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Feb 08 2014

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Skeletal Microstructures-Carbonate Petrology

Homogeneous Prismatic

– Size: 1 um wide and 5 – 20 um long.
– Use high power magnification in petrographic microscope.
– Identification: sweeping extinction. Microgranular: Dark coloured in ppl. Opaque in reflected light. Porcelaneous: dark or amber in ppl. Shiny white in xpl. Hyaline: speckled colours in xpl.
– Primary characteristic of foraminifera, molluscs (bivalve).
– Secondary characteristic of trilobites, ostracodes.

Normal Prismatic

– Size: ~ 10s to ~ 100s um long
– Use high power magnification in petrographic microscope, but may also be visible under low power.
– Identification: rod-like crystals that undergo unit extinction.
– Primary characteristic of brachiopods outer layer.
– Secondary characteristic of molluscs.

Complex Prismatic

– Size: < 100 um width. - Identification: rod-like structures with radiating fiber-like crystals. These radiations will make a V-shaped pattern within the rod. When the stage is rotated, the extinction will move across each rod-like structure (move across to one end and move back to the other end). - Primary characteristic of molluscs ONLY.

Composite Prismatic

– Size: ~250 um(?) in width.
– Identification: elongated/rectangular structures with fan-like radiations pointing in the elongation direction.
– Primary characteristic of molluscs ONLY.

Foliated and Nacrerous

– Foliated term is used on calcitic shells while nacreous term is used on aragonitic shells.
– Size: 0.1 – 0.5 um thickness, 4 – 20 um long and 2 – 4 um width
– Foliated identification: long thin lamination-like structures that are near parallel or parallel to the shell surface or short irregular fiber-like inclined structures.
– Primary characteristic of brachiopods.
– Secondary characteristic of molluscs, bryozoans, worm tubes.

– Nacrerous identification: almost similar to foliated but more polygonal and aragonites may be separated by a films of conchiolin
– Primary characteristic of molluscs ONLY.

Single-crystal

– Size: relatively larger than other microstructures.
– Identification: since these fragments are made of a single (usually calcite) crystal, they will undergo unit extinction. Typical calcite twining may observe (~ 60/120 type crosses) on the surface. In some samples it may look like a calcite crystal. In others you may observe some morphological features such as an outline of an echinoderm.
– Primary characteristic of echinoderms.
– Secondary characteristic of molluscs, fenestrate bryozoans, sponge spicules.

Crossed-lamellar

– Size: few mm or 100 um
– These are wedges made of 1st and 2nd order lamellae
– Identification: ridge-like elongated structures. Not to be confused with complex prismatic because they could look very very similar.
– Primary characteristic of molluscs (bivalve) ONLY.

Fascicular fibrous structure

– Size: fibers are very thin, but often found in large quantities (may be observed under low power).
– Identification: elongated fiber-like crystals that stacked around coalesced (side-by-side). They may have a bit of circular radiation pattern. Transverse sections will have fan-like growth lines.
– Primary characteristic of corals; Tabulata corals (may also be normal to tabulae). Rugose corals. Scleractinia: (trabecular structures in septal walls, crystals radiating out and upwards)

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