Geology: Silicate Minerals, Weathering, Soil Horizons & Sediment Sizes

Mineral Definition

Naturally occurring, inorganic solid with a regular internal crystalline structure and a defined chemical composition

Why Hardness is Not a Defining Criterion

Hardness is a physical property resulting from bond strength/structure, not a defining criterion like composition or atomic lattice

Silicon-Oxygen Tetrahedron

Fundamental building block of silicates consisting of 1 silicon cation surrounded by 4 oxygen anions (SiO4 4-)

Isolated Tetrahedra (Nesosilicates)

Tetrahedra share 0 oxygens; linked by interstitial cations (Example: Olivine)

Single Chain Silicates (Inosilicates)

Tetrahedra share 2 oxygens in a single chain (Example: Pyroxene / Augite)

Double Chain Silicates (Inosilicates)

Tetrahedra share alternating 2 and 3 oxygens in double chains (Example: Amphibole / Hornblende)

Sheet Silicates (Phyllosilicates)

Tetrahedra share 3 oxygens in continuous 2D sheets (Examples: Biotite, Muscovite, Glauconite, Clay minerals)

Framework Silicates (Tectosilicates)

Tetrahedra share all 4 oxygens in a 3D framework (Examples: Plagioclase, Potassium Feldspar / Orthoclase, Quartz)

Olivine

Isolated silicate mineral; high-temperature mineral that weathers first

Pyroxene

Single-chain silicate mineral (e.g., Augite); dark mafic mineral

Amphibole

Double-chain silicate mineral (e.g., Hornblende); dark mafic mineral with 60/120 degree cleavages

Biotite

Sheet silicate; dark-colored, iron/magnesium-rich mica

Muscovite

Sheet silicate; light-colored, potassium-rich mica

Glauconite

Green sheet silicate (mica group) formed specifically in shallow marine environment settings

Clay Minerals

Sheet silicates formed as secondary weathering products of silicate minerals (e.g., kaolinite, illite, smectite)

Plagioclase Feldspar

Framework silicate forming a solid solution series from Na-rich (albite) to Ca-rich (anorthite)

Potassium Feldspar (Orthoclase)

Framework silicate rich in potassium; more resistant to chemical weathering than plagioclase

Quartz

Framework silicate composed purely of SiO2; extremely hard and highly resistant to chemical and physical weathering

Bowen's Reaction Series (Crystallization Order)

Crystallization from magma: Olivine -> Pyroxene -> Amphibole -> Biotite -> Ca-Plagioclase to Na-Plagioclase -> K-Feldspar -> Muscovite -> Quartz

Goldich Stability Series (Weathering Susceptibility)

Weathering susceptibility matches reverse Bowen's order: Olivine/Ca-Plagioclase weather first; Quartz and ZRT weather last

Why Quartz/ZRT Resist Weathering

Framework silicates sharing all 4 oxygens have higher bond energy and stability than isolated/chain silicates

Physical Weathering (Disintegration)

Mechanical breakdown of rock into smaller fragments without changing chemical or mineralogical composition

Chemical Weathering (Decomposition)

Chemical breakdown and transformation of minerals into new, more stable minerals or dissolved ions

Thermal Expansion Weathering

Repeated heating and cooling causes differential expansion between rock exterior and interior, creating stress fractures

Frost Wedging

Water enters fractures and expands by 9% upon freezing, exerting immense pressure that shatters rock

Stress Release (Exfoliation)

Removal of overlying rock reduces pressure, causing underlying rock to expand and fracture parallel to surface

Talus Slope

An accumulation or pile of angular rock fragments at the base of a cliff produced by physical weathering

Solution (Dissolution)

Chemical weathering where soluble minerals dissolve completely in water or weak acids (e.g., calcite, halite)

Oxidation-Reduction (Redox)

Chemical reaction where elements lose/gain electrons; iron reacts with oxygen to form iron oxide oxides ("rust")

Hydrolysis

Reaction between H+ or OH- ions in water and silicate minerals; converts feldspar into clay minerals

