Earth materials

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• How rocks form

  Three classes, three routes — and each rock carries a legible record of the conditions that made it. Grain size is a clock; foliation is a compass.

• The rock cycle

  The rock cycle is usually drawn as a circle. It is better understood as a network: many possible pathways between materials, and no requirement that any particular rock travel the whole loop.

• What is a mineral?

  The definition is short and it does real work: it is what separates a mineral from a rock, and it is why a rock’s identity depends on how its minerals are arranged, not only on which ones are present.

Igneous rocks

Grouped by where they solidified, because cooling rate sets grain size and grain size is therefore a record of process.

• Rock

  Granite

  A coarse-grained intrusive igneous rock, typically an interlocking mixture of quartz, feldspar and mica. Its coarse crystals record slow cooling deep inside the crust.

• Rock

  Gabbro

  A coarse-grained intrusive igneous rock, the slowly cooled equivalent of basalt. It forms much of the lower oceanic crust.

• Rock

  Peridotite

  A dense, olivine-rich intrusive igneous rock. It is the principal rock of the upper mantle, and partial melting of peridotite is where most of Earth’s magma begins.

• Rock

  Basalt

  A common fine-grained extrusive igneous rock, formed where magma erupts and cools quickly at or very near the surface.

• Rock

  Rhyolite

  A fine-grained extrusive igneous rock of silica-rich composition. Magma of this kind is viscous, traps its gases, and tends to erupt explosively.

• Rock

  Andesite

  A fine-grained extrusive igneous rock of intermediate composition, characteristic of volcanic arcs above subduction zones.

• Rock

  Obsidian

  A volcanic glass formed when silica-rich lava cools too quickly for crystals to grow at all. Its texture is a direct record of the cooling rate.

Sedimentary rocks

Named by grain size for the clastic ones, because grain size records the energy of the medium that carried the material.

• Rock

  Conglomerate

  A clastic sedimentary rock of rounded pebbles set in a finer matrix, recording transport energetic enough to move coarse material.

• Rock

  Sandstone

  A clastic sedimentary rock made of sand-sized grains cemented together. Its grains are fragments of older rock, so a sandstone carries a record of a landscape that no longer exists.

• Rock

  Siltstone

  A clastic sedimentary rock of silt-sized grains, intermediate between shale and sandstone in the grain-size classification.

• Rock

  Shale

  A fine-grained clastic sedimentary rock made mostly of clay-sized particles, deposited where water was still enough for the finest material to settle.

• Rock

  Limestone

  A carbonate sedimentary rock formed from the remains of organisms or by direct precipitation from water. Under metamorphism it recrystallises into marble.

Metamorphic rocks

Grouped by whether directed pressure aligned their minerals — foliation records the orientation of the stress that produced it.

• Rock

  Slate

  A fine-grained metamorphic rock produced from shale, splitting along planes created by the alignment of platy minerals under directed pressure.

• Rock

  Schist

  A strongly foliated metamorphic rock with visibly aligned platy minerals, formed at higher grade than slate.

• Rock

  Gneiss

  A coarsely banded, high-grade metamorphic rock. The oldest rocks yet found on Earth are gneisses.

• Rock

  Marble

  A metamorphic rock produced from limestone or another carbonate rock, harder and more crystalline than its protolith. Its minerals are not flat or elongate, so it does not develop foliation.

• Rock

  Quartzite

  A hard, non-foliated metamorphic rock formed from quartz-rich sandstone.

Minerals

Accepted mineral species, each carrying its IMA-approved formula and status. Group names such as olivine, feldspar and mica are recorded separately, as groups, because that is what they are.

• Mineral

  Quartz

  A hard, chemically simple silicate mineral (silicon dioxide) that is among the most abundant minerals of the continental crust and the index mineral for hardness 7 on the Mohs scale.

