ROCKS AND MINERALS

Minerals and Crystals

 

Minerals are the building blocks of rocks. Most common minerals are made up of more than one element, but a few, like gold and silver, are made of single elements. Minerals are found as crystals; some crystals are picture–perfect, while others are hard to discern. Many minerals have more than one crystal form; in fact, calcite has more than 600.

 

More than 4,000 minerals are known on earth, although only a few are common. Minerals are formed from molten rock or liquid–rich solutions found within the earth and on its surface. A new mineral can also form from the alteration of other minerals; for example, a mineral formed deep in the earth may alter to clay when exposed to surface temperatures and pressures.

 

Most people have a sense of what a mineral is but may not know its definition.

A mineral:

•   must be found in nature,

•   must be inorganic,

•   has an orderly arrangement of atoms known as its crystal form,

•   has a specific range of chemical compositions; e.g. SiO2 is always quartz and quartz is never anything else; olivine can include iron or magnesium or some of each with its silicon and oxygen components;

•  has characteristic physical properties (see below) that can be used for identification.

 

Questions

For each of the following, state why the material is not a mineral.

1.  synthetic diamond

2.  coal

3.  obsidian, a volcanic glass

 

Answers

1.  Just like real diamonds, synthetic diamonds are made of pure carbon, they have the same crystal structure, and they’re formed at high pressures. But a synthetic diamond is made by humans so, by definition, it is not a mineral. 2.  Coal is organic; it is made of ancient plant and animal matter.  3.  Glass is amorphous; it does not have a crystal structure. Obsidian is no exception, so while it is a rock (by definition rocks don’t have to have crystal structure) it is not a mineral.

 

Minerals are held together by atomic bonds. The atoms of different elements have different numbers of electrons in their orbital shells. To be stable, an atom likes to have its outer shell full of electrons. The innermost shell contains two electrons, the rest of the shells have eight electron slots. To get a full outer shell, an atom may gain or lose some electrons, becoming an ion. When an atom gives electrons to or takes electrons from another atom, these atoms, now ions, are sharing atomic bonds. These bonds are called covalent bonds.

 

Minerals have a distinctive form because their atoms are arranged in an orderly structure. Given the space and time to grow, a mineral’s crystal form becomes obvious. For example, quartz forms beautiful prismatic crystals. It also takes time for crystals to grow. Obsidian cools quickly; its molecules do not have time to arrange themselves in a crystal structure.  Thus, obsidian is not a mineral. Other minerals have a crystal form, but may not be nicely defined.

 

Identifying Minerals

 

Geologists use the physical properties of minerals to identify them. They may need to identify minerals in the field or they may take them back to the lab where more sophisticated techniques can be used.

 

The physical properties of minerals are:

•  cleavage,

•  fracture,

•  specific gravity,

•  the color of its streak or powder,

•  the color of its surfaces,

•  hardness,

•  luster.

 

Because minerals have an orderly arrangement of atoms, when they are split, they tend to break apart along flat surfaces. This is called cleavage. For example, halite (salt) crystals are in the shapes of cubes; if you break a halite crystal, you will find a smaller cube. Micas have excellent cleavage in one direction; they can be peeled off in sheets. Fracture is the pattern in which a mineral breaks, but it is unrelated to cleavage. Minerals can have cleavage or fracture or both. Descriptions of fracture include splintery, jagged, and conchoidal— defined as the smooth curves found on broken glass.

 

Specific gravity is a measure of the density of a material relative to water. At specified conditions, water's density equals 1 gram per cubic centimeter. A mineral's specific gravity would correspond to a mineral's density as expressed in grams per cubic centimeter. Some minerals, especially metallic minerals, are extremely heavy. A simple way to estimate specific gravity of a mineral in the field is simply to lift the mineral.

 

Some minerals vary in their color, some don’t. Olivine is always olive green and biotite mica is always brownish black. Quartz can be clear, white, smoky–gray, pink, or a variety of other colors. Since minerals often are found in many colors, and color cannot always be used to identify a mineral, geologists sometimes use streak. Streak is a mineral’s fine powder. To look at streak, powder the mineral or run it along a ceramic plate. Not all minerals have identifiable streak; quartz, for example, would be more likely to scratch the ceramic plate. Metallic minerals have the best streak.

