Amygdaloidal texture

The oval feature in this photomicrograph is an amygdule: a formerly open vesicle which has been filled with a secondary mineral(s) precipitated from low-T ground waters which have penetrated into the rock. In this case, the amygdule is probably filled with a zeolite mineral.

UNC sample

H-64

Rock type

amygdaloidal basalt

Locality

unknown

Kimberlite

This is a severely altered sample from the only diamond-producing kimberlite in the United States. Sorry, no diamonds in this slide, though! The polygonal shapes are relict crystals of olivine which have completed decomposed to serpentine and other alteration products.

UNC sample

ARK-35

Rock type

kimberlite

Locality

Arkansas

Volcanic breccia

This breccia is composed of fragments of a variety of volcanic materials.

UNC sample

WYO-19

Rock type

volcanic breccia

Locality

Wyoming

Volcanic breccia

This breccia is composed of fragments of a variety of volcanic materials.

UNC sample

WYO-19

Rock type

volcanic breccia

Locality

Wyoming

Komatiite

Komatiites are rare ultramafic volcanic rocks. This sample displays the characteristic "spinifex texture" defined by extremely acicular olivine phenocrysts--probably a sign of rapid crystallization from a significantly-undercooled magma.

UNC sample

CAN-?

Rock type

komatiite

Locality

Canada

Nepheline basalt

Many of the phenocrysts in this basalt are nepheline.

UNC sample

H-99

Rock type

nepheline basalt

Locality

unknown

Rhyolite

This crystal-rich rhyolite contains phenocrysts of quartz, K-feldspar (sanidine), plagioclase, and biotite in a fine-grained groundmass. Note the interesting shape of the (partially resorbed? skeletal?) quartz grain in the center of the photograph.

UNC sample

R-15

("rhyolite" card"

Rock type

rhyolite

Locality

unknown

Scoria

Scoria is another name for a highly-vesicular (almost "frothy") basalt. The black, ovals features in this photomicrograph are vesicles. Note the acicular, white plagioclase laths throughout and the euhedral, white olivine phenocryst at the lower right.

UNC sample

HAW-?

Rock type

scoriaceous basalt

Locality

Hawaii

Spherulites in rhyolite

Spherulites are radiating masses of fibrous crystals in a glassy matrix. These spherulites are probably composed of alkali feldspars and some polymorph of SiO2, and in this cross-polarized shot, appear as round objects with dark crosses. Note the large phenocryst which forms the nucleus of one of the spherulites at center-left.

UNC sample

WYO-14

Rock type

rhyolite

Locality

Wyoming

Spinifex texture

This komatiite sample displays the characteristic "spinifex texture" defined by extremely acicular olivine phenocrysts--probably a sign of rapid crystallization from a significantly-undercooled magma. The texture is named after a type of grass which grows in South Africa in the region where komatiites were first discovered.

UNC sample

CAN-?

Rock type

komatiite

Locality

Canada

Lightly-compacted tuff

In this tuff, the irregularly-shaped glass shards are still relatively undeformed. Also note the phenocryst of quartz (clear) and biotite (dark red) in this slide.

Contrast with poorly-welded tuff and welded tuff.

UNC sample

NEV-8

Rock type

rhyolitic tuff

Locality

Nevada

Poorly-welded tuff

In this sample, the glass shards are starting to get deformed. Note the phenocrysts of quartz (clear) and biotite (dark red) in this rock.

Contrast with lightly-compacted tuff and welded tuff.

UNC sample

NEV-9

Rock type

rhyolitic tuff

Locality

Nevada

Welded tuff

In this sample, the glass shards are fused together in a swirly mass, and the large pumice fragment at center right is flattened. In contrast, quartz phenocrysts are relatively undeformed.

Contrast with lightly-compacted tuff and poorly-welded tuff.

UNC sample

NEV-11

Rock type

rhyolitic tuff

Locality

Nevada

Vesicles

The black, ovals features in this scoriaceous basalt are vesicles. Note the acicular, white plagioclase laths throughout and the euhedral, white olivine phenocryst at the lower right.

Want to see what happens to vesicles after some hydrothermal alteration?

UNC sample

HAW-?

Rock type

scoriaceous basalt

Locality

Hawaii

Vitrophyre

A vitrophyre is another name for a phenocryst-bearing obsidian. The phenocrysts in the above photomicrograph are mostly plagioclase. The groundmass is obsidian glass. Can you think of some possible explanations to account for the extremely large difference in grain size in this rock?

UNC sample

BB-16

Rock type

Obsidian vitrophyre

Locality

Elloston, MT