What is the difference between a mineral and a rock?

Rocks

The earth's crust is composed of rocks. A rock is a naturally occuring unit of the earth's crust and are nearly always composed of an aggregate of 1 or more minerals, and because they are formed in different ways (or even changed after they are formed) they can vary quite a lot. For example, granite is usually about 20% quartz, 75% feldspar and 5% mica, but these proportions vary and the rock often contains small amounts of other minerals as well.

Minerals

A mineral is a naturally occuring inorganic element or compound whose chemical composition and physical properties are either uniform or variable within defined limits. Minerals are the "building blocks" from which rocks are made. Some minerals, such as gold, sulphur and copper, consist entirely of one chemical element. These minerals are called Native Elements.

Minerals are formed in one of several ways:

by crystallization during cooling from molten rock, magma or lava as developed in igneous rocks;by crystallization and deposition from water as chemical precipitates, as formed in some sedimentary rocks; or by recrstallization or alleration due to heat and/or pressure as in metamorphic rocks, or by the process of weathering.

Common Mineral Identification

Chemical composition and physical properties are important characteristics of minerals. The most important diagnostiic physical properties include – structure, crystal form, cleavage, fracture, colour, luster, streak, hardness, specific gravity and magnetism.

Structure
Refers to the outward shape and form taken by the mineral, and can be described in many ways:

  • crystallized - showing crystal form or crstal faces
  • massive - not bound by crystal faces
  • fibrous - as in asbestos minerals
  • micaceous or platy - as in mica, which can be split into thin plates
  • earthy - as in limonite, an iron oxide mineral
  • granular - or formed of aggregates of grains as in certain varieties of apatite
  • radiating - as in some varieties of tremolite and in actinolite
  • botryoidal - or grape-like as in hematite or goethite
  • dendritic - tree-like, as shown by native copper, native silver or manganese stains on rock surfaces

Crystal Shapes:

Crystal Shapes - if you look at a box it has 3 dimensions: length, width and height. If you draw lines through the center of the box they all meet at 1 point. These lines are called axes or axis if only one line. Axes of a a box can be compared to axes on a crystal's unit cell.
Like boxes, unit cells can be several shapes with axes of different dimensions. Crystals grow by repeating the specific shape of their unit cell around axes. This is known as the crystal's structure.

There are 32 different types of crustal structures, which can be grouped into six different crystal systems.

Cubic (or Isometric) - Cubic crystals are shaped like cubes or boxes, axis are equal in length. Example, the minerals galena, halite (salt), pyrite, uranite, Gold, Diamond etc. belong to the cubic or isometric crystal system, and may form crystalls that are cubic, octahedral, dodecahedral or pyritohedral.
Tetragonal - Tetragonal crystals are rectangled and shped like a shoe box, two axis are equal and third is longer or shorter. (Zircon, wulfenite and rutile)
Hexagonal - These crystals have unit cells that combine to form shapes with four axes instead of three. Three are equal in length and the fourth can be longer or shorter. Emeralds, apatite, beryl, quartz, corundum and snowflakes are hexagonal.
Orthorhombic - The axes all have different measurements and are right angles to one another. Olivine, topaz and sulfur are examples of orthorhombic crystals.
Monoclinic - the axes in monoclinic crystals ahve different measurements, only 2 of these sit at right angles to each other. Selenite rose, malachite and mesolite are examples of monoclinic crystals.
Triclinic - The axes all have different measurements and none are right angle to one another. Turquoise, rhodonite and labradorite are examples of triclinic crystals.

Minerals may show prismatic or pyramidal form as do zircon, quartz, rutile, barite and some varieties of pyroxene and amphibole. They can be six-sided or hexagonal as are apatite, beryl, quartz and corundum.

Colour
Colour is an important key to identifying your specimen – just as its important to be able to tell an apple from a pear. Some minerals always have the same colour, however many minerals have impurities or imperfections that will affect the colour.

Example, when iron atoms are trapped in quartz, the quartz appears purple. Purple quartz is known as amethyst. Or when iron atoms are joined woth water, the quartz may appear yellow known as citrine. Radiation from radioactive minerals along with impurities of aluminum in the quartz give it a smoky brownish to black colour.

Streak
The streak made by a mineral is often a different colour than the mineral itself. The streak is found by rubbing the mineral on an unglazed piece of ceramic tile. The colour of the streak is important in helping you identify the mineral. Hematite, for example has a characteristic brownish red streak, although the colour of the specimen may be silver-red, through brown to black.

Luster
Another important thing to notice when identifying a mineral is the shine – known as luster – whether it looks like metal (metallic) or glass, like diamond or silky.

  • glassy luster - quartz
  • vitreous luster - like porcelain, feldspar
  • resinous luster - like resin, sphalerite, sulfur
  • metalic luster - like metal
  • brilliant - diamond
  • pearly luster - like pearls
  • adamanite luster - like diamonds
  • dull luster - like talc
  • waxy luster - serpentine
  • Greasy - graphite
  • silky - azurite

Hardness
Another important factor in knowing your rocks is the hardness. A common system of measuring hardness is the Mohs Scale. The scale runs fron the softest mineral, talc, as "Mohs 1" to the hardest mineral, diamond, as "Mohs 10". .
The Mohs Scale:

      1. Talc - is the softest
      2. Gypsyum
      3. Calcite
      4. Flourite
      5. Apatite
      6. Feldspar
      7. Quartz
      8. Beryl/Topaz
      9. Corundum
      10. Diamond - is the hardest.