Hydration and Dehydration

Addition (hydration) or removal (dehydration) of water molecules from a mineral lattice (e.g., anhydrite to gypsum)

Ion Exchange

Substitution of ions in solution for ions held on mineral surface lattices; especially effective in clay minerals

Climate Influence on Weathering

Chemical weathering is fastest in warm, wet climates; physical weathering dominates in cold, dry climates

pH Influence on Weathering

High H+ acidity accelerates hydrolysis and dissolution; silica dissolves more at high pH, iron/alumina dissolve at low pH

Eh (Redox Potential) Influence

High Eh (oxidizing) promotes rust/iron-oxide precipitation; low Eh (reducing) mobilizes soluble iron

Ion Mobility

The ease with which ions are leached and carried away in groundwater; highly mobile ions accelerate weathering pathways

Soil Profile

Vertical sequence of parallel layers (horizons) developed by chemical weathering, leaching, and organic accumulation

O Horizon

Topmost organic horizon made of carbon-rich humus; contributes organic acids and CO2 to percolating fluids

A Horizon

Topsoil zone where organic humus mixes with mineral matter; serves as the primary zone of leaching (eluviation)

B Horizon

Subsoil horizon consisting mostly of mineral matter; serves as the primary zone of accumulation (illuviation) of clays and oxides

C Horizon

Bottom soil horizon consisting of partially weathered parent bed rock material

Paleosol

An ancient, buried soil horizon preserved in the stratigraphic record indicating past climate, landscape, and exposure

Udden-Wentworth: Boulder

Sediment diameter > 256 mm (Phi < -8)

Udden-Wentworth: Cobble

Sediment diameter 64 mm to 256 mm (Phi -6 to -8)

Udden-Wentworth: Pebble

Sediment diameter 4 mm to 64 mm (Phi -2 to -6)

Udden-Wentworth: Granule

Sediment diameter 2 mm to 4 mm (Phi -1 to -2)

Udden-Wentworth: Sand

Sediment diameter 0.0625 mm (63 microns) to 2 mm (Phi 4 to -1)

Udden-Wentworth: Silt

Sediment diameter 0.0039 mm (4 microns) to 0.0625 mm (63 microns) (Phi 8 to 4)

Udden-Wentworth: Clay

Sediment diameter < 0.0039 mm (< 4 microns) (Phi > 8)

Phi Scale Formula

Phi = -log2(d), where d is grain diameter in mm. Coarser grains have lower/negative values; finer grains have higher positive values

Measuring Gravel Size

Direct measurement using calipers, rulers, or pebble-counting field methods

Measuring Sand Size

Sieve analysis using stacked mesh screens

Measuring Mud Size

Settling methods (hydrometer or pipette analysis) based on settling velocity

Stokes' Law

Calculates grain settling velocity in fluid; states settling speed increases with the square of particle diameter (used for mud analysis)

Grain Form

Overall 3D shape defined by relative lengths of long (a), intermediate (b), and short (c) axes

Grain Sphericity

A measure of how closely a grain's shape approaches that of a true sphere

Zingg Diagram Classification

Uses b/a and c/b axis ratios to classify grains into 4 form fields: Oblate (disc), Equant (sphere), Bladed, Prolate (roller)

Grain Roundness

The sharpness or curvature of grain corners and edges; increases with transport distance and abrasion

Grain Surface Texture

Micro-features on grain surfaces (e.g., frosted sand from wind, scratches/striations from glaciers)

Sorting

The measure of grain size uniformity and scatter around the mean size; improves with transport distance/energy

Framework Grains

Dominant sand-sized grains forming the structural skeleton of a sedimentary rock (mostly quartz, feldspar, lithic fragments)

Matrix

Fine-grained mud, silt, or clay filling pore spaces between larger framework grains

Chemical Cement

Precipitated minerals (quartz, calcite, hematite) that bind framework grains together post-deposition

Accessory Minerals

Minor minerals making up < 1-2% of a rock's volume

Mafic Minerals

Dark, iron/magnesium-rich silicate minerals like hornblende, pyroxene, and olivine

