Rocks and Minerals

Young Astronomers Blog, Volume 28, Number 19.

Walk outside and look down. You might see a few rocks. A casual glance might reveal that the rocks around you are gray, or they might have a hint of color. A few might even sparkle in the sun. Although they look simple, rocks are hard! A more concentrated look will reveal a great deal of complexity. And after some research, you will discover that rocks are classified based on the minerals they contain and the way they were formed.

Rocks are non-living material, composed of minerals, and held together as a solid mass.

Minerals, in turn, are made of molecules and have a specific chemical formula. All minerals have five specific characteristics. Minerals:

1. Occur in nature and are not artificially constructed.
2. Are inorganic, that is they are not living plants or animals.
3. Are solid objects.
4. Have atoms that exhibit an orderly structure.
5. Have a chemical composition, which uniquely defines a mineral.

Minerals are often described by certain physical properties.

1. Cleavage, the tendency to break.
2. Color, how it appears to the eye.
3. Hardness, the resistance to scratching measured by the Mohs Hardness Scale.
4. Luster, how light is reflected from its surface.
5. Streak, the powder color when scratched on a ceramic plate.
6. Specific gravity, their density relative to water.

A rock’s mineral structure can help identify the type of rock. Many rocks are composed of silicon.

• Silicon (Si) is an element with an atomic number of 14.
• Silica is the combination of silicon and oxygen (Silicon Dioxide/SiO₂).
• Silicate minerals (Silicates) are those composed of Silicon and Oxygen.

These are a few of the more common Silicates.

• Quartz is the most common mineral found in the Earth’s crust and is pure Silica (SiO₂).
• Feldspar is a common family of minerals containing Aluminum in addition to Silica. It also has varying combinations of Sodium, Potassium, and Calcium.
  • Alkali feldspar has a varying ratio of Sodium and Potassium, but no Calcium.
  • Plagioclase feldspar has a varying ratio of Sodium and Calcium, but no Potassium.
  • Orthoclase feldspar is Potassium feldspar (no Sodium or Calcium).
• Amphibole and Pyroxene are each a family of minerals with a similar crystal structure, but varying amounts of Sodium, Calcium, Iron, and Magnesium.
• Olivine is a green and glassy mineral with a concentration of Iron and Magnesium.

Carbonite minerals, containing Carbonate (CO₃), are relatively soft non-silicates found in some rocks. The “acid test”, where a diluted drop of hydrochloric acid will bubble as carbon dioxide is released, is often used to detect carbonite minerals.

• Calcite, found in Limestone, is Calcium Carbonate (CaCO₃).
• Dolomite, found in Marble, contains is Calcium Magnesium Carbonate (CaMgCO₃)₂.

In addition to looking at the minerals, rocks are classified into three primary types based on how they were formed.

• Igneous rocks are formed from magma and lava. They are further divided into Extrusive/Volcanic (from lava), Intrusive (from magma), and Plutonic (from deep magma).
• Sedimentary rocks are formed through erosion by either wind or water. They are further divided into Clastic (from sediment), Chemical (from dissolved material), and Organic (from living material).
• Metamorphic rocks are formed from heat and pressure. They are further divided into Foliated (with layers) and Non-foliated (without layers).

• Rocks melt, due to extreme heat from great pressure, creating volcanic lava and magma.
• Magma cools and solidifies becoming Igneous rocks.
• Rocks are broken down by weathering and are transported by erosion creating sediment.
• Sediment undergoes lithification (compaction and cementation due to pressure) creating Sedimentary rocks.
• Rocks undergo metamorphism (from great pressure and heat) and are transformed into Metamorphic rocks.