Igneous Rocks

Sedimentary Rocks

Metamorphic Rocks

Mineral Terms

Mineral - A naturally occurring inorganic compound with a specific chemical makeup and a defined crystal structure

Mineral Tests

Hardness/Scratch Test - This test determines the hardness of the material (or its ability to resist scratching) by taking two materials; one where you know the hardness and the other where you do not. By scratching them together you can determine which is harder. The material with the higher hardness will be able to leave a scratch mark. For a reference of items and their hardness one can look to the Moh- Hardness Scale. The Moh- Hardness scale takes 10 common minerals and rates them. The one that ranks number 10 is the most resistant and the one that ranks 1 is the least resistant. This scale also defines hard minerals from soft minerals. All minerals above the mark of 5.5 are known as Hard Minerals all those under the mark of 5.5 are known as Soft Minerals.

Moh - Hardness Scale



Object of known hardness















Streak Plate






Glass, knife









Penny (copper)






Finger nail







Streak Test - Often the color of a mineral is different form the color that is left by the minerals streak (aka the powdered residue) against an object (most often a streak plate). When trying to identify two similarly colored minerals this test can come in handy. Often the streak colors of the minerals will differ allowing for accurate identification. However this test only works on minerals that have a hardness of 6 or less.

Acid Test - If a mineral is a carbonate it will fizz when acid is dropped on it.

Magnetic Test - The mineral will be drawn to a magnet, therefore showing the presence of magnetic material.

Taste Test - Some minerals have specific tastes to them. By licking the mineral you can sometimes determine what it is. For example Halite tastes like salt.

Smell Test - For those who do not wish to lick a mineral sometimes a mineral can also be identified by the smell it gives off. For example sulfur has a very distinctive smell.

Mineral Proprieties

Luster - The appearance that a mineral gives off when light is reflected off it. There are many terms defining the type of luster a mineral gives off, most are self explanatory.

  • Metallic- looks like a metal
  • Non-metallic - does not look like a metal
    • Adamantine - looks like a diamond
    • Resinous - looks like resin
    • Vitreous - looks like glass
    • Pearly - looks like a pearl
    • Silky - looks like silk, meaning it has a very fine texture
    • Waxy - looks like wax
    • Earthy - looks like something that would come from off the ground, like dirt

Color - The color that the mineral is on the outside. This is not always a good way to determine what mineral is. Often minerals can be comprised of many colors or have a variety of colors.

Cleavage - Minerals that exhibit cleavage are able to break along parallel to sub parallel surfaces. A common example when explaining cleavage is Mica. Mica is made up of thin individual planes that can break off easily because of their weak bonds. When a mineral breaks and has cleavage it will break into pieces that haves the same geometry as each other. The type of Cleavage a mineral has is determined by the number of planes it has and at what direction they are placed. Minerals that do not have any cleavage will Fracture when trying to be broken.

Planes and Directions


1 (basal cleavage)


2 at 90°


3 at 90° (cubic cleavage)


3 not at 90° (rhombohedral cleavage)


4 (octahedral cleavage)


6 (dodecahedral cleavage)


Specific Gravity - Using the ratio of the weight of a mineral to the weight of an equal volume of water to determine what mineral it is. For example if a mineral weighs 5 times as much as an equal volume of water then its specific gravity is 5.

Crystal Form - Each mineral has a distinct crystalline structure within it. By identifying the crystal structure of a mineral you can narrow down the types of mineral it could be. There are about 64 different types of structures that are broken down into 6 groups.

  • Isometric - equal measure
  • Tetragonal - square cross sections with rectangular faces
  • Hexagonal - six sided
  • Orthorhombic - rectangular faces and profile
  • Monoclinic - rectangular faces and trapezoid faces
  • Triclinic - trapezoid faces

Like cleavage there are many terms used in defining crystal forms.

  • Crystal Face - the flat surface of a mineral
  • Crypto crystalline- crystals in a mineral to small to see with the naked eye
  • Amorphous - non-crystalline; due to rapid cooling

Mineral Groups

Silicates - Minerals that have silicone and oxygen in their composition. There are four types of silicate structured minerals.

  • Singe Chain Silicate (ex. Augite)
  • Double Chain Silicate (ex. Hornblende)
  • Sheet Silicate (ex. Micas)
  • 3-D Framework Silicate (ex. Feldspars)

Silicates are also broken up into two different groups based on their color. Light (nonferromangesian) Silicates are light in color and have a specific gravity around 2.7. Light Silicates contain amounts of aluminum, potassium, calcium and sodium. The other group of silicates is Dark (ferromagnesian) silicates. These silicates are dark in color and have a specific gravity ranging from about 3.2 - 3.6. They also contain mostly iron and magnesium.

