Cubic Fluorite Crystal

Mineral Basics What is a Mineral?

There is a classic five part definition for mineral.

Minerals: Are solid Are naturally occurring Are inorganic Have a specific chemical composition

(we call this a formula) Have a crystal structure Cubic Fluorite Crystal


Minerals are not man-made - they are produced by the natural 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. Because they are produced in laboratories, they do not meet the classic definition of a mineral.Tourmaline Crystal from Brazil

Naturally Occurring


Minerals are NOT produced by living organisms or processes.

As a result things like pearls, coral, coal and amber are not considered minerals.

Also included in the “NOT a Mineral List” are teeth, bones, sea shells and even kidney stones. Barite Rose - A flower like growth

of Barite crystals.

Inorganic

Mineral Identification Basics What is a Mineral?

Minerals form when atoms join together through electrical bonds to produce a definite internal structure or crystal.

Crystalline Pattern of Halite

Red = Sodium

Green = Chlorine

Crystal 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.

Can you SNIFC it?

SNIFC Solid Naturally Occuring Inorganic Formula Crystal

Mineral Identification Basics What is a Mineral?

The crystal structure of minerals means that there is a specific relationship in the number and type of atoms that makes up the mineral.

Minerals can be expressed by a chemical formula.

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 was rubbed across a glass plate. The result is that the glass plate was scratched. The quartz is 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 HARDNESS1. TALC2. GYPSUM3. CALCITE4. FLUORITE5. APATITE (*)

6. FELDSPAR7. QUARTZ8. TOPAZ9. CORUNDUM10. 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.


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.

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.

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 turned 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.

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 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. (*)


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

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 (*)

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 BASIC CRYSTAL SHAPES

Octahedron

Spinel

Cube

Fluorite Pyrite

Cube with Pyritohedron

Striations

TrapezohedronGarnetGarnet Garnet - Dodecahedron

These are all examples of ISOMETRIC Minerals.

(*)

Mineral Identification Basics HEXAGONAL CRYSTALS

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

(*)

Mineral Identification Basics HEXAGONAL CRYSTALS

RHOMBOHEDRON

Dolomite

SCALENOHEDRON

Rhodochrosite

QuartzHanksite

Pyramid Pyramid FaceFace

Prism Prism FacesFaces

Prism Prism FacesFaces

Pyramid Pyramid FacesFaces

Vanadinite (*)

Mineral Identification Basics TETRAGONAL CRYSTALS

WULFENITE

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

Mineral Identification Basics TETRAGONAL CRYSTALS

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

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

(*)

https://www.youtube.com/watch?v=8a7p1NFn64s


STAUROLITE (*)

Prism View (*)

Pinacoid View (*)

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

Mineral Identification Basics MONOCLINIC CRYSTALS

Gypsum Mica

Orthoclase

Top View (*)

Mineral Identification Basics TRICLINIC CRYSTALS

Microcline, variety Amazonite (*)

Cubic Fluorite Crystal

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