Many types of rocks contain crystals. Crystals form when minerals slowly precipitate out of liquid solutions inside rocks. The key factors that determine crystal growth are the chemistry of the liquid solution, the environmental conditions like temperature and pressure, and the time available for the crystals to form. Different rock types have characteristic crystals that help geologists identify them.
Igneous Rocks
Igneous rocks form from molten rock or magma. As magma cools slowly underground, atoms have time to arrange themselves into orderly crystal structures. Igneous rocks that cool underground to form intrusive igneous bodies tend to have large, well-formed crystals. Examples include granite, diorite, and gabbro. Extrusive igneous rocks like basalt cool more rapidly on the surface, resulting in smaller crystals.
Pegmatites
Pegmatites are exceptionally coarse-grained igneous rocks with interlocking crystals over 1 centimeter in size. They contain crystals of quartz, feldspars, and micas that crystallized from water-rich magmas. Giant pegmatite crystals can measure over a meter long and weigh hundreds of kilograms. Famous crystal-collecting localities include the Black Hills of South Dakota.
Granites
Granite is an intrusive igneous rock abundant in quartz and feldspars. Slow cooling underground allowed large interlocking crystals to grow. Quartz crystals in granite are translucent and usually gray or white, feldspars are pink, tan, or white. Dark minerals like biotite mica and amphiboles occur in some granites.
| Mineral | Crystal Habit |
|---|---|
| Quartz | Hexagonal prisms |
| Feldspars | Blocky |
| Biotite Mica | Flakes |
| Amphiboles | Prismatic |
Pegmatites
Pegmatite is an exceptionally coarse-grained intrusive igneous rock. It commonly contains giant crystals over 10 centimeters long. Primary minerals include quartz, feldspars, and micas. Accessory minerals like tourmaline, beryl, and topaz occur in some pegmatites. The giant crystals form from water-rich magmas that facilitate crystal growth.
| Mineral | Crystal Size |
|---|---|
| Quartz | 10-100 cm |
| Feldspar | 10-150 cm |
| Muscovite Mica | 10-150 cm |
| Biotite Mica | 10-100 cm |
Aplites
Aplite is a fine-grained intrusive igneous rock with a sugary texture. It consists mostly of quartz and feldspar crystals under 1 mm across. Aplites crystallize from the last liquids left over after larger crystals formed in the magma chamber. The rapid cooling only allowed small crystals to grow.
Rhyolites
Rhyolite is an extrusive igneous rock with high silica content. It has an aphanitic texture with crystals mostly unseen by the naked eye. Microscopic examination reveals tiny crystals of quartz, feldspar, and biotite mica. The quick lava cooling at the surface produced small crystals. Obsidian is an extremely rapid-cooled variety of rhyolite.
Basalts
Basalt is an extrusive mafic igneous rock commonly erupted from volcanoes. Rapid cooling of basaltic lava at the surface results in small crystals visible on close inspection. Common crystals include plagioclase feldspar, pyroxene, and olivine. Long columnar jointing reflects contraction stresses as the lava cooled.
Andesites
Andesite is an extrusive igneous rock intermediate in composition between rhyolite and basalt. It contains crystals of plagioclase feldspar and one or more dark minerals such as pyroxene, biotite, or amphibole. The crystal sizes result from rapid cooling at the surface, but andesite cools slightly slower than rhyolite and basalt.
Gabbro
Gabbro is a coarse-grained intrusive mafic igneous rock. Slow cooling within the crust allows large crystals to form, including plagioclase feldspar, pyroxenes, and olivine. Some gabbros contain gigantic crystals tens of centimeters long. The Palisades Sill in New Jersey is a classic gabbro locality.
Diorite
Diorite is an intrusive igneous rock with an intermediate composition. It commonly contains crystal sizes between those of gabbro and granite. Abundant plagioclase feldspar and hornblende crystals are visible on the surface. Some diorites also contain pyroxene and biotite.
Peridotite
Peridotite is a rare ultramafic intrusive igneous rock originating from the mantle. It consists largely of the minerals olivine and pyroxene. Accessory minerals include garnets, chromite, and magnesium ilmenite. Peridotite crystals can exceed several centimeters in size.
Sedimentary Rocks
Some sedimentary rocks contain crystals that grew within the sediments as they lithified into rock. Evaporites and diagenetic minerals fall into this category. Others contain clasts of pre-existing crystalline rocks.
Evaporites
Evaporites are sedimentary minerals that crystallized from evaporating seawater. They include halite (salt), gypsum, anhydrite, and sylvite. Clear tabular halite crystals precipitate from saltwater while gypsum forms star-like rosette crystals. Evaporites can form crystal layers and masses many meters thick in restricted marine basins.