Minerals vary greatly in their hardness, so hardness is commonly used for identification. To check a mineral’s hardness, scratch it with objects of known hardness, then see where it fits on the Mohs’ Scale. As you can see, diamond can scratch everything and everything can scratch talc. In this version of the Mohs’ scale, the hardness of some common objects that you might be carrying while in the field are included.

 

Question

You need the hardness of a mineral that you’ve found in the field. You forgot your Mohs’ kit (a collection of all the minerals on Mohs’ scale with their hardness labeled) but you have several other useful items with you. The unknown mineral can scratch your fingernail and a copper penny. It can barely scratch a knife blade but cannot scratch glass. What is the hardness of the mineral you’ve found?

 

Answer

Between 5, the hardness of the knife blade, and 5.5 to 6, the hardness of glass.

 

Luster is another identifying trait of minerals. It is how shiny the mineral appears. A mineral’s luster can be described as metallic, pearly, silky, waxy, and dull, among others. A common type of luster is vitreous, meaning “like glass.”

 

Some minerals have unique properties that aid in their identification. Calcite dissolves in a weak acid. Several minerals, such as fluorite and ruby, fluoresce under an ultraviolet (UV) light. Magnetite is the only mineral that is strongly magnetic.

 

Question

While in the field you find a mineral that you’d like to identify. Fortunately, you brought your mineral guide, a Mohs’ hardness kit, a hand lens, and a bottle of dilute hydrochloric acid.

 

The unknown mineral you picked up is white and glassy. It has a white streak and its specific gravity is average. You find some likely minerals in your guide and make a table of their physical properties.

 

Outline how you will go about identifying this mineral. What would be the easiest mineral to eliminate from your list of possibilities?

 

The hardness of the minerals is very different but it falls into two groups: calcite and muscovite mica have low hardness, plagioclase feldspar and quartz have much higher hardness. A hardness test could tell you which mineral you have, but it might be hard to distinguish between the two groups.

 

The easiest mineral to check for is calcite, since it is the only mineral that fizzes in acid. If you place a small drop of acid on the mineral and nothing happens, you can rule out calcite!

Question

Now that you’ve eliminated calcite, what is the next test you would do to narrow down your choices?

 

Answer

You would easily be able to distinguish the hardness of muscovite mica from that of the other two. But an even easier way to identify muscovite is by its cleavage. If you try to peel off a sheet of the mineral and nothing happens, the mineral is not muscovite mica!

 

Question

Your mineral is either plagioclase feldspar or quartz. What do you do next?

 

Answer

Take a good look at those minerals. Quartz forms prismatic crystals; plagioclase forms blocky crystals that have striations. Your mineral is not a well–formed crystal, but when you look with your hand lens you do not see striations. You think you have quartz.

Question

How could you confirm your guess?

 

Answer

You can check the hardness of the two minerals. Quartz is #7 in your Mohs’ kit; plagioclase is a relative of orthoclase, #6 in your Mohs’ kit. Your unknown mineral scratches the #6 but does not scratch the #7. You have found quartz.

 

There is more than one way to go about identifying your unknown mineral. This was one set of simple and accurate tests.

 

 

 

Activity

 

Find a mineral guide, a Mohs’ hardness kit or the items listed on the Mohs’ scale above, a hand lens or magnifying glass, and a bottle of dilute hydrochloric acid or vinegar and go outside. Have fun identifying any minerals you see!

 

Gemstones

 

A diamond is a diamond; it has no other name. But many minerals are gemstones only in some colors or crystal forms. When corundum contains chromium as a contaminant, it appears red and is known as ruby. When it contains iron and titanium, it is blue and is known as sapphire. Fancy sapphires contain vanadium, iron, or chromium plus iron, and can be pink, orange, yellow, green, or purple.

 

Here are some minerals and their gemstones.