physical properties 3

PHYSICAL SCIENCE 120PHYSICAL PROPERTIES OF

MINERALS

As you view the Power Points you will be prompted to advance to the next slide when you see this symbol (*).

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Mineral Identification Basics What is a Mineral?

There is a classic four part definition for mineral.

Minerals must be: (*) Naturally occurring (*)

Inorganic (*)

Possess a definite crystalline structure (*)

Have a definite chemical composition (*)

Cubic Fluorite Crystal


Minerals are not synthetic - they are produced by the natural geological processes working on Earth. For example, steel, brass, bronze and aluminum are not considered minerals in that they are not found in nature. (*)

Technically speaking, synthetic gemstones are not considered minerals. This area of mineralogy has a hazy boundary in that synthetic stones are in every way the same as the natural stones. But because they are produced in laboratories, they do not meet the classic definition of a mineral. (*)

Also note that many synthetic gemstones are “doped” with a fluorescent dye to distinguish them from natural stone. (*)

Tourmaline Crystal from Brazil

Naturally Occurring (*)


Minerals are NOT produced by organic processes. As a result things like pearls, coral, coal and amber are not considered minerals.

Also included in this

“NOT a Mineral List” are teeth, bones, sea shells and

even kidney stones. (*)Barite Rose - A flower like growth

of Barite crystals.

Inorganic (*)


Minerals are the result of atoms joining together through electrical bonds to produce a definite internal structure. (*)

Crystalline Pattern of Halite

Red = Sodium

Green = Chlorine

Internal Structure

Halite (salt) from Searles Lake, CA

It is the nature of the atoms and the strength of the chemical bonds that determine many of the minerals’ physical and chemical properties. (*)

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Minerals can be expressed by a chemical formula. The internal order of minerals means that there is a definite relationship in the number of atoms that makes up the mineral. (*)

Halite - NaCl

For every atom of Sodium there is an atom of Chlorine.

Definite Chemical Composition (*)

Mineral Identification Basics PHYSICAL PROPERTIES HARDNESS

HARDNESS is defined as the resistance a mineral has to being scratched - its “scratchability”. Hardness tests are done by scratching one mineral against another. The mineral that is scratched is softer than the other. (*)

Pyrite Crystals

Hardness of 6.5 (*)


In this photo, a quartz crystal will be rubbed across a glass plate. The result is that the glass plate will be scratched. The quartz is therefore harder than the glass. (*)

Quartz is harder than glass.

HINT: In doing a hardness test try to pick a smooth or flat surface on the mineral to be scratched. Try to pick a point or a sharp edge on the mineral that you think will do the scratching. Glass is usually a good place to start because it is in the middle of the hardness table, it has a flat, smooth surface and it is easily obtained. (*)


Care must be taken on some minerals that crumble easily. Remember that hardness is the resistance a mineral has to being scratched - NOT how easily it breaks apart. The physical property related to the ease in which a mineral breaks is tenacity. (*)

Also be sure to determine the hardness of a mineral on a fresh surface whenever possible. Some minerals have a tendency to oxidize or corrode. These surface deposits usually have a different hardness than the fresh mineral. (*)

Moh’s scale is a list of minerals with increasing hardness.(*)


MOH’S SCALE OF MINERAL HARDNESS

1. TALC

2. GYPSUM

3. CALCITE

4. FLUORITE

5. APATITE (*)

6. FELDSPAR

7. QUARTZ

8. TOPAZ

9. CORUNDUM

10. DIAMOND (*)

OTHER MATERIALS COMMONLY USEDOTHER MATERIALS COMMONLY USED::

2.5 - FINGERNAIL2.5 - FINGERNAIL 3 - COPPER PENNY 3 - COPPER PENNY

5.5 - GLASS 6-6.5 - STEEL FILE5.5 - GLASS 6-6.5 - STEEL FILE(*)

Mineral Identification Basics PHYSICAL PROPERTIES CLEAVAGE

CLEAVAGE is the property of a mineral that allows it to break repeatedly along smooth, flat surfaces. (*)

These GALENA cleavage fragments were produced when the crystal was hit with a hammer. Note the consistency of the 90o angles along the edges. (*) These are FLUORITE cleavage

fragments. (*)


Within this crystalline pattern it is easy to see how atoms will separate to produce cleavage with cubic (90o) angles. (*)

It is similar to tearing a piece of paper that has perforations in it. The paper has a tendency to tear along the perforations. They are zones of weakness. (*)

In this example the lines represent breaks between the atoms that make up the mineral. Cleavage is guided by the atomic structure. (*)


These pictures show different cleavage angles and the quality of cleavage.

