Mineral Identification Lecture 

                                    Illustrated                     
Cochise College
Geology Home Page 
Roger Weller, geology instructor  ( wellerr@cochise.edu )                

last edited:  12/15/15     copyright 2015-R.Weller 


transparent gypsum (selenite)
                         

How can minerals be identified?
   
Because minerals are defined as specific chemicals with a crystalline
    structure, they have both distinctive chemical and physical properties.
   
Mineral identification video

chemical properties
   
Geologists rarely use chemical properties of minerals for identification purposes,
    unless it is a rare mineral.


    The one exception is the use of hydrochloric acid for distinguishing between
     calcite and dolomiteThese two minerals fizz (produce gas bubbles) when cold
     hydrochloric acid is applied to them.  Calcite fizzes profusely and quickly, while
     dolomite fizzes slowly.
 
video of reaction of calcite to HCl
     (The rate of reaction is also controlled by the type of surface; porous substances
     have greater surface area and thus produce more bubbles).


physical properties
    Physical properties deal with the manner in which the atoms are bound within
    the crystal structure or how the mineral reacts with light.


hardness
   
Hardness is one of the most common properties used to identify minerals. 
    Hardness refers to the ability of a mineral to resist being scratched or its ability
    to scratch other materials. Mohs Scale of Hardness video


   
Mohs Scale of Hardness goes from 10 (hardest) to 1 (softest)
       
10- diamond     -diamond hardness-Opal's Pals
        
9- corundum
        
8- topaz
        
7- quartz

         6- feldspar
        
5- apatite      -apatite hardness-Opal's Pals

         4- fluorite

         3- calcite

         2- gypsum

         1 -talc           -talc hardness-Opal's Pals

 

    Glass and steel are about 5.5 on this scale and a fingernail is about 2.5.
 

toughness
    Toughness is not the same thing as hardness. 
    Toughness refers to the ability to take a beating without breaking. 
    There are many materials, such as diamond, that are very hard but are not tough;
    they are brittle. 
    Many physical properties of minerals are based on what happens when you try
    to damage a mineral with force.
        cleavage

        fracture

        flexibility

        elasticity

        malleability
        friability

        ductility

 

cleavage
   
Because the atoms in some minerals are arranged in distinct, widely separated planes,
    the mineral will break along smooth, flat surfaces parallel to these planes of atoms. 
    The directions of these flat surfaces are called cleavage planes and the smooth flat
    surfaces created  by cleaving are cleavage surfaces.
    1 direction of cleavage

        mica     mica and gypsum cleavage video

        gypsum (selenite)

        topaz
   
2 directions of cleavage

        feldspar

        augite

        hornblende

    3 directions of cleavage

        halite (salt)   halite (salt) cleavage video
        calcite   calcite cleavage video
        galena

    4 directions of cleavage

        fluorite
        diamond   diamond cleavage video

 

fracture
   
Fracture describes a broken surface that is not cleavage.
        fracture-Opal's  Pals     video on fracture
   
irregular fracture- the most common type of fracture

        cinnabar

        sodalite

    conchoidal fracture

        rose quartz

        sulfur
        conchoidal fracture-Opal's Pals

    fibrous fracture

        asbestos   asbestos video

    splintery fracture

        natrolite

 

flexibility

    Flexible means that the material can be bent.  
    As might be expected, most mineral cannot be bent without breaking.

    The most flexible mineral is mica.
     If pressure is applied slowly both selenite and stibnite can be bent.

elasticity

    A mineral is elastic if it can be bent without breaking and when released,
    it will snap back to its original shape.

    Again, as expected, most minerals are not elastic.

    Mica is flexible and elastic.

