Minerals and Rocks

• Quartz: $\mathrm{SiO_2}$, Mohs hardness $7$, commonly without cleavage and with conchoidal fracture. It is resistant to many surface-weathering conditions, but the form and condition of silica matter when evaluating concrete aggregates.
• Feldspars: abundant rock-forming silicates, including potassium feldspar and plagioclase solid solutions. Weathering can produce clay minerals; the product is not always kaolinite, and “clay” does not imply one swelling behavior.
• Micas: sheet silicates, including commonly dark biotite and pale muscovite. Strong basal cleavage can influence a rock's fabric and directional behavior, depending on mineral abundance and arrangement.
• Calcite: $\mathrm{CaCO_3}$, Mohs hardness $3$, with cleavage and a characteristic acid reaction under suitable test conditions. It is common in limestone and marble. Dissolution depends on water chemistry and flow; its presence does not prove a cavity beneath a site.

Silicates contain silicon–oxygen structural units, commonly described using the $\mathrm{SiO_4}^{4-}$ tetrahedron. Quartz, feldspars and micas are important examples. Other groups include:

• Carbonates: calcite and dolomite; relevant to carbonate-rock dissolution and construction materials.
• Sulfates: gypsum $\mathrm{CaSO_4\cdot2H_2O}$ and anhydrite; dissolution, hydration and chemical compatibility can matter. Not every sulfate mineral has the same solubility.
• Oxides: hematite $\mathrm{Fe_2O_3}$ and magnetite; useful mineralogical indicators with distinct physical properties.
• Sulfides: pyrite $\mathrm{FeS_2}$ can contribute to acidic drainage when oxidation and water conditions permit.

Mineral names indicate questions to investigate; they do not independently set an allowable stress, reaction rate or treatment.

• Hardness: resistance to scratching, distinct from compressive strength, toughness and excavation abrasivity.
• Cleavage: preferential breakage along crystallographically controlled planes.
• Fracture: breakage outside cleavage planes. A mineral with cleavage can also fracture in another direction.
• Luster and streak: reflected-light appearance and powder color; a surface color alone can be misleading.
• Specific gravity: solid density divided by the density of reference water, a dimensionless ratio.

Use clean, representative observations and known test references. A powder mark is not automatically a scratch, and a single property rarely identifies a mineral uniquely.

The standard sequence is: talc $1$, gypsum $2$, calcite $3$, fluorite $4$, apatite $5$, orthoclase $6$, quartz $7$, topaz $8$, corundum $9$, diamond $10$.

The numbers rank scratch resistance; their differences and ratios are not proportional physical hardness. A mineral at $8$ is not established as “twice as hard” as one at $4$. Scratch comparisons can bracket an unknown, but an exact identity needs other evidence. Everyday tools vary in composition and condition and are not universal calibrated hardness standards.

• Igneous: cooling and solidification of magma or lava. Intrusive examples such as granite and gabbro commonly have visible crystals; rapid surface cooling can produce fine grains or glass, as in basaltic lava or obsidian. Vesicles and cooling joints may occur. Neither texture nor the rock name establishes strength or excavation method.
• Sedimentary: accumulation and lithification of clastic sediment, chemical precipitation, biological accumulation, or combinations of these processes. Sandstone, mudstone, limestone, gypsum and coal do not all share one origin, pore structure or engineering behavior.
• Metamorphic: changes in mineralogy and texture in the solid state under appropriate temperature, deformation and fluid conditions. Slate, schist and gneiss show different fabrics; marble and quartzite are common nonfoliated examples. This is not one obligatory grade sequence for every protolith.

Volcanic glass is not a crystalline mineral, and organic-rich rocks such as coal are not defined by the mineral criterion. Rock classification therefore needs more than the assertion that every constituent is crystalline.

Rock can weather to sediment, melt to magma, or change through solid-state metamorphism where appropriate conditions occur. Clastic sediment can lithify into sedimentary rock; cooling magma can form igneous rock. A metamorphic rock can undergo further metamorphism without leaving that broad category.

These paths may be repeated, bypassed or interrupted. They have no universal duration. Chemical and biogenic sedimentary pathways add further possibilities beyond the clastic path illustrated below.

A common idealized cooling sequence shows olivine, pyroxene, amphibole and biotite on a discontinuous branch, while plagioclase compositions progress from calcium-rich toward sodium-rich on a continuous branch. Potassium feldspar, muscovite and quartz can occur among late or residual phases where the bulk composition permits.

Not every magma produces every listed mineral or ends with quartz. Composition, crystallization history and other conditions matter. The relative susceptibility of common silicates to chemical weathering in moist surface environments often broadly reverses the high-to-low crystallization order, but weathering rate also depends on climate, water chemistry, surface area and rock condition. This is not a time calculator or a universal durability ranking for construction.

For the simplified Wentworth-based divisions used here, with nominal diameter $d$:

• Clay size: $d<1/256\ \text{mm}$, approximately $0.0039\ \text{mm}$.
• Silt size: $1/256\leq d<1/16\ \text{mm}$.
• Sand size: $1/16\leq d<2\ \text{mm}$, with $1/16=0.0625$.
• Gravel size: $d\geq2\ \text{mm}$.

