful gems & minerals you'll ever see!

Minerals and Gems (National Geographic)

Minerals and Gems

When you hear the word minerals, what comes to mind? Do you picture a cereal box advertising extra vitamins and minerals? Do you think of miners spending years searching for a glimpse of a shiny nugget or a brilliant stripe across a rock face? Or the many-faceted beauty of a friend’s diamond ring? Rock found on the Earth’s crust is a solid material created by three main geological processes: magma solidification, sedimentation of rock layers, and metamorphism. As a result, three basic rock types are formed.

* Igneous rock (volcanic or plutonic) is formed by the solidification of molten magma from the mantle.

* Sedimentary rock is formed from the burial, compaction, and lithification of deposited rock debris or surface sediments.

* Metamorphic rock is created when existing rock is chemically or physically modified by intense heat or pressure.

Geologists usually consider rocks to be a jumble of naturally occurring materials, mainly minerals. They can contain a mix of minerals and other organic substances ranging from microscopic mineral grains or organic matter to rough mineral agglomerates. Rocks can range in size from pebbles to mountains.

When people talk about their ‘‘rock collection,’’ they usually mean their ‘‘mineral collection.’’ Although some people collect rocks, mineral collectors are more common. They are the people looking for the ‘‘perfect’’ example of a specific mineral or the ‘‘rarest’’ specimen within a mineral group.

Amateur mineralogists and collectors are a lot like people who show dog breeds, like German Shepherds or English Pointers, to name a few. They get more points for having a specimen that meets the standard characteristics for the rock type and is of a high priority.

People value things that are rare and perfect. Flawless diamonds are much more valuable than those with flecks and flaws.

In fact, people have decorated themselves with shells, pieces of bone, teeth, and pebbles for the past 25,000 years (Paleolithic Period). But at that time most of the stones they chose were soft and brightly colored. Red carnelian and crystals were common choices.

From the time between 3000 and 2500 BC, lapis lazuli from Dadakshan reached Egypt and Sumer (Iraq). China, Greece, and Rome got their gemstones from many of the same regional mines.

Then, as people traveled and traded more, stones were made into family or governmental seals. They had different textures and some were carved.

When rolled on damp clay, an imprint was made that identified a product. Seals were part of a leap in commercial trade. Some stone seals were worn around the neck and considered a status symbol. Kings and rulers had ring seals that were recognized as symbols of identity and power.

Ancient people thought gems and crystals had special powers. In an uncertain world, people wore them for protection. Color was important in the imagery. Gold was related to the Sun, blue to the sea, sky, or heavens, red to blood or the life force, and black for death. Wearing powerful gems was thought to protect the wearer’s health, and bring wealth, luck, and love.

When the mummy of King Tutankhamun (1341BC–1323BC) a Pharaoh of ancient Egypt, was discovered, it was decorated with gold, red carnelian, turquoise, crystals, jasper, obsidian, alabaster, amazonite, jade, and lapis lazuli. These were the amulets of wealth and strength at that time. These stones were worn during life and some placed on the mummy after death as a protection against harm in the afterlife.

Some minerals and gems were thought to be powerful by themselves, while others were thought to wield power through the figures and words written on them.

Minerals and gems were also thought to contain medicinal powers. The early Greeks recorded these claims in medical papers known as lapidaries. The Greek philosopher, Theophrastus (372–287 BC) wrote the oldest surviving book on minerals and gems, called On Stones. He grouped 16 minerals into metals, earths, and stones (gemstones). A natural geologist, he accurately described physical characteristics of color, luster, transparency, hardness, fracture, weight, and medicinal benefits.

Pliny the Elder (AD 23–79) pulled together everything that earlier scholars had written into his 37-volume series, Historia Naturalis. Pliny’s work provided a lot of useful information on sources, mining methods, uses, trade, and gem value.

Since the 1600s, scientists have become even more questioning. The study of minerals and gems has become a part of the study of chemistry, optics, and crystallography.

Minerals are often described by their chemical formulas in order to note the chemical substitutions of one or more atoms. For example, topaz, a prismatic crystal with the formula, Al2SiO4 (F5OH)2, has been found to be as large as 100 kg. It can be colorless, white, gray, yellow, orange, brown, bluish, greenish, purple, or pink.