    Selenite and stibnite, once bent, stay bent.

malleability

    Metals are malleable; they can be pounded flat with a hammer without breaking.

       malleability of gold video
    Naturally occurring malleable metals

        gold

        silver

        copper

friability
     A rock or mineral that crumbles easily is said to be friable.
        -friability-Opal's Pals

 

appearance

    There are many physical properties in this category that deal with the interaction of
    minerals with light.
        color

        transparency

        luster

        streak

        fluorescence

 

color

    White light is made up of a wide range of frequencies of light. 
    Each frequency represents a different color. 
    The color of a mineral is determined by the collection of light frequencies that are not
    absorbed by the mineral. 
    The color of a mineral is determined by the frequencies of light that are either reflected
    off the surface of the mineral or those frequencies which pass through the mineral without
    being absorbed.

    Many minerals possess distinct colors which aid in the identification of a mineral. 
    However, impurities, such as the presence of iron oxides, can alter the color of a mineral,
    often drastically.

    yellow minerals

        sulfur     video on sulfur crystals

        orpiment     video on orpiment

        citrine

        carnotite

    red minerals

        cuprite

        cinnabar      video on cinnabar

        corundum-variety ruby

        crocoite
        realgar
        vanadinite
   
blue minerals

        azurite      video on azurite

        sodalite

        aurichalcite

        turquoise

        chrysocolla

    green minerals

        malachite      video on malachite

        jade

        dioptase

        beryl-variety emerald
           epidote

    purple minerals

        quartz-variety amethyst

        fluorite       purple fluorite video

    pink minerals

        rhodochrosite

        rhodonite

        quartz-variety rose quartz 

    brown minerals

        quartz-variety smoky quartz

        barite

    black minerals

        graphite       video on graphite pencils       video on graphite

        pyrolusite

    white minerals

        feldspar-variety albite

        wollastonite

    colorless minerals

        quartz-variety rock crystal

        gypsum-variety selenite

        topaz

 

    Some minerals occur in a wide range of colors, sometimes even in the same specimen.

    multicolored minerals
        fluorite

        tourmaline

        tourmaline, polished cabochons

 

transparency

    A mineral that allows light to pass through, almost unimpeded, is called transparent. 
    Window glass is transparent.  Most faceted, high quality gemstones are transparent.

       -transparent-Opal's  Pals

        transparent minerals

        topaz

        halite

        gypsum-variety selenite , really transparent gypsum

    When some light can come through thin pieces of a material and it has a cloudy
    appearance the mineral is translucent.

       -translucent-Opal's Pals

        translucent minerals

        jade

        fluorite

        corundum-variety ruby
           amazonite cleavage

    When no light can pass through even a thin piece of a mineral, the material is opaque. 
    Minerals with a metallic luster are usually opaque.

       -opaque-Opal's Pals

        opaque minerals

        galena

        stibnite

        graphite

 

luster

    Luster describes the appearance of the surface of a mineral. 
    The greatest difference between groups of minerals is that some have a metallic luster
    while most minerals have a non-metallic luster.
        video on luster

    metallic luster

        galena

        stibnite

        pyrite
        bornite

    non-metallic lusters

        adamantine (extremely shiny)- diamond

        glassy (vitreous)- quartz

        silky- asbestos
       
pearly- talc  heulandite
       
resinous- sphalerite, amber

        waxy- common opal

        dull- brucite

        earthy- kaolinite

 

streak

    Streak is the term used for the color of the powdered form of a mineral. 
    The powder is obtained by dragging the mineral across a rough, white ceramic tile.
    The three common iron oxides all produce different colored streaks.
        mineral streak video

        hematite-reddish brown streak

        limonite-brownish yellow streak

        magnetite-black streak

        Native gold (real gold) can be separated from pyrite (fools gold) by a streak test. 
        Native gold leaves a yellow streak, while pyrite leaves a greenish black streak.

 

fluorescence      (visit Fluorescent Minerals(also, fluorescence-Opal's Pals)
        mineral fluorescence video

    Ultraviolet light is just like regular light except that it has a higher frequency and cannot be
    seen by the human eye.  Certain minerals have the ability to absorb ultraviolet light and the
    emit light at a lower frequency within the visible spectrum; this property is called fluorescence.

