Rocks

The whole world is literally built on rocks.

Introduction

Reading the Rocks VSI. So many new words! Here are some definitions and examples. Except the OED section, all content generated by GPT4o with no specific fact checking.

Mineral and Rock: OED Definitions

mineral (n)

A naturally occurring substance of neither animal nor vegetable origin; an inorganic substance

mineral (n)

A substance obtained by mining; a product of the depths of the earth, esp. one other than a native metal.

mineral (n) science

A solid, naturally occurring, usually inorganic substance with a definite chemical composition and characteristic physical structure and properties (such as crystalline form).

mineral (n) biology and medicine

Any of the chemical elements required by living organisms that are or can be obtained as inorganic compounds (other than water or molecular oxygen), i.e. those other than hydrogen, carbon, nitrogen, oxygen, and (sometimes) sulfur.

rock (n)

A large rugged mass of hard mineral material (see sense I.2a) or stone forming a cliff, crag, or other natural feature on land or in the sea.

rock (n)

The solid mineral material forming much of the substance of the earth (or any similar planetary body), whether exposed on the surface or overlain by soil, sand, mud, etc.

rock (n)

A particular kind of such material. Rocks are distinguished by their composition and their physical properties, and consist of aggregates of minerals (very commonly silicates or calcium carbonate) and occasionally also organic matter (as in, for example, lignite and oil shale). They vary in hardness, and include soft materials such as clays. Rocks form the substance of the earth’s crust and mantle, down to the upper surface of the metallic core. Those occurring at the earth’s surface are broadly divided into three classes according to their process of formation: igneous, metamorphic, and sedimentary.

Terminology

Structural Terms

Term	Definition	Example / Composition	Notes
Amorphous	Lacking long-range atomic order	Volcanic glass	Common in glasses and some fast-cooled lavas
Crystalline	Atoms arranged in a repeating 3D pattern	Quartz, feldspar	Most minerals are crystalline
Glass	Solid without crystal structure (amorphous)	Obsidian (volcanic glass)	Forms by rapid cooling of lava
Crystal	Solid with internal lattice structure	Any visible mineral grains	Can refer to shape or internal structure

Major Mineral Groups

Term	Definition / Group	Composition / Formula	Occurrence
Silica	SiO₂ in various forms (crystal or glass)	Quartz (crystalline), glass	Very common, in sand and rocks
Feldspar	Framework silicate group	KAlSi₃O₈–NaAlSi₃O₈–CaAl₂Si₂O₈	~60% of Earth’s crust
Mica	Sheet silicate group	e.g. Muscovite: KAl₂(AlSi₃O₁₀)(OH)₂	Common in metamorphic & igneous rocks
Pyroxene	Single-chain silicate group	(Mg,Fe,Ca)SiO₃	Found in igneous/metamorphic rocks
Amphibole	Double-chain silicate group	e.g. Hornblende	Common in metamorphic rocks
Olivine	Isolated tetrahedra silicate	(Mg,Fe)₂SiO₄	Common in mantle rocks (e.g. peridotite)
Corundum	Oxide mineral	Al₂O₃	Rare; ruby & sapphire varieties

Rock Types

Term	Definition	Notes / Examples
Basalt	Fine-grained volcanic rock (mafic)	Ocean crust; rich in pyroxene, feldspar
Igneous	Formed by solidified magma or lava	Granite, basalt
Sedimentary	Formed from particles or precipitates	Sandstone, limestone
Metamorphic	Altered by pressure/temperature	Gneiss, schist, marble

Soil & Derivatives

Term	Definition	Notes
Clay	Fine-grained minerals, often sheet silicates	Product of weathering
Soil	Mix of minerals, organic matter, water, air	Surface layer for life
Mineral	Naturally occurring crystalline solid	Building blocks of rocks

Sediment Size, Name, and Rock Type

Size Class	Diameter (mm)	Loose Sediment Term	Lithified Rock Type	Notes
Boulder	> 256	Gravel	Conglomerate (rounded) or Breccia (angular)	Often found in mountain or glacial deposits
Cobble	64 – 256	Gravel	Conglomerate / Breccia
Pebble	4 – 64	Gravel	Conglomerate / Breccia
Granule	2 – 4	Gravel	Conglomerate / Breccia	Transition to sand
Sand	0.0625 – 2	Sand	Sandstone	Named by dominant grain size
Silt	0.0039 – 0.0625	Mud (if wet) / Silt	Siltstone	Gritty texture when rubbed
Clay	< 0.0039	Mud (if wet) / Clay	Claystone or Shale	Smooth feel; shale = fissile

