Their Physical Traits

What is a Mineral?

• It must be Solid• It must be Made by Nature• It must have Never (Ever) Been Alive• It must be Made of the Same Stuff Throughout

Definition of a Mineral?

• Definition: A naturally occurring, inorganic solid with certain definite characteristics, such as Crystal Structure and Chemical Composition.

• QUESTION?– If the Best way to identify any mineral is by its Crystal Structure and Chemical Composition, why

don’t we just do that every time?

• ANSWER: – Because it would require heavy and expensive equipment that we can’t take with us easily.

• Instead, we identify minerals based on many of their Characteristics that are easy to see.

What is a Mineral Characteristic?

• It is a feature that helps you to identify a Mineral

• It will help you tell one Mineral apart from another

• It will help you describe it to someone else

Here is an Example

• How would you describe a car?– Would you say, “A four wheeled vehicle.”

OR– Would you say, “A 1963 Volkswagen Beetle, with two

red and blue racing stripes and the number 53 painted on the hood, doors, and trunk.”

• I bet we are all thinking of the same car now!

You can describe a Mineral in the same way

• You can describe how the Mineral – Looks (by describing its Colors, Crystal formation and Class)– Feels (by describing its Texture)– Smells (some minerals smell very strong)– Tastes (some minerals taste very salty)– Shines (by describing its Luster)– Breaks (by describing its Hardness, Cleavage and Fracture)– Writes (by describing the color of its Streak)– Weighs (by describing its Specific Gravity)– Dissolves (by describing is Solubility)

• You can also describe if a Mineral is Magnetic, what are its Uses, what Effects Temperature And Pressure have on it, and how it may React With Other Compounds

• The words in RED are the Characteristics of Minerals

Mineral Characteristics(The ones we will need to know)

• Luster• Color• Streak• Moh’s Scale Hardness• Specific Gravity • Magnetic Properties• Effects of Temperature and Pressure on Rock Formation• Cleavage and Fracture• Crystal Formation and Class• Reactions with other Compounds (concepts: Acid, Base, and Inert)• Solubility• Usage of the Mineral

Characteristic Descriptions

• Now lets look at each Characteristic and see how would we describe them.

• Reminder:– A Car Company, would be a Characteristic of a Car

– Ford, GM or Chrysler, would be a description of the Car Company Characteristic.

Luster

• Luster is the way the surface of a mineral reflects light. – Luster should be observed on a clean, or freshly broken, part of

the Mineral’s surface.

• There are two general types of luster– Metallic

• looks shiny like a metal. Usually opaque and gives black or dark colored streak.

– Non-Metallic• Non metallic lusters are listed on the next table.

Different Terms for Luster

TERMS DESCRIPTIONAdamantine (Brilliant) very gem like crystals, (e.g. Diamond)

Dull just a non-reflective surface of any kind (e.g. Bauxite)

Earthy the look of dirt or dried mud

Greasy appears to be covered with a thin layer of oil or grease (e.g. turquoise, nepheline, or Talc)

Metallic the look of metals (e.g. Gold)

Submetallic a poor metallic luster, opaque but reflecting little light

Pearly the look of a pearl, iridescent (e.g. Talc, apophyllite)

Resinous the look of resins such as dried glue or chewing gum, or Tree Sap (examples: sphalerite, sulfur)

Silky the look of silk, similar to fibrous but more compact (e.g. Asbestossome, gypsum, serpentine, malachite)

Vitreous Looks Glassy (the most common luster) (e.g. Quartz, Tourmaline)

Waxy the look of wax

Less Common Not important to memorize these

Color

• Even though we notice the color of a mineral first, color by itself is not a reliable Characteristic to use in identifying a mineral.

Color• So why is Color by itself not enough to identify a mineral?

• The color in a Mineral is usually caused by impurities– (certain Elements) in the Mineral’s Chemistry.– Example:

• Copper usually produces green and blue colors. • Iron usually produces red and yellow

• Most minerals, are usually white or colorless in their pure state.– The impurities change what wavelengths of light get absorbed

by the Mineral, and that changes what color the Mineral appears to be.

What Causes Color in Minerals• Color in minerals is due to the absorption, or

lack of absorption, of various wavelengths of light. – When white light (which contains all

wavelengths of visible light) enters a crystal it often is merely reflected back to our eyes and so appears as white.

• However, some elements have electrons that absorb certain wavelengths or colors. The absorbed wavelengths provide energy so the element then emits another wavelength to get rid of the extra energy and this is what we see (as a different color).

