1. Glassy - instantaneous cooling
• Obsidian = volcanic glass

Obsidian

2. Aphanitic - fine grain size (< 1 mm); result of quick cooling

Rhyolite

• Basalt
• Rhyolite
• Andesite
3. Phaneritic - coarse grain size; visible grains (1-10 mm); result of slow cooling

Granite - polished

• Granite
• Diorite
• Gabbro
4. Vesicular - contains tiny holes called vesicles which formed due to gas bubbles in the lava or magma. Very porous. May resemble a sponge. Commonly low density; may float on water.
• Vesicular basalt - basalt with a vesicles, which may be quite large. Sometimes lined with crystals to form geodes.

Vesicular basalt


Vesicular basalt with olivine phenocrysts, building stone at Hawaiian Volcano Observatory, Big Island of Hawaii

• Pumice - light in color; white to gray; may be glassy or dull. Fully riddled with holes. Very sponge-like. Floats. Used as an abrasive. (Pumice stone, Lava Soap).

Pumice

Composition of Igneous Rocks

Igneous rocks can be classified into four groups based on their chemical compositions:

1. Sialic (or granitic or felsic)
1. Dominated by silicon and aluminum (SiAl)
2. Usually light in color
3. Characteristic of continental crust
4. Forms a stiff (viscous) lava or magma
5. Rock types include:
1. Granite

Granite

2. Rhyolite

Rhyolite

6. Minerals commonly present include:
1. potassium feldspar (generally pink or white)
2. Na-plagioclase feldspar (generally white)
3. quartz (generally gray or colorless)
4. biotite
5. amphibole?
6. muscovite?
2. Intermediate (or andesitic)
1. Intermediate in composition between sialic and mafic
2. Rock types include:
1. Andesite (aphanitic)
2. Diorite (phaneritic)

Diorite

3. Minerals commonly present include:
1. plagioclase feldspar
2. amphibole
3. pyroxene
4. biotite
5. quartz
3. Mafic (or basaltic)
1. Contains abundant ferromagnesian minerals (magnesium and iron silicates)
2. Usually dark in color (dark gray to black)
3. Characteristic of Earth's oceanic crust, Hawaiian volcanoes
4. Forms a runny (low viscosity) lava
5. Also found on the Moon, Mars, and Venus
6. Rock types include:
1. Basalt (aphanitic)

Basalt

2. Gabbro (phaneritic)

Gabbro

4. Ultramafic
1. Almost entirely magnesium and iron silicates (ferromagnesian minerals)
2. Rarely observed on the Earth's surface
3. Believed to be major constituent of Earth's mantle
4. Commonly found as xenoliths in basaltic lavas
5. Rock types include:
Peridotite (phaneritic) - dominated by olivine - the birthstone is Peridot, which gives its name to Peridotite

Peridotite

6. Minerals commonly present include:
1. Olivine is dominant. (Olivine is olive green).
2. may have minor amounts of pyroxene and Ca-plagioclase

Igneous Rock Classification Chart

	Composition
Texture	Sialic	Intermediate	Mafic	Ultramafic
Fine-grained (aphanitic)	Rhyolite	Andesite	Basalt	--------------
Coarse-grained (phaneritic)	Granite	Diorite	Gabbro	Peridotite

Use this chart to name the igneous rocks. Determine the texture and the composition of the sample, and then read its name from the classification chart. Other igneous rocks which you may see in lab, but which are not on this chart are obsidian and pumice.

1. Clasts (larger pieces, such as sand or gravel)
2. Matrix (mud or fine-grained sediment surrounding the clasts)
3. Cement (the glue that holds it all together), such as:
1. calcite - fizzes in HCl acid
2. iron oxide - reddish brown color
3. silica - neither fizzes nor is reddish brown

Clasts and matrix (labelled), and iron oxide cement (reddish brown color)

Terrigenous sedimentary rocks are classified according to their texture (grain size):

• Gravel: Grain size greater than 2 mm
• If rounded clasts = conglomerate
• If angular clasts = breccia

Conglomerate	Breccia

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• Sand: Grain size 1/16 to 2 mm
• Sandstone is most commonly made of quartz and feldspar, but it may contain grains of nearly any mineral composition, and also grains of fine-grained rock types (such as basalt).
• If dominated by quartz grains = quartz sandstone (also called quartz arenite)
• If dominated by feldspar grains = arkose
• If dominated by rock fragment grains = lithic sandstone (also called litharenite or graywacke)

	Quartz Sandstone
	Arkose
	Graywacke (lithic sandstone)

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• Silt: Grain size 1/256 to 1/16 mm (gritty)
• Siltstone

Siltstone

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• Clay: Grain size less than 1/256 mm (smooth)
• Shale (if fissile)
• Claystone (if massive)

Note: Mud is technically a mixture of silt and clay. It forms a rock called mudstone (or mudshale if fissile).

	Shale - fissile
	Claystone (kaolinite) - not fissile

B. Chemical/biochemical Sedimentary Rocks

This group includes the evaporites, the carbonates (limestones and dolostone), and the siliceous rocks.

These rocks form within the depositional basin from chemical components dissolved in the seawater. These chemicals may be removed from seawater and made into rocks by chemical processes, or with the assistance of biological processes (such as shell growth). In some cases it is difficult to sort the two out (in carbonates or some siliceous rocks, for example), so they are grouped together as chemical/biochemical.

1. Evaporites - The evaporites form from the evaporation of water (usually seawater).
1. Rock salt - composed of halite (NaCl).
2. Rock gypsum - composed of gypsum (CaSO4.2H20)
3. Travertine - composed of calcium carbonate (CaCO3), and therefore, also technically a carbonate rock; travertine forms in caves and around hot springs.

Layered gypsum of the Castille Formation, Carlsbad, New Mexico	Gypsum crystals, Marion lake, Australia

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2. Carbonates - The carbonate sedimentary rocks are formed through both chemical and biochemical processes. They include the limestones (many types which you will learn about in a later lab) and dolostones.

Two minerals are dominant in carbonate rocks:

1. Calcite (CaCO3)
2. Dolomite (CaMg(CO3)2)
li>Remember which of these fizzes readily, and which of these must be scratched or powdered!
3. Siliceous rocks - The siliceous rocks are those which are dominated by silica (SiO2). They commonly form from silica-secreting organisms such as diatoms, radiolarians, or some types of sponges (you will learn about these in later labs).
1. Diatomite - looks like chalk, but does not fizz in acid. Made of microscopic planktonic organisms called diatoms. May also resemble kaolinite, but is much lower in density and more porous). Also referred to as Diatomaceous Earth.
2. Chert - Massive and hard, microcrystalline quartz. May be dark or light in color. Often replaces limestone. Does not fizz in acid.

C. Organic Sedimentary Rocks (Coals)

This group consists of rocks composed of organic matter (mainly plant fragments). Because of this, they lack minerals (which must be inorganic, be definition). These are the coals. There are several types of coal, which you will learn about in a later lab.