Fluorite has cleavage in four directions. (*)

A thin sheet of Muscovite seen on edge.

Mica has perfect cleavage in ONE direction. (*)


Common salt (the mineral HALITE) has very good cleavage in 3 directions. (*)

These 3 directions of cleavage are mutually perpendicular resulting in

cubic cleavage. (*)


Rhombohedral Cleavage - 3 directions

CALCITEEven these tiny fragments have rhombohedral cleavage. (*)

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Blocky Cleavage 2 directions

Orthoclase Feldspar (*)

Orthoclase feldspar has good cleavage in 2 directions.

The blocky appearance of this specimen is a hint that it has cleavage. The clue that the specimen has cleavage is the fact that numerous faces will reflect light at the same time. Each face is parallel and light will reflect of each face producing a flash of light. (*)

Note that the faces in the circle are at different levels. By adjusting the lighting, all of the parallel faces will reflect simultaneously. This results in a flash of light from all the parallel faces. (*)


TALC has micaceous cleavage. That is to say that it cleaves like mica (1 perfect direction) but, in talc the crystals are so small that they cannot easily be seen. Instead the effect is that the talc “feels soapy”. The second picture shows some of the talc that has cleaved onto the fingers. (*)

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Mineral Identification Basics PHYSICAL PROPERTIES FRACTURE

FRACTURE is defined as the way a mineral breaks other than cleavage. (*)

This is a piece of volcanic glass called OBSIDIAN. Even though it is NOT a mineral, it is shown here because it has excellent conchoidal fracture. (*)

If you try this yourself, use caution. Conchoidal fracture in obsidian can produce extremely sharp edges. (*)

Mineral Identification Basics PHYSICAL PROPERTIES FRACTURE

This Quartz crystal will be struck with a hammer to show how that the external form of the crystal does not repeat when broken. (The flat crystal faces are not cleavage faces.)

This is a good example of conchoidal fracture. (*)

Note the smooth curved surfaces. (*)

Mineral Identification Basics PHYSICAL PROPERTIES STREAK

STREAK is defined as the color of the mineral in powder form. (*)

Hematite on Streak Plate

Streak is normally obtained by rubbing a mineral across a “streak plate”. This is a piece of unglazed porcelain. The streak plate has a hardness of around 7 and rough texture that allows the minerals to be abraded to a powder. This powder is the streak. (*)

Hematite has a reddish brown streak. (*)

Mineral Identification Basics PHYSICAL PROPERTIES STREAK

Sphalerite is a dark mineral, however, it has a light colored streak. Next to the reddish brown streak of hematite is a light yellow streak. This is the streak of the sphalerite. (*)

Sphalerite has a light yellow streak. (*)

Light colored streaks are often difficult to see against the white streak plate. It is often useful to rub your finger across the powder to see the streak color. (*)

Mineral Identification Basics PHYSICAL PROPERTIES LUSTER

LUSTER is defined as the quality of reflected light. Minerals have been grossly separated into either METALLIC or NON-METALLIC lusters.

Following are some examples: (*)

Native Silver has a Metallic Luster. (*)

The basic idea for Metallic Luster is that the minerals look like metals. (*)

Mineral Identification Basics PHYSICAL PROPERTIES LUSTER METALLICLUSTER METALLIC

Stibnite Galena

Marcasite Pyrite (*)

Vitreous Luster means that the mineral has a “glassy” look. Normally we think of glass as being clear, but there are many different colors of glass and they are all very “glassy” looking. Even china plates and glazed porcelain are vitreous. Here are some examples: (*)

Mineral Identification Basics NON-METALLICNON-METALLIC LUSTER VITREOUSLUSTER VITREOUS

Olivine - Peridot Wulfenite

Spinel Quartz (*)

Mineral Identification Basics NON METALLICNON METALLIC LUSTER

Miscellaneous Lusters

Asbestos - Silky Apophyllite – Pearly (*)

Limonite - Dull or Earthy Sphalerite - Resinous

Graphite has a greasy or submetallic luster and easily marks paper. (*)

Mineral Identification Basics PHYSICAL PROPERTIES LUSTER

This piece of Native Copper is severely weathered. It does not

look metallic. (*)

This is the same piece but the left side has been buffed with a steel brush. Note the bright metallic

luster. (*)

The moral to this story is to look at a

fresh surface whenever possible.