Gems and minerals are at the heart of the study of geology. Whether in the Earth or found on other planets, minerals tell the story of a planet’s chemical and physical developments. They have specific characteristics with unique physical and chemical properties. This adds to their great variety and makes the study of minerals interesting.

The study of minerals, minerology, is usually focused on the external microscopic study of minerals in polished sections. People who hunt for and collect rough mineral specimens as a hobby are often called ‘‘rock hounds.’’

Mineral Groups and Properties

All minerals belong to a specific chemical group, which represents their affiliation with certain elements or compounds. The chemical structure of minerals is exact, or can vary slightly within limits. They have specific crystalline structures and belong to different groups according to the way the mineral’s atoms are arranged. Elements like gold, silver, and copper are found naturally and considered minerals.

A mineral is a naturally found, inorganic substance with a specific crystalline structure.

Minerals are classified into the following chemical groups: elements, sulfides, oxides, halides, carbonates, nitrates, borates, sulfates, chromates, phosphates, arsenates, vanadates, tungstates, molybdates, and silicates. Some of these chemical groups have subcategories, which may be categorized in some mineral references as separate groups.

Nine Classes of Minerals

Geologists have identified over 3000 minerals. In order to study them more closely, they have divided minerals into nine different groups.. Minerals occur naturally as inorganic solids with a crystalline structure and distinct chemical make up.

Major mineral groups are determined by chemical composition.

Type Chemical structure

1. Elements

2. Sulfides

3. Halides

4. Oxides and hydroxides

5. Nitrates, carbonates, borates

6. Sulfates

7. Chromates, molybdates, tungstates

8. Phosphates, arsenates, vanadates

9. Silicates

Minerals are divided by different groupings

Mineral – Group (element-e, halide-h, oxide-o, silicate-si, sulfide-su, phosphate-p, molybdate-m, borate-b, carbonate-c) – Hardness (Mohs’ scale) – Chemical (Composition)

Antimony – e - 3 – 3.5 – Sb

Arsenic e 3.5 As

Bismuth – e - 2–2.5 Bi

Carbon (diamond and graphite) –e - Graphite 1–2 Diamond 10 - C

Copper – e - 2.5 –3 - Cu

Gold - e - 2.5 –3 - Au

Nickel – iron - e - 4 –5 - Ni,Fe

Platinum - e - 4 –4.5 - Pt

Silver - e 2.5 – 3 Ag

Sulfur e -1.5 – 2.5 - S

Fluorite - h 4 - CaF2

Halite – h - 2.5 - NaCl

Corundum - o - 9 Al2O3 - (ruby, sapphire)

Cuprite - o - 3.5 – 4 - Cu2O

Hematite - o - 5 – 6 - Fe2O3

Albite – si - 6 – 6.5 - NaAlSi3O8

Anorthite – si - 6 – 6.5- CaAl2Si2O8

Beryl - si 7 –8 - Be3Al2(SiO3)6

Dioptase – si – 5 - CuSiO2 (OH) 2

Jadeite - si - 6 – 7 - Na (Al, Feþ3) Si2O6

Labradorite – si - 6 – 6.5 - (Na, Ca) Al1 – 2Si3 – 2O8

Microcline – si - 6 – 6.5 - KAlSi3O8

Olivine – si - 6.5 – 7 - (Mg, Fe) 2SiO4

Orthoclase – si - 6 – 6.5 - KAlSi3O8

Quartz – si - 2.65 - SiO2

Topaz – si – 8 - Al2SiO4 (F, OH) 2

Zircon - si - 7.5 - ZrSiO4

Cinnabar – su - 2 – 2.5 - HgS

Galena – su - 2.5 - PBS

Pyrite - su - 6 – 6.5 - FeS2

Molybdenite – su - 1 – 1.5 - MoS2

Gypsum - su - 2 CaSO4-2 - (H2O)

Lazulite - p - 5.5 – 6 - (Mg, Fe) Al2 (PO4) 2 (OH)

Turquoise – p - 5 – 6 CuAl6 (PO4) 4 (OH)8  4H2O

Wulfenite m - 2.5 – 3 - PbMoO4

Borax b 2 – 2.5 - Na2B4O5 (OH) 4 8H2O

Calcite - c - 3 - CaCO3

Malachite - c - 3.5–4 - Cu2 (CO3) (OH) 2

Rhodochrosite - c - 3.5–4 - MnCO3

Earth minerals are composed of different elements. Oxygen 47%, Silicon 28%, Alumnium 8%, Iron 5%, Calcium 4%, Sodium 3%, Potassium 3%, Magnesium 2%

ELEMENTS

The elements include more than 100 known minerals. Many of the minerals in this class are made up of only a single element. Geologists sometimes subdivide this group into metal and nonmetal categories. Of all of the elements, 80% are metals. Gold, silver, and copper are examples of metals.