Common Silicate Groups

Light Silicates

Group Name

Silicate Structure



Feldspar Group

3-D Framework

Two planes meeting at 90°



3-D Framework





1 plane (Basal)


Clay Minerals


1 plane (Basal)

Dark Silicates





Olivine Group

3-D Framework



Pyroxene Group

Single Chain

Two planes meeting at 90°


Amphibole Group

Double Chain

Two planes at 60° and 120°




1 plane (Basal)



3-D Framework


Non-Silicates - All other minerals that are not silicates are put into the non-silicate group then broken down into subgroups of non-silicates. There are 6 subgroups.

  • Carbonates - minerals that contain carbon and oxygen
  • Oxides - minerals with an oxygen base
  • Sulfides - minerals that contain sulfur
  • Sulfates - minerals that contain sulfur and oxygen
  • Halides - minerals that contain a metal and a halogen element
  • Native Metals - copper, silver, gold, zinc, iron, and nickel

Igneous Rock Terms

Igneous Rocks - made from the rapid or slow cooling of magma/lava.

Igneous Rock Types

Intrusive Igneous Rock - Igneous rock formed inside the earth. This type of igneous rock cools very slowly and is produced by magma. It has large grains, contains gas pockets, and usually lots of silicate minerals.

Extrusive Igneous Rock - Igneous rock formed on the surface of the earth. This type of igneous rock cools very fast and is produced by lava. It has small grains and contains little to no gas.

Igneous Rock Properties

Grain Size - The size of the grains in an igneous rock is an indicator on how fast the rock cooled. To be considered a coarse grained rock it has a ruff exterior with grains between 1 and 10 cm. There are also fine grained rocks which often have grains that are less than 1mm and hard to see with the naked eye. These rocks are very smooth to the touch. Rocks that are neither extremely coarse but are not fine grained are called medium grained. To say something is coarse, fine, or medium grained are informal terms. To learn the more formal terms move on to the next definition.

Texture - The feel of a rock based on the size, shape, and arrangement of the grains and other parts of the rock. For igneous rocks this is determined by the cooling rate. An igneous rock that cools faster will have smaller grains and therefore a smoother texture then one that cools slowly and forms bigger grains. There are many different terms to describe the type of texture an igneous rock has.

  • Pegmatitic - Very coarse, very slow cooling. Grains greater then 1 cm.
  • Phaneritic - Coarse. Grains 1 – 10 cm
  • Porphyritic - Has large crystals that are set in smaller crystals.
    • Phenocrysts -Large crystals (slow cooling)
    • Groundmass - Small crystals (fast cooling)
  • Aphanitic -Very fine and fast cooling. Grains less than 1mm
  • Glassy - no crystals
  • Vesicular - rock that contains gas pockets, look like little tiny voids in the rock
  • Frothy - porous texture
  • Pyroclastic - made of pyroclasts

Igneous Rock Composition

Mineral Composition - The mineral composition of an igneous rock is dependent on where and how the rock was formed. Magma around the world has different mineral make up. There are four different composition types; Felsic, Mafic, Ultramafic, and Intermediate. Color and mineral make up are indicators of each type. These compositions all have varying amounts of common minerals found in igneous rocks.

Common Minerals: Plagioclase feldspar, Olivine, Potassium feldspar, Pyroxene, Quartz, Amphibole, Biotite, Muscovite

Felsic - Felsic igneous rocks are light in color and are mostly made up of feldspars and silicates. Common minerals found in felsic rock include Quartz, Plagioclase feldspar, Potassium feldspar, and Muscovite. They contain about 0-15% mafic mineral crystals and have a low density.

Mafic - Mafic igneous rocks are dark colored and consist mainly of magnesium and iron. Common minerals found in mafic rocks include Olivine, Pyroxene, Amphibole, and biotite. They contain about 46-85% mafic mineral crystals and have a high density.

Ultramafic - Ultramafic igneous rocks are very dark colored and contain higher amounts of the same common minerals as mafic rocks. They contain about 86-100% mafic mineral crystals.

Intermediate - Intermediate are between light and dark colored. They share minerals with both felsic and mafic rocks. They contain 16-45% mafic minerals.

Sedimentary Rock Terms

Sedimentary Rocks - Rocks which are an accumulation of fragments of many pre-existing rocks.

Sediment - Fragment of a rock on the earth’s surface

Weathering - The process by which rocks are broken down into sediments. There are two types of weathering:

  • Mechanical - weathering in which physical process such as frost wedging and unloading break down rocks.
  • Chemical - weathering in which chemical processes such as oxidation break down rocks.

Transport - method by which sediments are moved across the surface. Types of transport include fluvial, glaciers, wind, and gravity.