Opaque Heavy Minerals

Dense minerals that do not transmit light, such as magnetite and ilmenite

Non-Opaque Heavy Minerals (ZRT)

Zircon, Rutile, Tourmaline; ultra-stable heavy minerals used to trace sediment provenance

Sediment Provenance

The ultimate source terrain, parent rock type, and tectonic setting of sedimentary grains

Textural Maturity

Degree to which sediment has lost clay, become well-sorted, and achieved well-rounded grains through transport

Chemical Maturity

Degree to which unstable minerals (feldspars, lithics) have been destroyed, leaving stable quartz

Conglomerate vs Breccia

Conglomerate contains rounded gravel clasts (> 30%); Breccia contains angular gravel clasts (> 30%)

Oligomict vs Polymict Gravel

Oligomict contains clasts of a single lithology (e.g., quartz); Polymict contains clasts of multiple lithologies

Epiclastic vs Volcanoclastic Rocks

Epiclastic rocks form from normal weathering/erosion; Volcanoclastic rocks form directly from volcanic activity

Clast-Supported Conglomerate

Gravel clasts touch each other; matrix occupies < 15% of total rock volume

Matrix-Supported Conglomerate

Gravel clasts float in fine matrix; matrix occupies 15% to 70% of total rock volume

Sandstone Framework Components

Quartz (Q), Feldspar (F), and Lithic rock fragments (L)

Arenite Sandstone

Clean sandstone containing < 10% fine mud/clay matrix

Wacke Sandstone

Dirty sandstone containing > 10% (up to 50%) fine mud/clay matrix

Quartz Arenite

Sandstone composed of > 90% quartz framework grains

Arkose Sandstone

Feldspar-rich sandstone (< 90% quartz, feldspar > lithic fragments) indicating arid climate or short transport

Lithic Arenite

Rock-fragment-rich sandstone (< 90% quartz, lithics > feldspar) indicating rapidly eroding tectonic source

Mudrock Subdivisions

Siltstone (silt-dominated), Mudstone (non-fissile mix), Claystone (clay-dominated), Shale (fissile mud-rich rock)

Fissility

The physical property enabling shale to split easily along thin, parallel bedding planes

Fluid Density

Mass per unit volume (g/cm3); affects buoyant forces and fluid momentum during transport

Fluid Viscosity

A fluid's internal resistance to flow and deformation; controls turbulence and grain settling rates

Laminar Flow

Smooth fluid flow with parallel, non-crossing streamlines; occurs at low speeds, high viscosity, or shallow depths

Turbulent Flow

Chaotic fluid flow with crossing, swirling streamlines and eddies; dominates natural stream sediment transport

Reynolds Number (Re) Formula & Meaning

Re = (Velocity * Hydraulic Radius * Density) / Viscosity; ratio of fluid inertial forces to viscous forces

Reynolds Number Thresholds

Re < 500 = Laminar flow; Re 500 to 2000 = Transitional flow; Re > 2000 = Turbulent flow

Froude Number (Fr) Formula & Meaning

Fr = Inertial forces / Gravitational forces; determines subcritical vs supercritical hydraulic state

Froude Number Thresholds

Fr < 1 = Lower flow regime (tranquil flow); Fr > 1 = Upper flow regime (rapid, supercritical flow)

Forces on Grain Entrainment

Resisting: Grain weight & friction. Encouraging: Boundary shear stress & hydraulic lift

Hydraulic Lift (Bernoulli Effect)

Fluid speeds up over a curved grain top, lowering pressure relative to the bottom, lifting the grain

Traction Transport

Sediment movement along the bed by rolling and sliding

Saltation Transport

Sediment movement along the bed via short, bouncing leaps

Bedload

Total sediment moving along the bed via traction and saltation

Suspended Load

Fine sediment (silt/clay) held in the water column by fluid turbulence

Dissolved Load

Ions carried in invisible chemical solution in water

Stream Competence

The maximum particle diameter or size a flow can entrain; depends directly on flow velocity (~V^2)

Stream Capacity

The total mass or volume of sediment a stream can carry; depends on discharge and volume of water