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Geology is the Way

Crystal shape (igneous rocks)

Igneous rocks derive from the solidification of magma or lava in crystals and, eventually, volcanic glass. When they crystallize from a melt, growing crystals compete for space and may get reabsorbed if their chemical composition is in disequilibrium with the surrounding melt. Consequently, the shape of crystals is an important textural hint of the crystallization history of an igneous rock.

Euhedral, subhedral, anhedral
Crystals in rocks are classified as euhedral, subhedral, and anhedral based on their crystal shape. These three terms comes from the same Greek root, hedron, meaning ‘face’ (of a solid figure). Eu means ‘good’, hence euhedral ‘good face’ (i.e. well-formed face). The term ‘anhedral’ contains the alpha privative and it means the opposite (no faces). Subhedral has an intermediate meaning between the two (some faces).

Euhedral crystals
Crystals are said to be euhedral (synonyms: idiomorphic, automorphic) when they are bounded by their characteristic crystal faces, in other words when they show their crystal habit. In igneous rocks, crystals are able to be euhedral only when they have been able to grow undisturbed within the magma, not interacting or interfering with neighboring crystals, and without being corroded by the melt. In general, euhedral crystals represent the earliest products of crystallization from the melt.

Anhedral crystals
Anhedral crystals (synonyms: xenomorphic, allotriomorphic) are crystals that completely lack their characteristic crystal shape. This includes crystals with no-well defined shape, for example embayed crystals with irregular outline, but also interstitial grains that grew in the space left between other, pre-existing crystals. In this latter sense, anhedral crystals may be bounded by sharp crystal faces but are never their crystal faces but the faces of surrounding crystals.

Subhedral crystals
Subhedral crystals (synonym; ipidiomorphic) are bound by only some of their characteristic crystal faces. This means that were able to crystallize with their characteristic habit only in part, due to the presence of neighboring crystals that partly constrained their growth.

Outcrop-scale examples

alkali feldspar phenocrysts in monzogranite

Phenocrysts of alkali feldspar showing well-defined crystal faces (euhedral). Monte Capanne monzogranite, Elba, Italy. [see post]

reabsorbed alkali feldspar in mafic enclave

Crystals of alkali feldspar with a rounded outline (anhedral) in a mafic enclave. Monte Capanne monzogranite, Elba, Italy. [see post]

Pyroxene Castiglioncello

Subhedral to anhedral crystals of pyroxene (metallic luster) intergrown with plagioclase (green) in a gabbro from Castiglioncello, Livorno, Italy. [see post]

Microscale examples

euhedral olivine

Euhedral olivine crystals with well-formed crystal faces in a basalt from Iceland. CPL. Width: 2mm. Photo © Alessandro Da Mommio.

subhedral orthoclase

Prismatic orthoclase crystals. The crystal faces are in part straight (mostly at the contact between neighboring orthoclase grain) and they are irregular at the contact with quartz, which makes up the fine-grained grains in the groundmass. Quartz has no defined shape (anhedral). Strathbogie Granite, Australia. CPL. Photo © Rockswhisperer.

nepheline

Nepheline with subhedral habit. Note that straight crystal faces are visible only in some places (i.e. lower-left corner). In most of the cases, this crystal is intergrown with the surrounding crystals. Cape Verde archipelago. CPL. Width: 2 mm. Photo © Alessandro Da Mommio.

embayed quartz

Embayed (reabsorbed) quartz phenocrystals with anhedral habit in a rhyolite. San Vincenzo, Tuscany (Italy). CPL. Width: 2 mm. Photo © Alessandro da Mommio.

References

        

it_IT Italiano
Igneous Minerals
Igneous Textures
Plutonic Rocks
Igneous Bodies

 

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