Carbon produces the minerals diamond and graphite, which are nonmetals. Elements like phosphorus and selenium are also nonmetals. For a complete listing of the known chemical elements, scientists use the Periodic Table of Elements. This is a chart that lists all the elements known today, along with a lot of other useful information. Besides the computer, the Periodic Table is probably the most important tool that scientists use.

Geologists use the Periodic Table to figure out the chemical composition of new minerals and to learn possible ways that different elements might bond.

The Periodic Table of Elements lists an element’s symbol (shorthand name, like C for carbon, Al for aluminum), atomic number (equal to the number of protons), atomic weight, and sometimes the atomic energy levels of the element. When a certain element is described, it is written with the atomic number in superscript and the atomic weight in subscript. On a Periodic Table, magnesium, with atomic number 12 and an atomic weight 24.31, is written as:

12

Mg

24:31

While the simplest of Periodic Tables show just an element’s atomic number and weight, complete charts give a broader amount of information. To give you an idea of the usefulness of the Periodic Table, the information listed for titanium in most Periodic Tables is shown below.

22

Ti

47:90

Ti

Titanium

Atomic Number – 22

Atomic Weight – 47.90

Group – 4

Period – 4

Transition Metal

Electrons per orbital layer – 2, 8, 10, 2

Valence electrons – 1s2 2s2p6 3s2p6d2 4s2

Knowing specifics about elements, like their electron arrangement, allows chemists and other scientists to figure out the bonding possibilities and types of compounds that can be formed with other elements. From this information, the mineral content of new and unknown samples is worked out. This information is also helpful when creating new compounds in the laboratory.

HALIDES

The halides are a group of nonmetals whose main chemical components include chlorine, fluorine, bromine, and iodine. Most halides are very soluble in water. They also form highly ordered molecular structures with a high degree of symmetry. Halite is the most common mineral of this group. It is known to most people as rock salt. Other halites include the minerals, cryolite, atacamite, fluorite, and diabolite.

OXIDES

A group of minerals, made up of one or more metals combined with oxygen, water, or hydroxyl (OH), is known as the oxides (and hydroxides) group. The minerals in this group show a great variety of physical characteristics compared to other more nonchanging groups. Some oxides are hard and others soft. Some have a metallic luster, while some are clear and transparent. Some of the oxide minerals include anatase, corundum, chromite, and magnetite, while hydroxides include manganite, goethite, tungstite, and diaspore.

SILICATES

The silicates encompass the largest mineral group. As the name implies, these minerals have varying amounts of silicon and oxygen. Silicates are often opaque and light weight. Silicate minerals are different from other groups in that they are all formed as tetrahedrons. However, it can be tough to identify individual minerals within the silicates group. A tetrahedron is a chemical structure where a silicon atom is bonded to four oxygen atoms (SiO4). Some representative silicates include albite, andesine, hornblende, microcline, labradorite, sodalite, leucite, and quartz.

SULFIDES

The minerals of the sulfide group are often made up of a metal combined with sulfur. They are recognized by their metallic luster. The sulfides are an economically important group of minerals. The extraction of sulfide ores from composite metals is a standard process in industry. Specific ores are known for certain metal extractions, like cinnabar (a major source of mercury), molybdenite (molybdenum, an alloy in steel), pyrite (iron source), and galena (lead, used in piping and pewter).

SULFATES

The sulfate mineral group usually combines one or more metals with the sulfate compound, SO4. Most sulfates are transparent to translucent, light in color, and soft. They usually have low densities. Gypsum, the most plentiful sulfate, is found in evaporite deposits. Common sulfates include anhydrite (CaSO4) and celestine (SrSO4).