Silicates

Structural Types

Structure Type	Description	Example Mineral Groups	Notes
Isolated (Nesosilicate)	Single SiO₄ tetrahedra, no shared oxygen atoms	Olivine, garnet	High-density, found in mantle rocks
Single-chain (Inosilicate)	SiO₄ tetrahedra linked in single chains (2 shared O)	Pyroxene	Common in mafic igneous rocks
Double-chain (Inosilicate)	Tetrahedra linked in double chains (alternating shared O)	Amphibole	Typical in metamorphic rocks
Sheet (Phyllosilicate)	Tetrahedra linked in 2D sheets (3 shared O)	Mica, clay minerals	Soft, flaky; common in sediments
Framework (Tectosilicate)	3D network of tetrahedra (all 4 O shared)	Feldspar, quartz (silica)	Very stable, abundant in crust

Silicate structural types describe how SiO₄ tetrahedra (a silicon atom bonded to 4 oxygen atoms) are linked together at the atomic level. Each tetrahedron is shaped like a pyramid: one Si atom in the center, four O atoms at the corners.

Structure	Oxygens Shared per Tetrahedron	Description
Isolated	0	Tetrahedra not touching
Single Chain	2	Each tetrahedron shares 2 O atoms → chains
Double Chain	2 or 3	Chains linked side by side
Sheet	3	Tetrahedra share 3 O atoms → form sheets
Framework	4	All 4 O atoms shared → 3D framework

Sheet and Framework Silicates

The arrangement of atoms affects the physical properties (e.g. cleavage, hardness), stability under pressure/temperature, and abundance (framework types dominate Earth’s crust).

• Sheet (Phyllosilicates):
  • Forms flat 2D layers.
  • Example: Mica, clays.
  • Layers are weakly bonded → flaky, soft, perfect cleavage.
• Framework (Tectosilicates):
  • Forms rigid, 3D network.
  • Example: Quartz, Feldspar.
  • Strong, hard, very stable.

Major Rock-Forming Silicate Mineral Groups

Mineral Group	Structure Type	Key End-Members / Examples	Notes
Feldspar	Framework	Plagioclase series: Anorthite (CaAl₂Si₂O₈) ↔︎ Albite (NaAlSi₃O₈)
		Alkali feldspar: Orthoclase (KAlSi₃O₈)	Most abundant group in the crust
Mica	Sheet	Muscovite (K-rich), Biotite (Fe/Mg-rich)	Flaky, soft, excellent cleavage
Pyroxene	Single-chain	Enstatite (MgSiO₃), Ferrosilite (FeSiO₃), Diopside (CaMgSi₂O₆)	Common in basalt, gabbro
Amphibole	Double-chain	Hornblende (complex), Tremolite (Ca₂Mg₅Si₈O₂₂(OH)₂)	Common in metamorphic rocks
Olivine	Isolated	Forsterite (Mg₂SiO₄), Fayalite (Fe₂SiO₄)	Mantle minerals, ultramafic rocks
Silica / Quartz	Framework	Quartz (SiO₂), Cristobalite, Tridymite	Polymorphs of SiO₂
Clay minerals	Sheet	Kaolinite, Smectite, Illite	Weathering products of feldspar
Feldspathoid	Framework (open)	Nepheline, Leucite	Occur in silica-undersaturated rocks

Plagioclase Feldspar Series

An end-member is a pure composition at one end of a solid solution series—a range where elements can substitute for each other in a mineral’s crystal structure. The Plagioclase Feldspar Series is a continuous series between two end-members:

End-member	Formula	Notes
Albite	NaAlSi₃O₈	Sodium-rich
Anorthite	CaAl₂Si₂O₈	Calcium-rich

Most natural plagioclase feldspars are somewhere in between, e.g.: Labradorite ≈ 50% anorthite, 50% albite and Oligoclase, Bytownite are other intermediate names.

Etymology

Etymological pointers for the major terms, especially mineral groups.