Depositional Environment - area in which the sediment comes to rest, there are many different groups and subgroups classifying depositional environments. Most often they are very straightforward. For example a marsh environment would be called a marsh depositional environment. The three main groups however are:

  • Continental - deserts, lakes, river beds, swamps, and caves
  • Continental and Marine - deltas
  • Marine - ocean

Lithification - process by which sediments come together to form a sedimentary rock. There are three ways in which this is done:

  • Compaction - intense weight and compression squishes sediments together to form a sedimentary rock
  • Cementation - process in which sediments are cemented together via a precipitation.
  • Crystallization - process where an existing solution creates a sedimentary rock.

Properties of Sedimentary Rocks

Texture - The feel of a rock based on the size, shape, and arrangement of the grains and other parts of the rock. Sedimentary rocks can be broken down into having five different textures:

  • Clastic - consists of broken fragments of preexisting rock. Well sorted.
  • Bioclastic - consists of the remains of organic material.
  • Crystalline (Nonclastic) - minerals are in a pattern of interlocking crystals.
  • Amorphous - no crystal structure
  • Oolitic - made of small round particles of calcium carbonate

Composition - materials commonly found in sedimentary rocks.

  • Silica
  • Carbonate
  • Clay Minerals
  • Organic Matter
  • Evaporites
  • Rock Particles
  • Heavy Minerals
  • Feldspar

Shapes of Sediments

  • Angular - sharp corners and edges
  • Rounded - smoothed edges

Sizes of Sediments

  • Clay - <1/256mm
  • Silt - 1/256mm – 1/16mm
  • Sand - 1/16mm – 2mm
  • Pebble- 2mm – 64mm
  • Cobble - 64mm – 256mm
  • Boulder - >256mm

Sorting of Sediments

  • Poorly Sorted - all different sized particles
  • Well Sorted - same sized particles

Sedimentary Structures - Features in sedimentary rocks that reflect on what type of transportation created the sedimentary rock.

  • Stratification - layers formed from sediment deposition
  • Cross Bedding - stratification at angle, usually caused by wind.
  • Graded Bedding - bedding in which the size of the sediment increases as the depth does; marine environments
  • Surface impressions - impressions left by nature
    • Ripple Marks - left by water flow in one direction
    • Oscillation Marks - left by water flow in a back and forth direction
    • Mud Cracks
    • Raindrops
    • Trace Fossils

Metamorphic Rock Terms

Metamorphic Rocks - rocks that change form through the influence of heat, pressure, and/or chemical activity

Types of Metamorphism

1. Contact metamorphism - changes in the rock due to heat from nearby magma

2. Regional metamorphism - causes change through intense heat and pressure

3. Hydrothermal metamorphism - chemical changes in the rock due to the circulation of hot liquids through the rock fractures

4. Fault Zone metamorphism - metamorphic changes caused by fault movements

Degrees of Metamorphism - The quality of the rock is based on the amount of heat and pressure it had applied to it during the metamorphic processes

A. High Grade - high amounts of heat and pressure

B. Intermediate Grade - medium amounts of heat and pressure

C. Low Grade - low amounts of heat and pressure

Changes That Occur during Metamorphism

  • Re crystallization - occurs when small crystals join together to create larger crystals of the same mineral
  • Neomorphism - new minerals are created from the original mineral composition
  • Metamorphism - new minerals are created by gaining or losing chemicals

Metamorphic Rock Properties

Texture - The feel of a rock based on the size, shape, and arrangement of the grains and other parts of the rock. Metamorphic rocks can be broken up into two texture groups:

  • Foliated Texture - layers due to heat and pressure; occurs mostly at regional metamorphism spots. Types of foliation can be used to correctly identify rocks.
  • Non-foliated Texture - no layers it is granular instead

Foliated Textures




Other Features


Low grade

Fine grained micas

Sheet separation


Low – intermediate grade

Fine grained mica and chlorite

Glossy with a wrinkled look


Intermediate grade

Medium to coarse grained micas, chlorites, and quartz



Intermediate grade

Medium to coarse grained

Alternating dark and light layers


High grade



Non-Foliated Textures



Other Features


Medium to Coarse

Equal sized visible crystals



Microscopic crystals


Medium to Coarse

Fused grains


No visible grains


There are also features that occur in both Foliated and Non-foliated Metamorphic Rocks

  • Stretched/Sheared grains - pebbles, fossils, or crystals that have been stretched, shortened, or sheared
  • Porphyroblastic - large crystals arranged in a fine grained groundmass
  • Hydrothermal veins - fractures filled in by minerals from fluids
  • Folds - bends in the rock layers
  • Lineations - lines on the rock at the edge of foliation, shear planes, slay cleavage, folds, or aligned crystals

Common Metamorphic Changes

Parent Rock

New Rock






Schist or Amphibolite