The Building Blocks of Earth: Minerals and Rocks
A rock is a naturally occurring solid aggregate of one or more minerals.
A mineral is a naturally occurring, inorganic solid with a definite chemical composition and a crystalline structure.
To be a mineral, a substance must satisfy all five criteria:
1.Naturally occurring
2.Inorganic (not from living things)
3.Solid
4.Definite chemical composition (e.g., SiO₂)
5.Ordered internal structure (crystalline)
Mineral Identification
Geologists use several physical properties to identify minerals:
Luster: How light reflects off the mineral's surface (e.g., metallic, glassy, dull).
Color: The apparent color of the mineral. Often unreliable as many minerals come in various colors.
Streak: The color of the mineral's powder when scraped on an unglazed porcelain plate. More reliable than color.
Hardness: A measure of the mineral's resistance to scratching, tested using the Mohs Hardness Scale (1=Talc, 10=Diamond).
Cleavage/Fracture: How a mineral breaks. Cleavage is the tendency to break along flat, planar surfaces. Fracture is irregular breaking.
Crystal Form: The external shape of a crystal, which reflects its internal atomic structure.
Major Mineral Groups
Silicates: The most common group, forming over 90% of the Earth's crust. They are based on the silicon-oxygen tetrahedron (SiO₄). Examples: quartz, feldspar, mica.
Non-silicates: All other groups, classified by their chemical composition. Examples include carbonates (calcite), oxides (hematite), and sulfides (pyrite).
The Three Rock Types Revisited
1.Igneous Rocks: Formed from the cooling and solidification of molten rock (magma or lava).
Key characteristic: Crystalline texture.
Examples: Granite (intrusive, large crystals), Basalt (extrusive, small crystals), Obsidian (extrusive, glassy).
2.Sedimentary Rocks: Formed from the compaction and cementation of sediments.
Key characteristic: Layered appearance (stratification), often contains fossils.
Examples: Sandstone (from sand), Shale (from mud), Limestone (from calcite, often from shells).
3.Metamorphic Rocks: Formed when existing rocks are altered by heat and pressure.
Key characteristic: Foliation (alignment of mineral grains into bands or layers) or distorted structure.
Examples: Marble (from limestone), Slate (from shale), Gneiss (from granite, high-grade metamorphism).