Rocks and Minerals Key Vocabulary

Phaneritic

Coarse-grained texture where crystals are visible to the naked eye, typical of slow-cooling plutonic rocks like granite.

Aphanitic

Fine-grained texture with crystals too small to see without magnification, from rapid surface cooling in volcanic rocks.

Porphyritic

Texture with large phenocrysts (early-formed crystals) in a finer groundmass, indicating two-stage cooling.

Glassy

Non-crystalline texture like obsidian, formed by very rapid quenching of lava.

Phenocryst

Large crystal embedded in a finer matrix of igneous rock.

Plutonic

Intrusive igneous rocks that cool slowly underground, producing phaneritic textures.

Volcanic

Extrusive igneous rocks that cool quickly at the surface, often aphanitic or glassy.

Felsic

Light-colored composition rich in silica (>65%), feldspar, and quartz; low in iron/magnesium.

Mafic

Dark-colored composition rich in magnesium, iron, pyroxene, and olivine; silica ~45-55%.

Foliation

Planar arrangement of minerals in metamorphic rocks due to aligned flaky or elongate crystals under directed pressure.

Schistosity

Type of foliation with strong parallel alignment of platy minerals like mica, allowing splitting into thin sheets.

Protolith

Original parent rock (igneous, sedimentary, or metamorphic) that undergoes metamorphism.

Contact Metamorphism

Local alteration near igneous intrusions from heat, producing non-foliated rocks like hornfels.

Regional Metamorphism

Widespread change over large areas from tectonic forces, yielding foliated rocks like schist and gneiss.

Metamorphic Grade

Relative temperature and pressure intensity; low-grade (slate) to high-grade (gneiss).

Index Mineral

Specific minerals like garnet or staurolite that indicate metamorphic conditions.

Porphyroblast

Large crystal grown during metamorphism amid finer matrix grains.

Bedding

Layers of sedimentary rock separated by bedding planes, representing periods of deposition.

Bed

A single layer of sedimentary rock with uniform composition and texture, bounded by bedding planes; thicknesses range from millimeters to meters.

Clastic

Sedimentary rocks made from fragments (clasts) of pre-existing rocks, like sandstone or conglomerate, formed by weathering, transport, and cementation.

Strata

A sequence of beds or layers in sedimentary rock, showing the order of deposition with older layers below newer ones.

Lamination

Thin layers (a few grains thick) within bedding, often from subtle changes in sediment supply, like varves in lakes.

Cross-bedding

Inclined layers within a bed, formed by currents like wind or water, such as in dunes or river channels.

Cementation

The process where minerals like calcite or silica bind sediment grains into solid rock.

Grain size

Refers to clast diameters, e.g., clay (

Sorting

Degree of uniformity in grain sizes; well-sorted means similar sizes, poorly sorted means mixed.

Conglomerate

Coarse clastic rock with rounded pebbles in a matrix.

Breccia

Coarse clastic rock with angular fragments.

Shale

Fine-grained, fissile rock from clay and silt.

Limestone

Biogenic rock often from shell fragments or precipitates.