Rock & Mineral Terms

Term	Language & Root	Meaning / Origin
Amorphous	Greek a- (not) + morphē	“Without form”
Crystalline	Greek krystallos	“Ice” or “rock crystal”
Glass	Germanic origin (glas)	Related to “shiny” or “amber-like”
Crystal	Greek krystallos	Same root as crystalline
Mineral	Latin minerale	“Something mined”
Soil	Latin solum	“Ground, bottom”
Clay	Old English clǣg	Related to “sticky earth”

Mineral Groups

Term	Language & Root	Meaning / Origin
Feldspar	German Feldspat	“Field rock”; spat = non-metallic mineral
Mica	Latin mica	“Crumb” or “grain” (due to flaky appearance)
Silica	Latin silex / silicis	“Flint” or “hard stone”
Olivine	Latin oliva	“Olive” (green color)
Pyroxene	Greek pyr (fire) + xenos (stranger)	“Fire stranger” (thought to be impurity in lava)
Amphibole	Greek amphibolos	“Ambiguous” (complex and variable forms)
Corundum	Sanskrit kuruvinda	“Ruby”
Anorthite	Greek an- (not) + orthos (straight)	Refers to crystal angles
Anorthosite	Derived from anorthite	Rock made of anorthite

Rock Types

Term	Language & Root	Meaning / Origin
Igneous	Latin ignis	“Fire”
Metamorphic	Greek meta- (change) + morphē (form)	“Changed form”
Sedimentary	Latin sedere	“To sit” → settle out of water
Basalt	Latin basaltes (from Greek or Ethiopian origin)	Possibly “very hard stone”

General Naming Principles

Mineral vs. Rock

• Minerals:
  • Use the -ite suffix.
  • Names often reflect chemistry, structure, or appearance.
  • E.g.:
    • Anorthite = a specific calcium-rich feldspar mineral.
    • Olivine, Quartz, Muscovite, Hornblende are mineral names (though not all end in -ite).
• Rocks:
  • Use the -ite or -ite-derived name if based on a dominant mineral (but not always).
  • Use -stone, -rock, or -ite depending on tradition and grain size.
  • Plutonic/igneous rock names often end in -ite or -ite-derived terms based on content.
  • E.g.:
    • Anorthosite = igneous rock composed mostly of anorthite (mineral).
    • Diorite, Gabbro, Granite = rock types named for mineral content.
    • Peridotite = rock rich in peridot (a gem variety of olivine).
    • Dunite = rock composed almost entirely of olivine.

Mineral/Rock Pairs

Mineral	Rock Made Mostly of That Mineral	Notes
Anorthite	Anorthosite	Plagioclase feldspar
Olivine	Dunite, Peridotite	Ultramafic rocks
Calcite	Limestone, Marble	Marble = metamorphic
Quartz	Quartzite, Sandstone	Sandstone = clastic; quartzite = metamorphosed
Muscovite	Schist (mica schist)	Metamorphic rock rich in mica
Hornblende	Amphibolite	Metamorphic rock from mafic precursors

IUGS System

The IUGS (International Union of Geological Sciences) rock classification schemes are international standards used to classify igneous, metamorphic, and sedimentary rocks.

Igneous Rocks

• Plutonic rocks (slow-cooled, coarse-grained) are classified by mineral content, especially proportions of:
  • Quartz (Q)
  • Alkali feldspar (A)
  • Plagioclase (P)
  • Feldspathoids (F)
• Example:
  • >20% quartz → Granite
  • High plagioclase, little quartz → Diorite, Gabbro
  • >90% olivine → Dunite
• Volcanic rocks (fine-grained) use texture and chemistry instead:
  • Basalt = mafic, low-silica
  • Rhyolite = felsic, high-silica

Sedimentary Rocks

• Clastic rocks classified by grain size:
  • Conglomerate, Sandstone, Siltstone, Shale
• Chemical/biochemical rocks classified by composition:
  • Limestone, Chert, Evaporites (e.g., gypsum, halite)

Metamorphic Rocks

• No single universal scheme.
• Classified by:
  • Texture: foliated (schist, gneiss) vs. non-foliated (marble, quartzite)
  • Protolith: original rock before metamorphism
  • Mineral assemblage: index minerals indicating pressure/temperature

Clay

Clay is a group of very fine-grained minerals, typically defined by particle size (<2 μm) and distinctive sheet-like atomic structure. These minerals are phyllosilicates, built from layers of SiO₄ tetrahedra arranged in broad, thin sheets, often only a few nanometres thick. Individual clay particles are commonly flake-like, meaning they are flat and irregular in shape, with a very high aspect ratio—like microscopic torn bits of paper. These flakes may occur singly, as loosely connected book-like stacks, or, less commonly, in strand-like or fibrous forms. Because the particles are so small and their layers expose both external and internal surfaces, clays can have enormous surface areas—sometimes exceeding 100 m²/g, and even up to 700 m²/g for expansive clays like smectite. This vast surface area, along with surface charge properties, gives clays extraordinary chemical reactivity and water retention.

