Identifying Rocks – M

PROPERTIES OF MINERALS

We can identify rocks and minerals by looking at different characteristics.  Let’s look at some of these characteristics a little closer.

1) COLOR

Color can vary greatly depending on the chemicals in the rocks and minerals.  However, situations like weathering can cause changes in the color.  Color by itself is not the best way to identify a rock or mineral.

2) LUSTER

Luster refers to the how the surface reflects light.  Some words used to describe this characteristic include:

  • DULL – very dull, typically porous
  • WAXY – like wax paper or a candle
  • OILY – has a sheen or slight shine
  • PEARLY – like a pearl
  • SILKY – has a shiny or shimmery surface
  • GLASSY – shiny like glass
  • RESINOUS – some shiny, but tarnished look
  • ADAMANTINE – highly reflective
  • SUB-METALLIC – silvery but see-through (or nearly see through)
  • METALLIC – very shiny and reflective, like metals

3) STREAK

Streak is the characteristic that describes the color of a line made if you rub the rock or mineral across a rough surface.

4) CLEAVAGE

Cleavage is the characteristic that describes the patter or the way a rock or mineral breaks.  Some will break in sheets.  Some break into cubes.  Some will break leaving jagged edges.

5)  SPECIFIC GRAVITY

Specific gravity refers to an object’s weight compared to the weight of an equal volume of water.  This characteristic describes the density of the rock or mineral.

Because most of the rocks found in the earth’s crust would be quartz, calcite and feldspar, most rocks you would find will have a specific gravity around 2.75.  We use this characteristic to describe when a rock or mineral feels lighter or heavier than you would expect for its size.  Here are some examples:

  • very light: less dense than water, like pumice
  • average: quartz or turquoise
  • heavy: garnet
  • extremely heavy: gold or silver

6)  HARDNESS

This characteristic refers to how easy it is to scratch a rock/mineral (and what it can scratch).  There is a scale called the Moh’s Hardness scale that shows how to group rocks or minerals by this characteristic.

Moh’s Hardness Scale

Hardness

Example Mineral

Description

1 Talc Easily scratched, even by a fingernail.
2 Gypsum Can be scratched with a fingernail
3 Calcite Can be scratched by copper penny
4 Fluorite Easily scratched by steel knife.
5 Apatite Can be scratched by steel knife
6 Feldspar Cannot be scratched easily by steel knife.  It scratches glass.
7 Quartz Easily scratches steel and glass.  (This is the hardest commonly found mineral.)
8 Topaz Harder than commonly found minerals.
9 Corundum Can scratch topaz.
10 Diamond Among the hardest of all minerals.

 

 

7) CRYSTAL FORM

The shape a rock or mineral would grow naturally if not confined by the space it is in.  Some of the groups for this characteristic are:

  • radiating needles ~ crystals that grow in fine needles
  • cubic ~ 6 equal, square faces
  • dendritic ~ branching, tree-like, looks like the veins in a leaf or like a painted “tree shape”
  • dodecahedron ~ 12-sided, like a 12-sided die
  • dog-tooth ~ shaped like the canine tooth, like a dog’s tooth
  • fibrous ~ looks like fibers or threads, forms in parallel lines
  • geode ~ round shape, hollow inside, often lined with crystals
  • hexagonal prism ~ hexagonal cross-section, sometimes with pointy ends and sometimes with rounded ends
  • massive ~ a chunk of mineral with no crystal shape evident
  • octahedral ~ 8-sided
  • prismatic ~ like a prism with flat ends
  • tabular ~ divide easily into thin plates or sheets, a stack is know as a “book”
  • termination ~ the end of a complete crystal

8) TENACITY or TOUGHNESS

This characteristic describes whether the rock or mineral tends to fall apart.

9) TRANSPARENCY

This characteristic describes how well the rock or mineral allows light through it.

There are some special properties common to only a some minerals.  Some of these unique properties include: MAGNETISM, CHATOYANCY, FLUORESCENCE, ODOR, CONDUCTIVITY and BURN PROPERTIES

10) MAGNETISM

This is a rare characteristic among minerals.

11) CHATOYANCY

This characteristic describes minerals that have luminous bands that can seem to move with the mineral is moved.

12) FLUORESCENCE

Some minerals, when exposed to ultraviolet light, will emit light.

13) ODOR

There are minerals that emit an odor.  Sulfur, for example, is known to have a smell like that of rotten eggs.