Introduction to Petrology

• Rocks are solid masses of naturally occurring geological materials
• Petrology is the study of the compositions, structures, and origins of rocks
• Petrologic research combines field studies and lab work, drawing from chemistry, mineralogy, physics, geological mapping, and sometimes biology
• Petrography is a subdiscipline focused on the description and classification of rocks
• Petrologic research includes examining rocks in outcrops and hand samples, using a petrographic microscope, and geochemistry
• Rocks are divided into igneous, sedimentary, and metamorphic types
• Rocks and minerals can transition between these categories

Importance of Petrology

• Provides insights into Earth's history
• Key to discovering and developing mineral resources
• Principles from petrology are applicable in modern industry

Rocks

• Rocks include mineral aggregates, fossils, biological materials, or volcanic glass
• A rock is a solid mass of naturally occurring geological and related materials

Petrology

• Petrology studies the compositions, structures, and origins of rocks
• Petrologists interpret information rocks contain
• Traditionally involves fieldwork and mapping
• Modern research includes lab analysis

Petrologic Research

• Begins with examining rocks in outcrops
• Considers rock compositions, minerals present, textures, and relationships between different rocks
• A hybrid science that involves aspects of chemistry, mineralogy, physics, geological mapping, and sometimes biology
• Expertise varies among petrologists

Broader Scope

• Involves the nature and evolution of Earth, including surface features, the crust, and the mantle
• Surface petrology is crucial due to daily interactions
• Determines which areas are suitable for agriculture

Mineral Resources

• Key to discovering and developing mineral resources
• Understanding rocks helps guide mineral exploration and development

Industrial Applications

• Metallurgy and crystallography originated from studies of rocks and minerals
• Lessons apply to synthetic minerals
• Crystallography led to the creation of microchips and modern electronics

Petrography

• Deals with the description and classification of rocks
• Involves examining rocks in outcrops, hand samples, and at high magnification

Microscopic Examination

• High magnification reveals otherwise unseen details
• Thin sections are studied using a polarizing petrographic microscope
• Petrographic microscopes reveal mineral properties and rock crystallization history
• Petrologists interpret rock petrogenesis using textures and mineral relationships

Thin Sections

• Thin slices of rock (30 μm) are glued to glass slides
• Light passes through mineral grains
• Mineral colors are more pronounced at high magnification
• Reveals details that permit accurate mineral identification
• Mineral colors in thin section differ from hand specimen colors

Geochemistry

• Understanding rock formation in different environments helps study Earth's evolution, natural resource distribution, and volcanic hazards
• Analyzing rock (and magma) compositions provides crucial insights, especially for igneous rocks

Analytical Methods

X-Ray Fluorescence Spectroscopy (XRF)

• Measures element concentrations from major elements down to parts per million (ppm)
• X-ray beam causes atoms in the sample to fluoresce and emit X-radiation
• Elements emit characteristic secondary X-rays of different wavelengths
• Intensities at each wavelength are proportional to the amount of the element present
• Rapid compared to other techniques
• Cannot easily analyze sodium and lighter elements
• Analyzes elements with atomic numbers between 11 and 41 (sodium-niobium) and some heavier ones

Neutron Activation Analysis

• Instrumental Neutron Activation Analysis (INAA) is the most common
• A neutron beam converts elements to radioactive nuclides
• Radioactive specimens "cool" to allow radioactive elements to decay
• Analyzed by measuring gamma rays emitted during nuclide decay
• Gamma ray wavelengths indicate elements present
• Intensities are proportional to amounts present
• Requires a research nuclear reactor
• Good for analyzing rare earth, platinum group, and other elements
• Initial analyses take place days to months after activation

Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

• Samples must be dissolved in caustic acids
• Liquid passes through a plasma torch
• Flame emits light with wavelengths specific to the elements present
• Elemental concentration is determined by light intensity (ICP)
• Emitted ions are sent to a mass spectrometer (MS) for precise measurement and greater sensitivity
• Allows measurement of specific isotope concentrations
• Primarily used for trace elements and isotopes, but can analyze major elements
• Measures the concentrations of specific isotopes
• Primarily used for trace elements and isotopes, but some variations permit major element analyses

Method Selection

• Petrologists combine techniques for thorough characterization
• Choice depends on purpose and available instrumentation
• Commonly combine XRF and INAA for a complete chemical characterization

Three Kinds of Rocks

• Igneous rocks, sedimentary rocks, and metamorphic rocks
• Form by different processes

Igneous Rocks

• Created when magma cools and hardens
• Can be crystalline with visible mineral crystals or fine-grained
• May contain volcanic glass (obsidian)
• Can contain single or multiple components
• May have structures indicative of flowing magma and an intrusive origin

Sedimentary Rocks

• Clastic rocks contain compressed and cemented mineral crystals, rock fragments, fossils, or organic materials
• Lithification involves compression, cementation, and recrystallization during burial
• Chemical sedimentary rocks form when minerals precipitate from water, sometimes biologically
• Organic sedimentary rocks form from biological debris
• Sedimentary rocks are often in thick horizontal beds
• Individual beds range from centimeters to meters thick

Metamorphic Rocks

• Form by alteration of preexisting rocks due to pressure, heat, or fluids
• Metamorphism can change minerals, texture, or composition
• Protolith: the original rock, limits the type of metamorphic rock that can form
• Temperature ranges from Earth-surface to deep-Earth temperatures
• Metamorphism: does not involve melting
• Pressure may not be uniform, leading to foliations (layered appearance)
• Foliations result from parallel alignment of mineral crystals or layers of different mineral composition

The Rock Cycle

• Any preexisting rock may melt to form an igneous rock, weather to form a sedimentary rock, or be metamorphosed
• Rocks and minerals cycle between igneous, sedimentary, and metamorphic pedigrees
• Geological materials change and evolve over geologic time

Scope of the Book

• Focus on igneous and metamorphic petrology
• These rock types are often found together
• Studies involve rock chemistry, thermodynamics, and phase diagrams
• Sedimentary petrology requires separate study due to low temperatures, lack of chemical equilibrium, and the need to discuss environments of deposition, stratigraphy, paleontology, tectonics, and Earth history