Rocks & Minerals

A mineral is

  • Naturally occurring
  • Inorganic
  • Definite chemical composition & crystalline structure
  • Solid

All physical properties of minerals come from the “internal arrangement of atoms”

Mineral Identification Tests

The Color Test- easiest test to do but not always reliable

The Streak Test

  • The color of the powdered mineral.
  • Performed by rubbing the unknown mineral on an unglazed tile.

The Luster Test

  • the way a mineral shines or doesn't shine
  • the only way to really learn the different lusters is to see them for yourself.

Types of Luster

  • Metallic- looks like shiney metal
  • Non-metallic- all the other ways that a mineral can shine
    • Glassy/vitreous- shines like a piece of broken glass (most common non-metallic)
    • Dull/earthy- no shine at all
    • Resinous/waxy- looks like a piece of plastic or dried glue
    • Pearly- looks oily it may have a slight rainbow like an oil slick on water. Also looks like the inside of some clam shells
    • Adamantine- brilliant, sparkling shine like a diamond

Hardness- a minerals resistance to scratching. This should not be confused with brittleness. A diamond is very hard and will scratch a hammer but a hammer will smash a diamond. Likewise, talc, one of the softest minerals, is not squishy. It will still put a serious hurting on you if you get hit in the head with it.

Moh’s Scale of Hardness

  1. Talc (Softest)
  2. Gypsum
  3. Calcite
  4. Fluorite
  5. Apatite
  6. Feldspar (AKA Albite)
  7. Quartz
  8. Topaz
  9. Corundum
  10. Diamond (Hardest)

Key Points of a Hardness Test

  • Choose one mineral to be the scratcher and one to be the scratchee.
  • Pick a smooth, flat surface to scratch.
  • After doing the test, wipe the powder away to confirm that the scratchee really got scratched.
  • If the scratchee did not get scratched, switch the two rocks and repeat.

Hardness Tools

  • Fingernail 2.5
  • Penny 3.5
  • Iron Nail 4.5
  • Glass Plate 5.5
  • Steel File 6.5
  • Streak Plate 7

Cleavage -To break along flat surfaces.

Examples of Cleavage

  • Cubic- To break into cubes

  • Rhombihedral- to break into “pushed over cubes”

  • Basal- to split into thin sheets

Fracture -The way a mineral without cleavage breaks.

Examples of Fracture

  • conchoidal- to break in a scooped out bowl shape- like a conch (sea snail)
  • hackly fracture- to have irregular sharp edges
  • splintery- to break into long, thin needles

Miscellaneous Tests

  • Acid- Calcite and powdered dolomite will effervesce (fizz) in dilute hydrochloric acid (HCl)
  • Smell- Sphalerite will give off a rotten-egg smell when streaked on a streak plate. (Note: pure sulfur does not smell like rotten eggs!)
  • Magnetism- Magnetite (AKA Lodestone) will pick up paper clips (weak samples will only be able to pick up staples)
  • Taste- Halite is rock salt and will taste salty. *Do not taste the samples since some have been tested with acid to see if it is calcite.
  • Fluorescence- some minerals (mostly forms of calcite) will glow in fluorescent colors under a black (UV) light.

Roll your mouse over to see the change from visible to Fluorescent

  • Double refraction- some clear forms of calcite (Iceland Spar) will make a double image of words.

A little about Quartz:

  • One of the most abundant minerals in the world (Quartz and feldspar fight for #1)
  • Makes beach sand
  • Makes glass (melted beach sand)
  • Chemical formula: SiO2
  • Since it is SiO2, there is twice as much oxygen as silicon.
  • According to the Reference Tables pg 11, Oxygen is the #1 element in the crust with Silicon #2 (and about half that of Oxygen)
  • Crystal shape is a pyramid called a “tetrahedron”
    • Tetra = 4
    • hedron =“sided solid


A rock is a mixture of one or more minerals. They are classified by the way that they are made.

Monominerallic- a rock made of only one mineral. In this case, the rock is both a rock and a mineral.

Polyminerallic- a rock made from more than one mineral.

Rock Cycle

Igneous Rocks

  • “Fire Formed”- melted rock material cools and solidifies (“freezing”)
  • Intrusive- rock formed inside the Earth
  • Extrusive- rock formed on the surface
  • Texture- the size of the crystals- NOT HOW IT FEELS
INtrusive Extrusive
Rocks INSIDE the Earth Rocks OUTSIDE the Earth
Plutonic Volcanic
Formed from magma Formed from lava
Usually dark Usually light colored
Usually dense Usually low density (light)
Mafic: (magnesium and Iron (Fe) Felsic: feldspar (aluminum)
Cools slowly Cools quickly
Large grains Small or no grains (fine or glassy)

Igneous rocks have “Intergrown Crystals”



Not Intergrown...

Sedimentary Rocks- Made from sediments or rock material that has been broken down in some way.

  • Sedimentary rocks are usually formed in a watery environment.
  • Often layered
  • Are the only rocks that normally contain fossils

Types of Sedimentary Rocks

  • Clastic (fragmental) –made by compaction and cementation of sediments.
    • Clastic rocks are identified by the size of the fragments.
    • Conglomerate has rounded fragments...

    • Breccia has angular fragments...

  • Chemically formed rocks: evaporites
    • Formed when water carrying minerals evaporates and leaves the minerals behind.
    • Are identified by the minerals present (ie halite hardness of 2.5)
  • Bioclastic-
    • bio= life
    • clastic= fragments
  • Made from accumulated shells (limestone) or plants (coal)

Metamorphic Rocks

  • changed from a pre-existing rock
  • caused by extreme heat and/or pressure
  • may result in a distorted structure like this...

or this...

Regional Metamorphism

  • Caused by extreme pressure and heat.
  • Happens over wide “regions”.

Rocks go from this...

To this...

Contact Metamorphism

  • Caused by contact with extreme heat.

Metamorphic rocks will often have foliation or a distorted structure.

Foliation is a “grain” to the rock.

Gneissic Foliation

  • minerals are squished into bands.
  • Zebra Stripes”

Photo by Phil Medina Colorado 2005

Banding vs. Layers:

Bands do not go all the way through and across your rock. Layers, like in sedimentary rocks go all the way through like a layer in a cake.



Schistose foliation- minerals have been squashed into flakes of mica.

Schist (actually it's holey schist)

Slaty foliation- rock splits into flat, thin layers.

Speed Links

Rock Cycle





Vocabulary Words

Worksheet on Sedimentary Rocks

Worksheet on the Rock Cycle

Percent Deviation on Rocks

Igneous Worksheet I

Igneous Worksheet II

Igneous Worksheet III

Quartz DBQ

Minerals DBQ

Metamorphic Rocks

Chemical Composition chart

Rocks Review


Fifteen years of
Medina On-Line