Geology: Dikes and Sills Formation

Characteristics of Minerals

• Minerals occur naturally as inorganic substances.
• Their composition can be expressed in terms of chemical formula.
• Minerals have a definite geometric arrangement of their constituent atoms, which is a crystalline solid.
• Minerals have a definite set of physical properties that are fixed within certain limits.

Physical Properties of Minerals

• Hardness is the property of a mineral to resist abrasion or scratching.
• Mohs scale of hardness: talc, gypsum, calcite, fluorite, apatite, feldspar, quartz, topaz, corundum, and diamond.
• Non-metallic minerals: carbonates, sulfates, and other compounds or native elements such as sulfur.
• Examples of non-metallic minerals: calcite, dolomite, sulfur, halite, kernite, gypsum, apatite, fluorite, graphite, pearls.

Special Properties of Minerals

• Magnetism: ability of minerals to be attracted to magnets (e.g., iron).
• Asterism: an ore is a rock or mineral deposit from which a metal may be extracted on a commercial scale.
• Fluorescence: ability to convert x-rays or ultraviolet light to visible light (e.g., fluorite, barite, and celestite).
• Thermoluminescence: ability to give-off light when heat is applied to a mineral.
• Radioactivity: minerals such as uranium and plutonium produce invisible radiation through the spontaneous breakdown of their atomic nuclei.
• Double refraction: some crystals have the ability to split light waves into two waves (e.g., calcite).
• Specific gravity: refers to the weight of the mineral compared to the weight of an equal volume of water.
• Fracture: the way a mineral breaks; words used to describe fractures are splintery, fibrous, irregular, uneven, and earthy.

Classification of Minerals

• Siliceous minerals: contain the chemical element silicon; examples: quartz, feldspar, and calcite.
• Metallic minerals: contain metals; examples: gold, silver, iron, copper, lead, zinc, tin, aluminum, mercury, titanium, uranium.
• Non-metallic minerals: do not contain metals; examples: calcite, dolomite, sulfur, halite, kernite, gypsum, apatite, fluorite, graphite, pearls.
• Gem minerals: minerals made into precious and semi-precious stones; examples: fluorite, calcite.

Classification of Rocks

• Igneous rocks: originate from magma or lava.
• Sedimentary rocks: form from the accumulation of sediments, such as sand, silt, and clay.
• Metamorphic rocks: originate from either igneous or sedimentary rock which has changed in texture or mineral composition due to temperature change.

Rock Cycle

• The rock cycle describes how rocks change, or evolve, from one another.
• It begins as rocks are uplifted or pushed up by tectonic forces.
• The exposed rocks erode as a result of surface processes, such as rain and wind.
• The eroded particles or sediments travel by wind or water until they are deposited, and the deposited material settles into layers.
• Additional sediment may bury in these layers until heat and pressure metamorphose, or change the underlying sediment to metamorphic rock.
• Rocks can also be subducted (sunk down into the lower layers of the earth) by plate tectonic processes.
• Buried and subducted rocks may also melt and recrystallize into igneous rocks.
• Metamorphic, sedimentary, and igneous rocks may then be uplifted, starting the rock cycle again.

Classification of Metamorphic Rocks

• Foliated: minerals are flattened and arranged in parallel layers or bands which tend to form flakes or needles.
• Non-foliated: fracture without definite pattern.

Uses of Metamorphic Rocks

• Marble: a recrystallized limestone; used for buildings, monuments, and sculpture.
• Slate: formed from shale; used for blackboards, roofs, sinks, and tops of laboratory tables.