Sometimes, sulfates contain substituted groups like chromate, molybdate, or tungstate in place of the sulfate group. Chromates are compounds in which metals combine with chromate (CrO4). The minerals crocoite (PbCrO4), wulfenite (PbMoO4), and scheelite (CaWO4) are all examples of different group replacements that form different minerals. These compounds are usually dense, brittle, and brightly colored.

PHOSPHATES

The mineral group, known as the phosphates, is made up of one or more metals chemically combined with the phosphate compound (PO4). The phosphates are sometimes grouped together with the arsenate, vanadate, tungstate, and molybdate minerals. These minerals have substituted arsenic, tungsten, and molybdenum elements, respectively.

Although geologists list several hundred different types of these minerals, they are not common. Apatite is the most common phosphate mineral. Most minerals in these groups are soft, but their hardnesses can range from 11/2 to 5 or 6 (turquoise). Although brittle, they have well-formed crystals in beautiful colors like lazulite (blue) and vanadinite (red or orange).

CARBONATES

This is an easy one. Carbonates are minerals which contain one or more metals bonded with carbon in the compound (CO4). Most pure carbonates are light colored and transparent. All carbonates are soft and brittle. They are usually found as well-formed rhombohedral crystals. Carbonates react with, bubble up, and dissolve easily in hydrochloric acid. Calcite is the most common carbonate. Other colorful carbonate minerals include rhodochrosite (pink to red), smithsonite (blue green), and azurite (deep blue), and malachite (medium to dark green).

Nitrates and borates are often thought of as a subgroup of carbonates. They are formed when metal compounds combine with nitrogen and boron. When metals bond with nitrate, minerals like nitratine, a rare rhombohedral, transparent, often twinned mineral is formed.

When metals bond with borate, minerals like borax, kernite, and ulexite are formed. Most people have seen white borax, but it can also be colorless, gray, greenish, or bluish. Borax forms near hot springs, in ancient inland lakes, and places from which water has evaporated.

ORGANIC MINERALS

Minerals originally from organic sources (plants) are not usually classified as true (pure) minerals. However, some crystalline organic substances look and act like true minerals. These substances, formed primarily from carbon, are called organic minerals. Amber (petrified tree sap) is an example of an organic mineral.

Metamorphic Rocks Video

Metamorphic Rocks

Chemical Changes

Characteristics that cause chemical changes in rocks also add to the formation of metamorphic rocks. Very hot liquids and gases can, under extreme pressures, fill the pores of existing rocks. These liquids and gases cause chemical reactions to occur, and over time, change the chemical composition of the existing rock. Metamorphism can take place instantly as in rock shearing at plate boundaries or can take millions of years as in the slow cooling of deeply buried magma.

It is important to remember that the changes that go on in metamorphism are mostly in rock texture. The chemical composition of metamorphic rock is altered very little. The basic changes that do occur include the addition or loss of water and carbon dioxide. The biggest changes of metamorphic transformation, then, have to do with the way minerals are rearranged.

A chemical shift in the composition of metamorphic rock can also be changed by the addition or removal of different elements. This can happen as a result of the intrusion of magma bringing new minerals into contact with existing rock. Sometimes this can be seen through color changes in minerals of the same basic chemical composition.

When hot, mineral-rich waters rise through magma, they carry a variety of elements. Some of these elements include sulfur, copper, sodium, potassium, silica, and zinc ions to name a few. These minerals come from magma and intruded rock, during the time that water is filtering upward through the crust. On this journey, they interact with other minerals and chemicals replacing some of their own minerals with others. This type of chemical interaction and substitution is called metasomatism. Metal deposits like copper and lead are formed in this way.

Index Minerals

A Scottish geologist, George Barrow, noticed that rocks having the same overall mineral make up (like shale) could be seen to go through a series of transformations throughout specific zones in a metamorphic region. He found that minerals in individual zones had specific mineral configurations. As he studied minerals across a zone, he found that when new metamorphic mineral configurations were created, it was predictable.

The first appearance of index minerals marks the boundary of lowto high-grade metamorphic rock changes in a specific regional zone.