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The moral to this story is to look at a

fresh surface whenever possible.

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Mineral Identification Basics PHYSICAL PROPERTIES COLOR

The COLOR of a mineral is usually the first thing that a person notices when observing a mineral. However, it is normally NOT the best physical property to begin the mineral identification process. (*)

Following are some examples of color variation within mineral species followed by minerals that have a distinctive color: (*)

Various colors of CALCITE. (*)

Clear - Without Impurities

Mineral Identification Basics PHYSICAL PROPERTIES COLOR

Various colors of Quartz.

Hematite Inclusions Chlorite inclusions

Amethyst

Ionic Iron

Quartz comes in a wide range of colors. It is very easily colored by even trace amounts of impurities. (*)

Some minerals do have a certain color associated with them. Here are some examples: (*)

Mineral Identification Basics INDICATIVE COLOR

Turquoise

SulfurMalachite

RhodochrositeAzurite (*)

Mineral Identification Basics PHYSICAL PROPERTIES SPECIFIC GRAVITY

The SPECIFIC GRAVITY of a mineral is a measure of the mineral’s density. It is related to the types of elements that make up the mineral and how they are packed into the mineral’s atomic structure. (*)

Gold in Quartz

Gold has a Specific Gravity of 19.2. It is 19.2 times the weight of an equal volume of water. Water has a Specific Gravity of 1. (*)


The SPECIFIC GRAVITY of a mineral is determined by weighing the specimen in air and then weighing it in water. Here is the formula: (*)

(Weight in air) - (Weight in water )

Weight in airSpecific Gravity =

(*)

(divided by)


Selecting the right material. (*)

Sphalerite

Opal in Rhyolite

Calcite with Garnet

HaliteLimonite

Not just any mineral will do. In determining the specific gravity of a mineral it must be pure, free of pockets or cracks (places that can trap air) and it should not easily dissolve in water. (*)


Sphalerite

Opal in Rhyolite

Calcite with Garnet

HaliteLimonite

The Limonite is full of pore spaces. It is almost like a sponge. When it is weighed in water it has numerous trapped air pockets that will make it lighter that it should be. (*)

It would be difficult to get an accurate weight. (*)


Sphalerite

Opal in Rhyolite

Calcite with Garnet

HaliteLimonite

This is not a pure specimen. It is a combination of two minerals. The result of the specific gravity process would only give you an average of the two minerals. (*)


Sphalerite

Opal in Rhyolite

Calcite with Garnet

HaliteLimonite

The opal in rhyolite has the same problem as the calcite with garnet. It is not a pure sample (*)


Sphalerite

Opal in Rhyolite

Calcite with Garnet

HaliteLimonite

Halite is a salt. When weighed in water it dissolves. It would be difficult to get an accurate reading as it would become lighter and lighter as it slowly dissolved. (*)


Sphalerite

Opal in Rhyolite

Calcite with Garnet

HaliteLimonite

Sphalerite (pronounced: sfal er ite) is a good choice. It is a pure sample with no crack or pore spaces. And, it does not dissolve in water. (*)

Sphalerite


Weight in Water

The weights are in the same place but now that the sphalerite is submerged in water it is lighter, and the balance is again out of balance. (*)


Weight in Water

It is important to note that the specimen being weighed is not resting on the bottom of the beaker or touching its sides. It is also completely submerged beneath the water. (*)


Weight in Water (*)

0 grams

0.94 grams

7 grams20 grams

The weight of the sphalerite in water is 27.94 grams. (*)






37.00 grams

27.94 grams37.00 grams

4.06Specific Gravity = Note that there are no units. The grams cancel out. This is a ratio of how heavy the mineral is compared to an equal volume of water. The sphalerite is 4.06 times heavier than water. (*)

Mineral Identification Basics PHYSICAL PROPERTIES TASTE

IT IS NOT RECOMMENDED THAT A TASTE TEST BE PERFORMED ON MINERALS AS A STANDARD PROCESS. SOME MINERALS ARE TOXIC.

