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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.

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