Diagenetic Crystals
After sediments are deposited, diagenetic minerals can grow in pore spaces by precipitation from mineralizing solutions. Diagenetic minerals include quartz, calcite, hematite, pyrite, kaolinite, and barite. They form interlocking crystal mosaics that cement the sediment grains together into sedimentary rock.
Conglomerates
Conglomerates are sedimentary rocks with rounded pebble, cobble, and boulder clasts over 2 mm in size. The clasts originally formed as crystals in igneous, metamorphic, and pre-existing sedimentary rocks. Conglomerates consist of transported, worn, and redeposited clasts in a finer sediment matrix.
Breccias
Breccias are coarse sedimentary rocks made of angular broken rock fragments instead of rounded clasts. They form in place by fragmentation rather than transport. The fragments consist of crystals, rock pieces, and minerals cemented back together after brecciation.
Sandstones
Sandstones are sedimentary rocks composed of sand-sized grains from 0.0625 to 2 mm across. Many sand grains are mineral and rock fragments containing crystalline components. Sandstones consist of detrital quartz, feldspar, rock fragments, and accessory heavy minerals.
| Component | Mineral Crystals |
|---|---|
| Quartz | Hexagonal Quartz |
| Feldspar | Plagioclase, K-Feldspar |
| Lithic Fragments | Various Igneous, Metamorphic, Sedimentary |
| Heavy Minerals | Zircon, Garnet, Epidote, Staurolite |
The sand grains were liberated by weathering and erosion of crystalline source rocks. Sand-sized crystals were transported and deposited to form sandstones.
Crystalline Limestones
Most limestones consist of calcite and aragonite crystals precipitated from seawater. Oolitic, fossiliferous, and lithographic (micrite) limestones exhibit microcrystalline textures. Coarser recrystallized metamorphic marbles have interlocking calcite crystals millimeters to centimeters in size.
Chert
Chert is a microcrystalline or cryptocrystalline sedimentary silica mineral. It has an extremely fine-grained texture with interlocking quartz crystals under 30 micrometers in size. Flint, agate, and jasper are colorful compact varieties of chert.
Metamorphic Rocks
Metamorphic rocks form when heat and pressure transform pre-existing rocks. New minerals grow as crystals in the solid state during metamorphism. The conditions determine the mineral assemblages. Higher grades develop larger crystals.
Slate
Slate forms from the low-grade metamorphism of mudstone and shale. Tightly packed microscopic mica crystals aligned perpendicular to cleavage planes give slate its fissility. Quartz and chlorite crystals may also be present. Slate lacks large visible crystals.
Phyllite
Phyllite is a fine-grained metamorphic rock with a silky sheen from very fine-grained mica crystals. Phyllites form at slightly higher temperatures and pressures than slate. Microscopic examination reveals tiny mica flakes and elongate quartz crystals.
Schist
Schist is a medium-grade metamorphic rock with aligned mica crystals visible to the naked eye. The micas biotite, muscovite, and chlorite form flattened crystals that elongate parallel to foliation planes during metamorphism.
Gneiss
Gneiss is a high-grade metamorphic rock with visible bands rich in minerals with differing compositions and textures. Light bands consist of quartz and feldspars. Darker bands contain abundant micas and other mafic minerals. The minerals recrystallize into bands during metamorphism.
| Felsic bands | Mafic bands |
|---|---|
| Quartz | Biotite mica |
| Feldspars | Amphiboles |
| Muscovite mica | Garnet |
Amphibolite
Amphibolite is a metamorphic rock composed mainly of amphibole minerals and plagioclase feldspar. It forms by the recrystallization of mafic igneous rocks like basalt and gabbro. Amphibolites exhibit granular textures with interlocking hornblende and actinolite crystals.
Marble
Marble forms by the metamorphism of limestone and dolostone. Recrystallization results in a coarse texture of interlocking calcite and/or dolomite crystals. Marble crystals can exceed several centimeters in size. Impurities cause coloring and banding.
Quartzite
Quartzite starts out as quartz sandstone before metamorphism recrystallizes the quartz grains into interlocking crystals. Quartzites are extremely hard metamorphic rocks that fracture through the quartz crystals. They consist almost entirely of quartz.
Serpentinite
Serpentinite forms when ultramafic rocks like peridotite alter into greenish metamorphic rocks. The original olivine and pyroxene crystals convert to serpentine group minerals and magnetite or brucite. Massive and fibrous serpentinite textures occur.
Conclusion
Many different rock types contain visible crystals that developed in the molten, sedimentary, or metamorphic environments where the rocks formed. Igneous rocks typically contain the largest crystals since their mineral constituents crystallized slowly from magmas. Metamorphic crystals grow by recrystallizing pre-existing rocks. Sedimentary crystals precipitate directly from liquids or consist of eroded crystals in clastic rocks. The mineralogy and crystal size provide clues to the origin of the rock.