However, the mineral HALITE is common salt and has a unique taste. (*)

Halite cubes from Trona, CA (*)

Mineral Identification Basics PHYSICAL PROPERTIES MAGNETISM

MAGNETISM is the ability of a mineral to be attracted by a magnet. This most commonly is associated with minerals rich in

iron, usually magnetite. (*)

This is a piece of MAGNETITE with a magnet adhering to it. Magnetite is a mineral that is strongly magnetic in that a magnet will easily be attracted to it. (*)


More sensitivity is achieved if instead of a large sample, small pieces are used. In this way, even weakly magnetic minerals will be attracted to the magnet. (*)


This is a sample of “black sand” from Lynx Creek, Arizona. Its dark color is due to its high concentration of magnetite. See what happens when a magnet is place beneath the bottom right portion of the paper. (*)

This technique is used to separate out much of the unwanted material in the search for gold in placer deposits. (*)


LODESTONE is a variety of Magnetite that is naturally a magnet. (*)

Mineral Identification Basics DOUBLE REFRACTION

DOUBLE REFRACTION: Is a property shared by many minerals ( but not those in the isometric crystal system). It is produced by the separating of a beam of light as it passes through the crystal structure. It is best displayed in the mineral CALCITE. This image clearly shows the double image below the calcite. (*)

Mineral Identification Basics CHEMICAL PROPERTIES

REACTION TO HYDROCHLORIC ACID

Some minerals, notably the carbonates, react to cold dilute HCl. In this illustration a piece of CALCITE is shown to react (fizz) after HCl is applied. (*)

Calcite Reacts to HCl (*)

This ends the basic introduction to the Physical Properties of Minerals. The rest of this presentation takes you through a few other properties

as well as crystal structures of minerals. Feel free to take a look.

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Mineral Identification Basics PHYSICAL PROPERTIES DIAPHANEITY

The manner in which minerals transmit light is called DIAPHANEITY and is expressed by these terms: (*)

TRANSPARENT: A mineral is considered to be transparent if the outline of an object viewed through it is distinct. (*)

TRANSLUCENT: A mineral is considered to be translucent if it transmits light but no objects can be seen through it. (*)

OPAQUE: A mineral is considered to be opaque if, even on its thinnest edges, no light is transmitted. (*)

Quartz with Spessartine Garnets


TRANSPARENT: A mineral is considered to be transparent if the outline of an object viewed through it is distinct. (*)

Topaz from Topaz Mountain, Utah (*)


Sylvite from Salton Sea, California (*)

TRANSLUCENT: A mineral is considered to be translucent if it transmits light but no objects can be seen through it. (*)

Backlit Apophyllite Crystals (*)


Schorl - The black variety of Tourmaline (*)

OPAQUE: A mineral is considered to be opaque if, even on its thinnest edges, no light is transmitted. (*)

Mineral Identification Basics PHYSICAL PROPERTIES CRYSTALS

A CRYSTAL is the outward form of the internal structure of the mineral.

The 6 basic crystal systems are: (*)

ISOMETRICHEXAGONAL

TETRAGONALORTHORHOMBIC

MONOCLINICTRICLINIC (*)Drusy Quartz on Barite

Mineral Identification Basics PHYSICAL PROPERTIES CRYSTALS

ISOMETRIC - Fluorite Crystals

The first group is the ISOMETRIC. This literally means “equal measure” and refers to the equal size of the crystal axes. (*)

Mineral Identification Basics ISOMETRIC CRYSTALS

ISOMETRICIn this crystal system there are 3 axes. Each has the same length as indicated by the same letter “a”.

They all meet at mutual 90o angles in the center of the crystal.

Crystals in this system are typically blocky or ball-like. (*)ISOMETRIC

Basic Cube

a3

a2

a1


ISOMETRIC Crystal Model (*)

Within this ISOMETRIC crystal model is the OCTAHEDRAL crystal form (yellow) and the TETRAHEDRAL crystal form (shown by the black lines). (*)


ISOMETRIC - Basic Cube (*)

a1

a3

a2

a3

a2

a1

Fluorite cube with crystal axes. (*)

Mineral Identification Basics ISOMETRIC BASIC CRYSTAL SHAPES

Octahedron

Spinel

Cube

Fluorite Pyrite

Cube with Pyritohedron

Striations

TrapezohedronGarnetGarnet Garnet - Dodecahedron

These are all examples of ISOMETRIC Minerals.