    To observe this phenomenon, you need a source of ultraviolet light; these are commonly called
    black lights.  The room needs to be darkened because the effect is often not very strong.

    Only a small number of minerals are fluorescent.  The most fluorescent of all minerals come
    from Franklin, New Jersey.

    fluorescent minerals (the following examples were observed under ultraviolet light)

        aragonite

        willemite

        fluorite

 

crystal form       (visit Crystals and Crystal Models)
    If minerals are allowed to crystallize in an unrestricted environment, they will form geometric
    shapes with flat surfaces; these forms are called crystals. 
    The flat surfaces run parallel to planes of atoms located within the crystalline material.

    Since each mineral is a unique chemical compound, each mineral prefers to grow in specific forms. 
    These crystal forms can be used to identify a mineral.
        crystalline structure video

 

    classic crystal shapes

        quartz-six-sided prism capped by a pyramid
        beryl-six-sided prism with a flat top (basal pinacoid)

        galena-cubic crystal

        fluorite-cubic crystal

        magnetite-octahedron

        spinel-octahedron

        calcite-scalenohedron

        pyrite-pyritohedron

        garnet-dodecahedron
        garnet-trapezohedron

       

mineral habit

    Some minerals are characterized by the way in which they commonly crystallize.

    acicular (needle-like): natrolite, cuprite-variety chalcotrichite
   
botryoidal (grape-like): malachite, azurite, siderite, mimetite, marcasite

    drusy (mass of small, sparkly crystals): azurite, sulfur

    radiating: wavellite, brochantite

    banded (layered): malachite, fluorite, quartz-variety agate

    arborescent (tree-like): copper

    dendritic (plant-like): pyrolusite/psilomelane

 

density   

    Density is the measure of the mass of an object compared to its volume. 
    Density can be expressed as grams per cubic centimeter or kilograms per cubic meter. 
    Density can be calculated by first measuring the volume of an object by the amount of
        water it displaces in a container and then compared to its mass.

    Many ore minerals are noticeably denser than common rock.

    Specific gravity is another way of describing the density of a mineral. 
    Instead of specifying mass per volume, the density of the mineral is compared to the density
        of water, which is established as 1.0. 
    Normal rock materials, like quartz and calcite, have specific gravities in the 2.6 to 2.7 range.

    Minerals such as cinnabar, scheelite, and galena have densities much greater
        than most common minerals.
       density-Opal's Pals        
video on density

    

magnetism     

    There is only one mineral that is both commonly occurring and strongly magnetic, magnetite.

    If you tie a string to a small, very strong magnet an drag it along the ground, it will often
    collect small, black particles of magnetite.     
 video on magnetism and magnetite
       magnetic-Opal's Pals

 

radioactivity

    Most minerals are not even slightly radioactive. 
    However, if there is uranium or thorium present within the mineral, it will be radioactive.

       Carnotite is a common greenish-yellow mineral.
       radioactivity-alpha decay-Opal's Pals

 

Does it burn?

    There is only one mineral that readily burns, a transparent to translucent lemony-yellow mineral
    known as sulfurThe fumes from burning sulfur smell like a burnt match.
        sulfur dioxide-Opal's Pals

 

taste and smell

    Tasting minerals it not recommended; you might be unknowingly licking an arsenic mineral.

    Taste can be used to distinguish halite (sodium chloride) from sylvite (potassium chloride);
    sylvite has a more bitter taste than halite (table salt).
        halite and sylvite-Opal's Pals

    Most minerals do not smell.  A fresh broken surface of chrysocolla may produce a strange,
    sweet smell.

        Marcasite produces a sulfur smell as it decomposes.

 

mineral associations
   
Many minerals can be identified by the company they keep with other minerals because
    they form in the same type of environment.

    minerals commonly occurring together

        azurite and malachite

        chalcopyrite and dolomite

        orpiment and realgar

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