Mineral and Rock Properties

©2010 Dr. B. C. Paul

Note most of the material for these slides was collected from a wide range of available sources and do not represent the organizers original work.

Difference Between Minerals and Rocks

• Mineral has the same composition (atoms of different kinds) and structure (arrangement of those atoms) all the way through.– About 3,000 known minerals

• Thank goodness you will not be ask to learn them all – We are interested in the ore minerals

• Rock is made up of 2 or more different minerals– Remember your igneous rocks had more or less of

the dark minerals depending on the iron content

Destructive and Non Destructive

• Many characteristics depend on fresh unweathered surfaces to see– If you try breaking the class samples I’ll get irate– I’m not going to push things like crystal form because

a lot of ore deposits are massive or crystal structures are very small

• Are tests like looking at color and luster, feeling how heavy it is, or checking with a magnet or fluorescence that do not hard the specimen

Rock Density

• Some Rocks are Heavy for their Size – Some are Light

• Density or Specific Gravity– Water at near freezing temperature is given a

specific gravity of 1 (62.428 lbs/cubic foot)– Specific Gravity of a Mineral is Ratio of the

weight of 1 unit of the mineral to one unit of water

Examples

You can use this to help clue you in on what mineral you may have byHow heavy it is.

Hardness

• How Easy it is to scratch and what will scratch it.

• Mohs Hardness ScaleTalc =1

Gypsum =2--fingernail at 2.5

Talc

Gypsum

More Hardness

Calcite =3--copper (old penny) at 3.5

Mohs Hardness Scale Continued

Fluorite =4Apatite =5

--window glass or typical knife blade at under 5.5

Fluorite

Apatite

The Hardness Scale

Orthoclase--streak plate or good steel file at over 6.5

Really Hard Stuff

Quartz =7Topaz = 8

Corundum = 9Diamond = 10

Quartz Topaz

Corundum

Diamond

Mohs Hardness Scale is Relative – Not Absolute

ColorActually less useful than you would hope because you can throw mineral colorsA great deal with simple impurities – can be a hint when used with how shiny orClear it is. Also some things can have pretty distinctive colors

Streak

Streak is closely related to color, but is a different property because the color of the mineral may be different than the color of the streak. Streak is actually the color of the powder of a mineral. It is called streak because the proper way to test for streak is to rub a mineral across a tile of white unglazed porcelain and to examine the color of the "streak" left behind. It has proven to be a powerful property because it is generally very consistent from specimen to specimen for a given mineral.

Its kind of a tie breaker because only 20% of minerals have a unique streak(but a lot of those that do are ore minerals)

When Exposed to Short Wave Light Many Minerals Develop Unique

Glows

Property is Called Luminescence

• Minerals contain certain elements that are called “activators”

• Electrons in the activator element can be bounced up into higher electron shells– When they drop back down they put out a

unique colored glow.

• If the something that bounces electrons up into higher shells is light we call it “Fluorescence”

The Light Spectrum

Most Fluorescence is to light or the wrong color for us to see

Real shortWave stuffWill makeYou glow inThe dark too(of courseYou’ll also die)

Things that Can Build Distinct Fluorescence

• X-Rays are great

• (of course X-Ray machines aren’t very cheap and you really can’t be around them much – back to the you glow and die problem)

• Ultraviolet Light Will Work on a Lot of Minerals.

UV Light

We call this Black Light (probably have had some fun with it)

It will activate about 15% of FluorescentMinerals

The Short and Midrange UV will activate90% (and also Sun-Burn you)

UVAUVBUVC

Some Typical Fluorescent Minerals

More Fluorescence

Magnetic

• Not a lot of minerals attracted to magnet

• Most of them that are are ore minerals

• Many minerals are only weakly magnetic so it takes a fairly impressive magnet to pull them