Barrow found that mineral (shale) configuration changes happened with regard to index minerals. These index minerals acted like milestones in the low- to high-grade metamorphic rock transformation process. Barrow found that the domino effect of metamorphism happened in the following series:

chlorite -> biotite -> garnet -> staruolite -> kyanite -> sillimanite ->

Low grade->High grade

When Barrow and his team studied the geological maps of the Scottish Highlands, they were able to plot where certain minerals started and stopped. They marked the locations of certain minerals and called these connected places isograds.

An isograd is a marker line on a map connecting different areas of certain minerals found in metamorphic rock.

Metamorphic Rock Textures

Metamorphic rocks are divided into two categories, foliated and nonfoliated. Foliate comes from the Latin work folium (meaning leaf ) and describes thin mineral sheets, like pages in a book. Metamorphic minerals that align and form bands, like granite gneiss and biotite schist, are strongly banded or foliated.

When metamorphic mineral grains align parallel in the same plane and give rock a striped appearance, it is called foliation or foliated rock.

Initially, the weight of sedimentary rock strata keeps the sheet-like formation of minerals parallel to the bedding planes. As the mineral layers are buried deeper or compressed by tectonic stresses, however, folding and deformation take place. The sedimentary strata are shoved sideways and are no longer parallel to the original bedding. In fact, metamorphism changes the texture enough that when broken, the metamorphic rock breaks in the direction of the foliation not the original mineral’s composition.

Foliates are made up of large concentrations of mica and chlorite. These minerals have very clear-cut cleavage. Foliated metamorphic rocks split along cleavage lines that are parallel to the alignment of the rock’s minerals. For example, mica can be separated into thin, flat nearly transparent sheets.

Mica is said to have good schistosity, from the Latin word schistos meaning easily cleaved.

Schistosity is the parallel arrangement of coarse grains of sheetstructure minerals formed during metamorphism and increasing pressure.

For fine-grained rocks with microscopic mineral grains, the breakage property is known as rock cleavage or slaty cleavage.

Slaty cleavage is found in an environment of low temperature and pressure. In these less-intense conditions, grain sizes increase and single grains are easily seen. Foliation is present with slaty cleavage, but not in a flat plane. Intermediate and high-grade metamorphic rock commonly breaks along rolling, or somewhat distorted surfaces similar to the orientation of the grain of quartz, feldspar, and other minerals.

Rock cleavage or slaty cleavage describes the way rock breaks into plate-like pieces along flat planes.

Large crystal textures can also be formed in a fine-grained, support rock during metamorphism. When this happens, crystals found in both contact and regional metamorphic rock are called porphyroblasts. They grow as the elements are rearranged by heat and temperature.

We learned that structural deformation goes on during metamorphism. When two rock surfaces deep in the Earth’s crust grind against each other, crushing and stretching into bands, myolites are formed. These rocks are deformed under very high pressure. This deformation can take place before, during, or after metamorphic changes have happened and is part of the ongoing recycling of rock.

For example, shale may be changed into schist during deep burial without any deformation. Then, much later, when tectonic action hauls the schist layer upward in mountain building, higher-grade metamorphism may cause foliation and deformation. Then, if the rock is living a really interesting life, it may be heated during contact metamorphism and change yet again.

Naming

When naming metamorphic rock, the rules are more flexible than that of igneous or sedimentary rock naming. Since metamorphic rock tends tochange in composition and texture as temperatures and pressures change, the naming changes.

For example, shale is a fine-grained, clastic sedimentary rock containing quartz, clays, calcite, and some feldspar. With the start of low-grade metamorphism, muscovite and chlorite begins to form. Transformed shale is called slate. If the slate meets with further metamorphism, the mineral grains grow and intermediate-grade metamorphism happens with foliation and mica forms. Continued metamorphism causes the formation of even larger, coarsegrained rock with high schistosity and is known as schist. Then at high-grade metamorphism, the minerals group into separate bands with layers of mica-like minerals such as quartz and feldspar. This type of high-grade metamorphic rock is called gneiss from an old German word, gneisto, meaning to sparkle. So naming then depends on what can be seen. Slate and phyllite describe textures, while gneiss is described by the large mineral grains (that are easily seen) being named first. So specific gneiss might be named, quartz–plagioclase–biotite–garnet gneiss. In this way, another geologist would have a pretty good idea of all that the rock contained. Nongeologists would probably just call it garnet!

The Internet has several sites that provide photos of metamorphic rock types. There are even photos that illustrate the complete metamorphic rock series.