• Visible light is only a small portion of the Electromagnetic Spectrum with wavelengths of 375 to 740 nanometers – a nanometer = one billionth of a meter or 10-9

Absorption and Emittanceof Light

• The elements that produce colors through absorption and emittance of wavelengths are usually transition metals such as Manganese(Mn), Vanadium(V), Chromium(Cr), Titanium(Ti), Nickel(Ni), Cobalt(Co), Iron(Fe), Copper(Cu), plus a few other metals such as Uranium(U).– When one of these elements is a necessary component of

the chemistry of a mineral then the mineral will always have a particular color (such as Azurite always being blue due to the Copper in its formula).

– Even if it is only a very small component (and is only an impurity) in an otherwise colorless mineral it can still strongly change the color of the mineral • An Example is the very small amount of Chromium that causes the

Kämmererite and Ruby to appear Red.

Color

• These Chemical impurities can change the color of a mineral without changing its basic make-up. For example,

– Quartz in its purest form is colorless and clear as glass.

– Quartz with traces of iron becomes violet (amethyst).

– Quartz with traces of manganese, turns pink (rose quartz)

– Quartz exposed to radiation, turns brown (smoky quartz).

Color

• There are some minerals that are easily identified by color because they are never any other color. – For example, Malachite is always green.

Streak

• Streak is much more reliable than Color, because the color of the streak is always the same, whether or not the mineral has impurities.

• When a mineral is rubbed firmly across an unglazed tile of white porcelain (a streak plate, hardness 6.5), it leaves a line of powder. This is called the streak.

• For example, quartz leaves a white streak, whether it's violet (amethyst), pink (rose quartz), or brown (smoky quartz).

Moh’s Scale Hardness

How to use the Moh’s Scale Hardness

• To use the hardness scale, try to scratch the surface of an unknown sample with a mineral or substance from the hardness scale (these are known samples). – If the unknown sample cannot be scratched by feldspar (6) but it can be scratched by quartz

(7), then it's hardness is between 6 and 7. – An example of a mineral that has a hardness between 6 and 7 is pyrite (6 to 6.5).

• If you don't have minerals from the hardness scale on hand, here are some common objects and their hardness values:

– If an unknown sample can not be scratched by your fingernail (2.5) but it can be scratched by a penny (3.5), then it's hardness is between 2.5 and 3.5. • An example of a mineral that has a hardness between 2.5 and 3.5 is calcite (3).

Hardness of Minerals• Strength of a Mineral:

– Mohs hardness (or the ability to withstand scratching) is one of the components of the strength of a mineral. Other components are

– Tenacity [ability to withstand stress by deformation (such as compressing, shearing, stretching, and twisting forces),

– Elasticity (ability to return to the original shape after the stress is removed), – Sectility (ability to be cut, into ‘sections’), – Malleability (ability to be shaped without breaking)] and – Cleavage which is the tendency minerals have to break or split along the planes of their crystal structure.

• Strength is essentially determined by the power of the chemical bonds and the structural geometry of the mineral. A substance’s structure can include planes and locations where fractures and cleavages are likely to occur. – For example, diamond, which is the hardest to scratch, can, nevertheless, cleave for the diamond cutter (or

the unwary wearer) along several cleavage planes.

• Bond strength correlates with hardness. – Covalent Bonds - Diamond has small atoms packed tightly together with strong covalent bonds in three

dimensions, – Ionic Bonds - Corundum also has small atoms, but strong ionic bonds.

• Many minerals have a mixture of ionic and covalent bonds. – Metallic Bonds – Copper, and others, have metallic bonds with electrons able to move freely in all directions– Van Der Waals Bonds – Graphite has even weaker van der Waals bonds as components of their structure.

How to Test Hardness• Test hardness by scratching

– a scratch on a mineral is actually a groove produced by microfractures on the surface of the mineral. • Either bonds break or there is a displacement of atoms (as with the metallic bonded minerals, e.g. dents).

– A mineral can only be scratched by a harder mineral (or other substance), making it possible to establish a relative scale comparing all minerals.

– In 1812, German mineralogist Friedrich Mohs established such a scale starting with the softest, talc, at one; and the hardest, diamond, at ten.

– In addition, since it is difficult to always have all ten of these minerals on hand in the field, we can use some everyday things to assist us.

–

• Generally speaking, gemstones have a hardness of 7 or more.

• The mineral quartz (which is the primary ingredient of dirt and sand, and almost everywhere, or ubiquitous) has a hardness of 7– This is why it is very important that any gem that is to be worn (as jewelry) can handle ordinary wear and

tear without being scratched (in a quartz world).

• Those gems with a hardness of less than 7 such as opal and turquoise must be carefully handled.

How to Test Hardness– It is also of interest to note that the Mohs scale, although

correct in its order, is not accurate about the exact differences in hardness between the substances. • Measurements show, for example, that diamond is four times

harder than corundum and calcite is three times harder than gypsum.