These explicit boundary conventions prevent ambiguity in exercises. Geological and engineering soil classifications need not use identical divisions. Clay-sized describes a size, while clay mineral describes mineralogy.

Rock naming also considers composition, sorting, clast shape, matrix, cement and fabric. Abundant rounded versus angular gravel clasts help distinguish conglomerate and breccia. Sand-sized clastic grains characterize sandstone; fissility distinguishes shale from many nonfissile mudstones. Not every clay-rich rock is fissile, expansive or weak in every loading direction.

Foliation may involve aligned minerals or compositional bands developed during deformation and recrystallization. Slaty cleavage is a rock fabric, distinct from cleavage within an individual mineral crystal. Fabric orientation, persistence, condition and the imposed loads influence directional behavior.

Quartzite can be hard and abrasive; marble can undergo carbonate dissolution. Neither description establishes site-wide excavation cost, bearing capacity or slope stability. A surface that can daylight is a geometric observation to investigate, not proof that sliding will occur without the required release geometry, loading and strength/water conditions.

Intact rock is material between larger discontinuities, sampled and tested at a specified scale and condition. It may itself be weathered, porous, anisotropic or microcracked; “intact” does not mean unweathered or strong.

A rock mass includes that material, joints, bedding, faults, other structures and their spatial relationships. Discontinuities may govern a mechanism, but their effect depends on orientation, spacing, persistence, infill, water and stress; some are tight or cemented. Intact strength and field behavior are therefore different questions, not values linked by one universal reduction factor.

A tunnel method, support system or foundation requires the relevant geological model, material tests, geometry, groundwater, loading, construction constraints and project criteria. Granite, basalt, shale or slate alone cannot select that solution.

Wetting, drying and abrasion can disintegrate some weak rocks. A slake-durability result describes retained dry material under the specified test conditions. It is not a compressive strength, a swelling percentage or a prediction of embankment settlement.

For the two-cycle retained-mass index used in the worked example:

Both masses exclude the container/drum tare. The first-cycle retained mass is not the denominator. Report the method, cycles and specimen observations; actual laboratory qualification requires the specified full procedure. Unexpected mass gains or inconsistent measurements should be investigated rather than silently clipped to an index limit.

Damaging alkali–silica reaction (ASR) involves susceptible silica, sufficient available alkali and moisture under the relevant exposure and time conditions. Some amorphous, microcrystalline or strained forms of silica can be reactive. Chalcedony is microcrystalline; strained quartz remains crystalline. They should not all be labelled amorphous silica.

Ordinary well-crystallized quartz is generally much less reactive than opaline or other susceptible forms under common service conditions. Resistance to ordinary surface weathering does not prove that every quartz-bearing aggregate is innocuous in alkaline concrete.

Petrography and appropriate reactivity/performance assessment, representative sampling, the proposed binder system and exposure inform acceptance. A rock name, a Mohs value or the presence of one silica constituent alone does not establish a guaranteed expansion, an automatic rejection or a universal mitigation dosage.

The following examples use $\rho_w=1000\ \text{kg/m}^3=1.000\ \text{g/cm}^3$ and $g=9.81\ \text{m/s}^2$, giving $\gamma_w=9.81\ \text{kN/m}^3$. With volume in $\text{cm}^3$, solid mass in grams is $m_s=G_sV_s$ under this water-density reference.

Dry weight means the water mass has been excluded. “Natural” mass is not automatically dry mass. The saturation-dependent expression applies where pores exist; at $n=0$, directly use $\gamma=G_s\gamma_w$ without assigning a saturation.

The additive-volume model does not predict swelling, dissolution, drainage accessibility, stress response or a change of specimen volume during wetting. A measured state that violates its assumptions requires a different interpretation.

Under the stated assumptions, water uptake measures pore volume reached by that method. Unreached or isolated pores, incomplete wetting, retained surface water, dissolution and volume error can alter the interpretation. Even a water-accessible pore is not necessarily part of a connected path carrying flow through the specimen. Report the method and uncertainty instead of treating uptake porosity as a direct permeability measurement.

• National Park Service, Minerals — mineral criteria and physical identification properties.
• National Park Service, Rocks and Minerals — rock origins, rock-cycle and Bowen teaching references.
• USGS, Petrologic and Mineral Physics Database for Use with the U.S. Geological Survey National Crustal Model, OFR 2019-1035 — idealized Bowen-series context.
• USGS, Lithogeochemical Character of Near-Surface Bedrock, WRI 99-4000 metadata — relative silicate-weathering context and its limitations.
• USGS, Geologic Characteristics of Benthic Habitats in Glacier Bay, OFR 2006-1081, Table 5 — Wentworth grain-size fractions and dimensions.
• FHWA, Alkali-Aggregate Reactivity Facts Book, FHWA-HIF-13-019 (2013) — silica forms, concrete environment and reactivity assessment.
• FHWA, Soil and Rock Properties, GEC No. 5, archived reference manual — material characterization and slake-durability discussion; not a claim of compliance with a current test edition.