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Mineral and Gem Characteristics Help (page 2)

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By — McGraw-Hill Professional
Updated on Apr 25, 2014

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).

A sampling of minerals with different habit types are listed in Table 9-3. Depending on the conditions present at the time crystals are formed, broad differences in a mineral or aggregate’s habit are possible.

Table 9-3 A mineral’s habit is a good visual way to identify it.

Mineral

Habit

Actinolite

Bladed

Alabaster

Granular, massive

Anhydrite

Fibrous, granular, massive

Barite

Prismatic

Beryl

Prismatic

Chalcanthite

Tabular, fibrous

Copper

Dendritic

Gypsum

Rosette, radiating

Hematite

Reniform

Limonite

Massive

Mica

Lamellar

Scolecite

Acicular

Titanite

Massive, lamellar

Silver

Dendritic

Zircon

Prismatic

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. Table 9-4 shows the relative hardness of minerals on the Mohs’ Scale of Hardness. To give you an idea of how common items compare in hardness to the Mohs’ scale, several common things are listed with their Mohs values.

Table 9-4 Mohs’ Scale of Hardness is used to test the comparative hardness of samples.

Mineral hardness (Mohs’ scale)

Mineral

Common hard stuff

1

Talc

Pencil lead (1–2)

2

Gypsum

Fingernail (2½)

3

Calcite

Penny

4

Fluorite

 

5

Apatite

Knife blade (5½)

6

Orthoclase

Glass (6)

7

Quartz

Garnet

8

Topaz

 

9

Corundum

Ruby, sapphire

10

Diamond

 

 

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.

Table 9-5 gives you a few hardness hints to look for when testing different mineral samples for 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 ( t alc, g ypsum, c alcite, f luorite, a patite, o rthoclase, q uartz, t opaz, c orundum, and d iamond). It can be anything. Mine is, “ T he G eologist’s C at F ound A n O ld Q ueen’s T offee C olored D iamond.”

Table 9-5 When testing for hardness, check for these common characteristics.

Characteristic

Hardness hints

Orientation

Most minerals have small di.erences in hardness depending on the direction and orientation of the scratch. Kyanite samples have a hardness range of (5½–7)

Size

A 1500 kg specimen is often softer than a single crystal because of the crystal structure. Hardness is truer when tested on individual crystals

Purity

Some minerals have a range of hardness values because of impurities or ion substitution

Dust

Sometimes there is a dust trail on a harder, una.ected mineral after being “scratched” by a softer mineral. Always blow or rub across a scratch to be sure there is a real scratch

Scratching ease

Relative hardness is a.ected by ease of scratching. Both diamond and quartz scratch glass, but diamond scratches glass with extreme ease, like a knife through butter

 

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 Minerals and Gems Hardness , 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 Minerals and Gems Hardness 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. Table 9-6 shows a comparison between the absolute hardness values and the relative Mohs’ hardness values for the same minerals.

Table 9-6 There is a big difference between the Mohs’ relative hardness scale and absolute hardness.

Mineral

Mineral hardness (Mohs’ scale)

Mineral hardness (Absolute scale)

Talc

1

      1

Gypsum

2

      3

Calcite

3

      9

Fluorite

4

    21

Apatite

5

    48

Orthoclase

6

    72

Quartz

7

  100

Topaz

8

  200

Corundum

9

  400

Diamond

10

1600

 

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 Minerals and Gems Hardness 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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