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Mineral Identification Basics HEXAGONAL CRYSTALS

HEXAGONAL - Three horizontal axes meeting at angles of 120o and one perpendicular axis. (*)

a1

a2

a3

HEXAGONAL Crystal Axes

c


HEXAGONAL Crystal Model (*)

HEXAGONAL

This model represents a hexagonal PRISM (the outside hexagon - six sided shape). The top and bottom faces are called PINACOIDS and are perpendicular to the vertical “c” axis.

Within this model is the SCALENOHEDRAL form. Each face is a scalenohedron. Calcite often crystallizes with this form. As the model rotates, the flash of light seen is from a scalenohedral face.(*)


These hexagonal CALCITE crystals nicely show the six sided prisms as well as the basal pinacoid. (*)

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RHOMBOHEDRON

Dolomite

SCALENOHEDRON

Rhodochrosite

QuartzHanksite

Pyramid Pyramid FaceFace

Prism Prism FacesFaces

Prism Prism FacesFaces

Pyramid Pyramid FacesFaces

Vanadinite (*)

Mineral Identification Basics TETRAGONAL CRYSTALS

TETRAGONAL

Two equal, horizontal, mutually perpendicular axes (a1, a2) (*)

TETRAGONAL Crystal Axes

a1a2

c

c

a2a1

This is an Alternative Crystal Axes (*)

Vertical axis (c) is perpendicular to the horizontal axes and is of a different length. (*)


TETRAGONAL Crystal Model (*)

TETRAGONAL

This model shows a tetragonal PRISM enclosing a DIPYRAMID. (*)


WULFENITE

Same crystal seen edge on.Same crystal seen edge on. (*)


APOPHYLLITE (clear) on Stilbite (*)

This is the same Apophyllite crystal looking down the “c” axis.

The red square shows the position of the pinacoid (perpendicular to the “c” axis). (*)

C axis line

Mineral Identification Basics ORTHORHOMBIC CRYSTALS

ORTHORHOMBIC

Three mutually perpendicular axes of different lengths. (*)

ORTHORHMOBIC Crystal Axes

ab

c

a

c

b

An Alternative Crystal Axes Orientation (*)


ORTHORHMOBIC Crystal Model (*)

ORTHORHOMBIC

This model shows the alternative axes where the vertical “c” axis is not the longest axis. (*)

The model shows the outside “brick” shape of the PRISM and the inner shape is a DIPYRAMID. The top and bottom faces are called PINACOIDS and are perpendicular to the “c” axis. (*)


Topaz from Topaz Mountain, Utah. (*)


The view above is looking down the “c” axis of the crystal. (*)

C axis

B axis

A axis

BARITE is also orthorhombic. (*)

C axis

A axis

B axis

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STAUROLITE (*)

Prism View (*)

Pinacoid

View (*)

This is a Staurolite TWIN with garnets attached. (*)

Mineral Identification Basics MONOCLINIC CRYSTALS

MONOCLINIC

In this crystal form the axes are of unequal length. (*)

MONOCLINIC Crystal Axes

a

b

c

But a and c make some oblique angle and with each other. (*)

Axes a and b are perpendicular. (*)

Axes b and c are perpendicular. (*)


MONOCLINIC Crystal Model

MONOCLINIC

In this model the outside shape is the PRISM. It looks like a distorted brick - flattened out of shape.

Inside is the DIPYRAMID. (*)


Gypsum Mica

Orthoclase

Top View (*)

Mineral Identification Basics TRICLINIC CRYSTALS

TRICLINIC

In this system, all of the axes are of different lengths and none are perpendicular to any of the others. (*)

TRICLINIC Crystal Axes

a b

c


TRICLINIC Crystal Model (*)

TRICLINIC

Again in this model the outside shape is the PRISM.

Located within the prism is the DIPYRAMID. (*)


Microcline, variety Amazonite (*)

Mineral IdentificationMineral Identification RESOURCESRESOURCES

http://www.gc.maricopa.edu/earthsci/imagearchive/index.htm

For lots of useful images of minerals

and more facts about minerals,

check out this web site:

http://www.gc.maricopa.edu/earthsci/imagearchive/minerals.htm












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