Photomicrographs of Rock Fragments

This photograph shows a sediment with many rock fragments. The two fragments in the center of the photograph above the large quartz grain are made up of fine-grained material which cannot be resolved at this magnification. They are fragments of shale or slate, and the characteristic platy shape is a result of derivation from a cleaved source rock containing abundant platy mimerals. The sediment is very poorly sorted, containing many small rock fragments, quartz grains and at least one twinned feldspar(in the center, near the top), as well as the large quartz grain, part of which is seen at the base of the photograph.(*)

This photograph shows a fragment of muscovite-bearing quartz-rich rock. The mica flakes show a preferential alignment resulting in a schistose texture. Such fragments are sometimes classified as schistose quartz rather than metamorphic rock fragments.(*)

This photograph shows two different igneous rock fragments. To the left and above the center of the field of view is a fine-grained, probably volcanic, basic rock. It consists of microphenocrysts of plagioclase feldspar set in a groundmass of feldspar, very small pyroxene crystals and opaques. Pale green chlorite occurs, possibly filling original vesicles. The lower part of the field of view is mostly occupied by a coarse-grained plutonic rock fragment consisting mainly of plagioclase feldspar and pyroxene.(*)

Sedimentary rock fragment-chert. Note the very uniform microcrystalline quartz with no visible relict texture. Grain is surrounded by carbonate matrix and cement. Chert derived from sedimentary sources can be mistaken for very finely crystalline volcanic rock fragments or clay clasts if not carefully examined(Cretaceous Travis Peak Cgl., Texas).(**)

A detrital dolomite fragment(SRF). Note the rhombic shape of constituent crystals as well as their pronounced zoning with cloudy centers and clear rims. These are good criteria for the recognition of dolomite although many dolomites do not show either characteristic. Staining is the most reliable technique for identification of detrital or authigenic dolomite(Cretaceous Rieselberger Sandstone, Germany).(*)

A large grain of definte metamorphic origin. Consists of numerous, elongate, crenulate quartz crystals welded together. Most probably this is a fragment of a sheared metaquartzite(Oligocene Tongriano Fm., Alaska).(**)

This photograph shows a schistose texture commonly seen in high-rank metamorphic rock fragments. Elongate quartz grains are separated by thin mica plates. Detrital fragments of such rock types are normally quite soft and rarely survive extensive transport; however, when such fragments are found, they are excellent indicators of a metamorphic source(Paleozoic andalusite schist, New Hampshire).(**)

Vocanic rock fragments(FRF's). The large grain in upper center shows laths of plagioclase set in a very finely crystalline matrix. The other dark grains are also VRF's but are much more difficult to identify because of the lack of phenocrysts. Such VRF's must be carefully distinguished from detrital chert or clay clasts. The cement in this example is calcite(Cretaceous Ildefonso Fm., Puerto Rico).(**)

Abundant volcanic glass shards composed of opalline silica. Fragments of shard-filled sediment can be found and also are excellent indicators of a volcanic source area(primarily acidic volcanism)(Tertiary Horse Springs Fm., Nevada).(**)

A volcanic feldspar grain. This grain would not be classified as a VRF, but it would be useful in confirming a volcanic source or in deciding whether associated fine-grained rock fragments are of volcanic origin. This example shows a plagioclase with well developed twinning, euhedral outline, and faint(but very diagnostic) compositional zoning set in a glassy groundmass(Tertiary Needles Range Fm., Nevada).(**)