In the beginning you can use four categories of hardness:

softer than 2.5
between 2.5 and 5.5
between 5.5 and 7 and
harder than 7

Common objects can be used as hardness indicators:

- fingernail H: 2.5
- copper coin H: 3.5
- knife blade H: 5.5
- metal file H: 6.5

Your fingernail has a hardness of 2.5. If you can scratch your mineral or rock with your fingernail, you know it is softer than 2.5. A pocket knife has a hardness of 5.5 and quartz has a hardness of 7. If you can scratch it with your pocket knife, you know it is softer than 5.5.

These four tests – colour, luster, streak and hardness – will go a long way to help you identify the mineral you find. All the mineral field guide books around the world use these characteristics in describing minerals.

Cleavage
The tendancy of a mineral to split along certain smooth planes that have definite geometric relationships one to another is known as cleavage. How the crystal breaks indicates how its atoms are structured. If the crystal breaks smmothly in one or more directions, it has perfect cleavage. If the break is not smooth the clevage is described as good, fair or poor. Some crystals such as metals crystals have no cleavage.

  • basal cleavage - one direction of cleavage that is singularly good (mica)
  • cubic cleavage - two good cleavage planes (feldspar) or three good cleavages at right angles (galena, Halite) and splitting of a sample of the mineral producing perfect cubes of varying sizes
  • rhombic cleavage - splits in three good cleavage directions that are not at right angles (calcite)

Fracture:

If a mineral possesses cleavage, it splits easily along certain smooth plains; if it lacks cleavage in certain directions, it will break along irregular surfaces or breaks/fractures. Fracture is how the mineral breaks once the tenacious limit has been exceeded.
Tenacity is the resistance that a mineral offers to breaking, crushing, bending, cutting, or other acts of destruction.
Minerals broken surfaces can resemble broken glass, smooth, irregular

 

Fracture Fracture Description
Brittle - Conchoidal Very brittle fracture producing small, conchoidal fragments.
Brittle - Irregular Very brittle fracture producing irregular fragments
Brittle - Subconchoidal Brittle fracture with subconchoidal fragments
Brittle - Splintery Brittle fracture leaving splintery fragments.
Brittle - Sectile Brittle fracture with slightly sectile shavings possible.
Brittle Generally displayed by glasses and most non-metallic minerals.
Brittle - Uneven Very brittle fracture producing uneven fragments.
Conchoidal Fractures developed in brittle materials characterized by smoothly curving surfaces, (e.g. quartz).
Conchoidal - Irregular Irregular fracture producing small, conchoidal fragments.
Conchoidal - Uneven Uneven fracture producing small, conchoidal fragments.
Elastic Fragments which spring back after bending.
Earthy Dull, clay-like fractures with no visible crystalline affinities, (e.g. howlite).
Even Flat surfaces (not cleavage) fractured in an even pattern.
Fibrous Thin, elongated fractures produced by crystal forms or intersecting cleavages (e.g. asbestos).
Fragile Crystals with a delicate and easily injured structure.
Flexible Flexible fragments.
Friable The crumbly disintegration of earthy materials or highly fractured minerals.
Granular Fracture surfaces produced by aggregated minerals, (e.g. granite).
Hackly Jagged, torn surfaces, (e.g. fractured metals).
Irregular Flat surfaces (not cleavage) fractured in an irregular pattern.
Micaceous Fracture of flexable micaceous cleavage fragments.
Malleable Deforms rather than breaking apart with a hammer.
None No fractures
Plastic Deforms like soft, plastic materials.
Plastic - Splintery Thin, soft flexable, elongated fractures produced by intersecting good cleavages or partings (e.g. hornblende).
Regular Flat surfaces (not cleavage) fractured in a regular pattern.
Sub Conchoidal Fractures developed in brittle materials characterized by semi-curving surfaces.
Sectile Curved shavings or scrapings produced by a knife blade, (e.g. graphite).
Splintery Thin, elongated fractures produced by intersecting good cleavages or partings (e.g. hornblende).
Sugary Fracture surfaces produced by finely aggregated minerals, (e.g. massive anhydrite).
Tough Difficult to break apart as shown by fibrous minerals and most metals.
Unknown Minerals too small to observe fractures.
Uneven Flat surfaces (not cleavage) fractured in an uneven pattern.
Weak Hard to handle without causing serious harm or damage.

Specific Gravity

The weight of a mineral compared with the weight of an equal volume of water is its spcific gravity.

Magnetic Properties

The magnetic properties of magnetite and pyrrhotite are diagnostic, a magnet will attract powdered magnetite or pyrrhotite.

For more information on rock and minerals visit:
http://www.canadianrockhound.com/junior/minerals.html