Clay particles in suspension are often negatively charged, causing them to repel each other and remain dispersed. However, when ions (especially multivalent cations like Ca²⁺) are present in solution, this repulsion can be reduced or neutralized, allowing the particles to flocculate—that is, form loose, porous clumps known as flocs. Flocculation makes clays settle out of water more readily and affects the physical behavior of soils, suspensions, and clays in industrial processes. This interplay between surface chemistry, particle shape, and water interaction is central to clay’s roles in soils, sedimentary rocks, engineering, and environmental systems.

Table 1: Terms used to describe clay.

Term	Meaning	Notes
Book-like	Stacks of flat clay sheets (like pages of a book)	Common in kaolinite; layers tightly bound, not easily swollen
Flake-like	Thin, flat, irregularly shaped particles—usually one or a few layers thick	General term for small, 2D particles (think torn pieces of paper)
Strand-like	Long, threadlike, fibrous particles—elongated in one direction	Less common in typical clays; may refer to halloysite tubes or weathered edges of sheets

Sand vs. Clay

Clay and sand differ in both particle size and mineral composition.

Sand is mostly composed of quartz (silicon dioxide, SiO₂) and has relatively large, gritty particles (0.05 to 2 mm in diameter). Sand grains are typically rounded and do not cohere when wet.

Clay is made of phyllosilicate minerals (such as kaolinite, illite, or smectite) and has extremely fine particles (< 0.002 mm). But the key difference is structure and composition, not just size. Clay particles are flat and plate-like, and they bind together when wet, becoming highly plastic. Clay minerals form by chemical alteration, not just physical breakdown—typically when feldspar-rich rocks weather under the action of water and acids.

You can’t make clay just by grinding sand. You need the right parent rock, chemical weathering, and geological time.

Concrete vs. Cement (vs. Mortar)

Cement is an ingredient; concrete is the final product.

Cement is a fine powder (usually Portland cement) made by heating limestone and clay. When mixed with water, it hardens through a chemical reaction called hydration.

Concrete is a mixture of cement, sand, gravel (or crushed stone), and water. The cement binds the other materials together when it hardens. Thus, concrete = filler (sand + gravel) + cement + water.

Typical concrete mix by volume (approximate):

Component	Proportion
Gravel	40%
Sand	30%
Cement	10–15%
Water	15–20%

These vary depending on strength, durability, and workability requirements. A common ratio by mass is 1:2:4 (cement:sand:gravel) with water–cement ratio ~0.45–0.60.

For a brick wall, you usually use mortar, not concrete. Mortar is cement + sand + water (sometimes with lime). It has no gravel, which makes it workable and easy to spread. Mortar is smoother and stickier, ideal for bonding bricks. Concrete is stronger in bulk and better for slabs, footings, and structural elements.

Romans used pozzolanic cement, which could set underwater. It was made from: Lime (calcium oxide), Volcanic ash (pozzolana, especially from the Bay of Naples); Seawater; and Volcanic rock or tuff as aggregate.

The volcanic ash reacted with lime and seawater to form calcium-aluminum-silicate hydrates, making a durable binder even underwater. Modern research shows this reaction actually strengthens over time, explaining the remarkable longevity of Roman marine structures.

Cement + water alone is called neat cement paste. It’s rarely used on its own because it shrinks a lot when it dries, cracks easily, and is expensive compared to concrete or mortar. But it does have a few uses: grouting (e.g. in rock or soil injection), sealing old oil wells, priming surfaces before plastering (a very thin layer, with extra water), and patching tiny cracks (with additives). Still, in most cases, it’s mixed with sand or aggregates to improve strength, reduce shrinkage, and lower cost.

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Other examples of composite materials combining a strong filler plus binding mixture are shown in Table 2.

Table 2: Materials made of strong stuff embedded in a matrix that holds it together and transfers stress.

Composite	Filler (Reinforcement)	Binder (Matrix)
Concrete	Sand + gravel	Cement paste
Carbon fiber epoxy	Carbon fibers	Epoxy resin
Fiberglass	Glass fibers	Polyester/epoxy resin
Plywood	Wood veneers (layers)	Glue (resin adhesive)
Adobe	Straw or grass	Clay
Reinforced concrete	Steel bars (rebar)	Concrete
Laminated composites	Fibers or fabrics	Polymer resins