• Mineral

  Forsterite

  The magnesium end member of the olivine series and the dominant mineral of the upper mantle. Its structure is isolated silicate tetrahedra bound by magnesium and iron, which is why its composition varies continuously toward the iron end member.

• Mineral

  Albite

  The sodium end member of the plagioclase feldspar series, and one of the most abundant minerals of the continental crust.

• Mineral

  Muscovite

  The common white mica: a sheet silicate whose perfect basal cleavage follows the planes of its structure, and whose platy grains align under directed pressure to make foliation.

• Mineral

  Diopside

  A calcium–magnesium clinopyroxene: a single-chain silicate and one of the principal minerals of the mantle and of calcium-rich metamorphic rocks.

• Mineral

  Almandine

  The iron–aluminium garnet, common in metamorphosed shales and a standard indicator of metamorphic conditions.

• Mineral

  Calcite

  The commonest carbonate mineral and the principal constituent of limestone. Its solubility depends on the acidity of the water it meets, which makes it the mineral through which carbon moves between rock and ocean.

• Mineral

  Magnetite

  A strongly magnetic iron oxide. Its presence in ocean-floor basalt is what allowed the sea floor to record the polarity of Earth’s magnetic field as it cooled — and so what made seafloor spreading detectable.

Processes

How material moves between the three classes.

• Geological process

  Magmatic crystallisation

  The solidification of molten rock into an igneous rock. How fast it happens sets the grain size: slow cooling at depth gives coarse crystals, rapid cooling at the surface gives fine-grained or glassy rock.

• Geological process

  Metamorphism

  The transformation of existing rock by heat, pressure and chemically active fluids, without melting. It rearranges minerals and fabrics; a rock that melts is no longer being metamorphosed.

• Geological process

  Lithification

  The conversion of loose sediment into rock through burial, compaction and cementation as pore water is expelled.

• Geological process

  Partial melting

  The production of liquid from only part of a solid rock, leaving a residue behind. Almost all magma on Earth is generated this way; the conditions that trigger it differ from setting to setting.

Mineral groups and series

Names in constant scientific use that the IMA List of Minerals does not accept as species. Keeping them apart from species is the difference between a mineralogical record and a folk taxonomy.

• Mineral group

  Olivine

  A group of orthosilicate minerals, not a species. Natural olivine is a solid solution between forsterite and fayalite, and it is those end members — not “olivine” — that the IMA list accepts.

• Mineral group

  Feldspar

  The most abundant mineral group of the crust — and a group, not a species. Its members split into the plagioclase series and the alkali feldspars, three of which share one formula and differ only in structural order.

• Mineral group

  Plagioclase

  The continuous solid-solution series between albite and anorthite. A plagioclase composition is a position along that series, which is why the series and not a single formula is the right unit of description.

• Mineral group

  Alkali feldspar

  The potassium- and sodium-rich feldspars, including microcline, orthoclase and sanidine — one composition, three structural states, three species.

• Mineral group

  Pyroxene

  Single-chain silicates divided into orthopyroxenes and clinopyroxenes. Like olivine, a group name rather than a species.

• Mineral group

  Mica

  Sheet silicates whose structure produces perfect basal cleavage. Muscovite is a species within it; “biotite” is not.

• Mineral group

  Biotite

  A name in universal scientific use that the IMA list does not accept as a species: it denotes the series between annite and phlogopite. Lithoen records it as a series so that the name remains usable without being mistaken for a species.

• Mineral group

  Garnet

  Cubic orthosilicates sharing one structure type across a wide range of compositions, including almandine, pyrope and grossular.

• Mineral group

  Amphibole

  Double-chain silicates. “Hornblende” — the name most often used for them in the field — is not an accepted species either, and Lithoen does not record it as one.

• Mineral group

  Clay minerals

  A functional grouping of fine-grained hydrous sheet silicates defined as much by grain size and behaviour as by structure. It is not an IMA grouping, and Lithoen marks it informal rather than implying an authority that does not exist.