1. Education
Send to a Friend via Email

Discuss in my forum

Crysts, Blasts and Clasts

The terminology of large particles

By

Here are three simple words related to a very basic concept in geology: big particles in rocks. Actually, they're pieces of words—suffixes—that are worth knowing about. This isn't especially easy to keep straight, but it's what geologists have to know.

Crysts

The "-cryst" suffix refers to grains of a crystalline mineral. A -cryst can be a fully formed crystal like your typical garnet, or it can be an irregular grain that, even though its atoms are all in rigid order, has none of the flat faces that mark a crystal. The most important -crysts are the ones that are much larger than their neighbors; the general name for these is megacryst. As a practical matter, "-cryst" is used only with igneous rocks, although a crystal in metamorphic rocks may be called a metacryst.

The most common -cryst you'll see in the literature is the phenocryst. Phenocrysts sit in a groundmass of smaller grains like raisins in oatmeal. Phenocrysts are the defining feature of porphyritic texture; another way to say it is that phenocrysts are what define a porphyry.

Phenocrysts generally consist of one of the same minerals found in the groundmass. (If they were brought into the rock from elsewhere, they may be called xenocrysts.) If they're clean and solid inside, we may interpret them as being older, having crystallized earlier than the rest of the igneous rock. But some phenocrysts formed by growing around and engulfing other minerals (creating a texture called poikilitic), so in that case they weren't the very first mineral to crystallize.

Phenocrysts that have fully formed crystal faces are called euhedral (old papers may use the terms idiomorphic or automorphic). Phenocrysts with no crystal faces are called anhedral (or xenomorphic), and in-between phenocrysts are called subhedral (or hypidiomorphic or hypautomorphic).

Blasts

The "-blast" suffix refers to grains of metamorphic minerals; more precisely, "-blastic" means a rock texture that reflects the recrystallizing processes of metamorphism. That's why we don't have a word "megablast"—both igneous and metamorphic rocks are said to have megacrysts. The various -blasts are described only in metamorphic rocks. Metamorphism produces mineral grains by crushing (clastic deformation) and squeezing (plastic deformation) as well as recrystallization (blastic deformation), so it's important to make the distinction.

A metamorphic rock made of -blasts of uniform size is called homeoblastic, but if megacrysts are also present it is called heteroblastic. The larger ones are usually called porphyroblasts (even though porphyry is strictly an igneous rock). So porphyroblasts are the metamorphic equivalent of phenocrysts.

Porphyroblasts may be stretched out and erased as metamorphism continues. Some large mineral grains may resist for a while. These are commonly called augen (the German for eyes), and augen gneiss is a well-recognized rock type.

Similar to -crysts, -blasts can display crystal faces in different degrees, but they are described with the words idioblastic, hypidioblastic and xenoblastic instead of euhedral or subhedral or anhedral. Grains inherited from an earlier generation of metamorphism are called paleoblasts; naturally, neoblasts are their younger counterpart.

Clasts

The suffix "-clast" refers to grains of sediment, that is, pieces of pre-existing rocks or minerals. Unlike -crysts and -blasts, the word "clast" can stand alone. Clastic rocks, then, are always sedimentary (one exception: a clast that is not yet wiped out in a metamorphic rock is called a porphyroclast, which, confusingly, is also classified as a megacryst). There's a deep distinction drawn among clastic rocks between holoclastic rocks, like shale and sandstone, and pyroclastic rocks that form around volcanoes.

Clastic rocks are made of particles ranging in size from microscopic to indefinitely large. The rocks with visible clasts are called macroclastic. Extra-large clasts are called phenoclasts—so phenoclasts, phenocrysts and porphyroblasts are cousins.

Two sedimentary rocks have phenoclasts: conglomerate and breccia. The difference is that the phenoclasts in conglomerate (spheroclasts) are made by abrasion whereas those in breccia (anguclasts) are made by fracture.

There is no upper limit to what can be called a clast, or megaclast. Breccias have the largest megaclasts, up to hundreds of meters across and larger. Megaclasts as big as mountains can be made by large landslides (olistrostromes), thrust faulting (chaoses), subduction (mélanges) and "supervolcano" caldera formation (caldera collapse breccias). Megaclasts are where sedimentology meets tectonics.

  1. About.com
  2. Education
  3. Geology
  4. Rocks & Rock Collecting
  5. Petrology and Geochemistry
  6. Crysts, Blasts and Clasts - Large Particles in Rocks

©2014 About.com. All rights reserved.