• SEE THE ABSOLUTE HARDNESS TO SEE HOW THEY COMPARE

Specific Gravity • Specific gravity is the weight of the Rock or Mineral

divided by the weight of an equal volume of water.– This is not the same as density.

• Some mineral samples will feel heavier than others, even if all your samples are the same size.

• The heavier ones have a greater specific gravity. • Here are some examples of common minerals and

their specific gravity ranges:

Magnetic Properties

• A materials that can become magnetized, and stay magnetized permenantly.– Also called ‘Ferromagnetic ‘minerals

• Examples of such minerals are – Magnetite– Hematite– Pyrrhotite

Cleavage• When a mineral sample is broken with a hammer, it breaks along a planes of

weakness that are part of its crystalline structure. – These breaks are cleavages.

• Some minerals break only in one direction. • Others break in two or more directions.

• Some common forms of cleavage are – cubic, rhombohedral, and basal.

• Cubic cleavages form cubes (example, halite). • Rhombohedral cleavages form six-sided prisms (example, calcite). • Basal cleavages occur along a single plane parallel to the base of the mineral (example, topaz).

• If a mineral breaks easily and cleanly in one or more directions, its cleavage is considered perfect. – For example, calcite cleaves perfectly along three planes.

• As the quality of the break decreases, cleavage may be described as good, distinct, and poor or none.

• Some minerals cleave perfectly in one direction and poorly in others. – For example, gypsum cleaves perfectly on one plane and poorly along two others.

Fracture• Not all minerals cleave easily, some fracture instead. • Unlike cleavages, which are usually clean, flat breaks,

fractures can be – smoothly curved– Irregular– jagged or – splintery.

• The most common types of fracture are – Conchoidal , smooth and curved (obsidian, malachite, quartz)– Fibrous or Splintery– Hackly (copper)– Uneven or – Irregular.

Reactions with other Compounds The Acid Test

• This simple test helps you tell rocks that are made of calcite (or contains calcium carbonate) from similar looking rocks made of other minerals.– All you need is some white vinegar

• Hydrochloric Acid (weak) will work better. • Put a large drop of vinegar (or a small drop of acid) on your

rock and watch what happens for a minute or so.– If the rock is made of calcite, small bubbles will start to form

inside the drop of vinegar. – Or, if you use weak acid (like in this picture), the acid will start

to fizz like crazy!

Reactions with other Compounds The Acid Test

• If your rock is made of dolomite, the bare rock won't make bubbles.

• However, if you scratch a dolomite rock into powder, the powder will make bubbles in vinegar or acid.

• Rocks such as limestone, marble, calcite, and chalk react with the acid.– Be careful to scrape a clean surface before testing

Solubility• Minerals differ in their tendency to dissolve in water. • Solubility is the amount of mineral that will dissolve in

a fixed amount of water, at a certain temperature, and is a property unique to each mineral composition.

• If a mineral in a rock dissolves, it leaves behind an open space, which increases the porosity (open space) of the rock. Permeability (ability of water to move through a rock) may be increased as minerals dissolve and connections between pores are widened.

• Example Salt is soluble because it dissolves in Water

Crystal Formation and Class• Minerals grow in specific shapes, and usually crystallize into one of six crystal systems. The axes of the crystal, the

angles at which the axes intersect, and the degree of symmetry define each system.

1. Isometric -- Also called the cubic crystal system. Crystals are usually shaped like blocks, with similar and symmetrical faces. The crystal has three axes of symmetry, all at right angles to each other, and all of the same length.Example: pyrite.

2. Tetragonal -- Typically, the crystals are shaped like four-sided prisms and pyramids. Each crystal has three axes, all perpendicular to one another. Two axes are the same length and lie on a horizontal plane. The third axis is not the same length and is at a right angle to the other two.Example: zircon.

3. Hexagonal -- These crystals are usually shaped like six-sided prisms or pyramids. Each crystal has four axes of symmetry. Three lie in the same plane, are the same length, and intersect at 120° angles. The fourth axis is not the same length, and is perpendicular to other three.Example: beryl.

4. Orthorhombic -- These crystals are short and stubby. Each crystal has three unequal axes, all at right angles to one another.Example: topaz

5. Monoclinic -- Crystals are short and stubby with tilted faces at each end. Each crystal has three unequal axes. Two axes lie in the same plane at right angles to each other. The third axis is inclined.Example: gypsum.

6. Triclinic -- Crystals are usually flat with sharp edges, but exhibit no right angles. Each crystal has three unequal axes. None are perpendicular to one another. Example: feldspar.