Grain Identification

GRAIN IDENTIFICATION GUIDE

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Aragonite	Biogenic/detrital/authigenic CaCO₃ Calcium Carbonate
	Aragonite needles and needle fragments Lower Pleistocene, Black Sea Plane-polarised light. Scale bar equals 0.05 mm (42º05.94'N, 29º36.82'E) DSDP Leg 42, 380A-63-4, 4 cm

Aragonite

Biogenic/detrital/authigenic
CaCO₃
Calcium Carbonate

Aragonite needles and needle fragments

Lower Pleistocene, Black Sea

Plane-polarised light. Scale bar equals 0.05 mm
(42º05.94'N, 29º36.82'E)

DSDP Leg 42, 380A-63-4, 4 cm

Low magnification view of aragonite needles seen in plane polarised light (left) and cross-polarised light (right). Although fine-grained, the acicular form is clearly visible
Lower Pleistocene, Black Sea (42º05.94'N, 29º36.82'E)

High power view of tritiform aragonite grains. Note the resemblence of the larger grains to grains of rice or wheat
Sapropel layer, Pleistocene, Black Sea (43º00.29'N, 36º00.68'E), DSDP Leg 42, 379A-11-5, 47 cm

Scale bar equals 0.05 mm


Plane-polarised light. Scale bar equals 0.05mm	Cross-polarised light (different field of view)

Biogenic silica	Biogenic SiO₂
	Siliceous clay with centric diatom fragments and a silicoflagellate test (top left) Holocene, Angola Basin, Southeast Atlantic Plane-polarised light. Scale bar equals 0.05 mm
	Siliceous ooze containing abundant centric diatom frustule fragments and broken, mainly monaxon, sponge spicules South Atlantic Plane-polarised light. Scale bar equals 0.05 mm
	Siliceous sponge spicules Tyrrhenian Sea, Mediterranean Sea Plane-polarised light. Scale bar equals 0.05 mm
	Siliceous sponge spicules with a broken triaxon spicule top right, note the clear central canals Tyrrhenian Sea, Mediterranean Sea Plane-polarised light. Scale bar equals 0.05 mm

Calcite	Biogenic/detrital/authigenic CaCO₃
	Scatter of coccolith plates seen in cross-polarised light. Note the black interference crosses shown by each plate. Form of the coccolith plates as seen in cross-polarised light is considered species specific. Early Cretaceous (Aptian), lower continental rise, SE of New York, NW Atlantic, (34º54'N, 69º10'W) DSDP Leg 11, 105-13-CC (core-catcher) Cross-polarised light. Scale bar equals 0.05 mm
	Biogenic calcite largely consisting of foraminifer tests and test fragments Pleistocene, Kings Trough, NE Atlantic (43º52'N, 21º57'E) Plane-polarised light. Scale bar equals 0.05 mm

Biogenic calcite largely consisting of foraminifer tests and test fragments
Pleistocene, Kings Trough, NE Atlantic, (43º52'N, 21º57'E)
Left: Plane-polarised light. Scale bar equals 0.05 mm; Right: Cross-polarised light. Note the interference crosses seen on the individual test chambers.

Dolomite	Authigenic CaMg(CO₃)₂ Calcium magnesium carbonate
Clay containing euhedral dolomite rhombs Cenomanian-Albian, Angolan continental slope, SE Atlantic (11º39.10'S, 11º53.72'E) DSDP Leg 40, 365-2-3, 8 cm Plane-polarised light. Scale bar equals 0.05 mm

Dolomite

Authigenic
CaMg(CO₃)₂
Calcium magnesium carbonate

Clay containing euhedral dolomite rhombs
Cenomanian-Albian, Angolan continental slope, SE Atlantic (11º39.10'S, 11º53.72'E)
DSDP Leg 40, 365-2-3, 8 cm
Plane-polarised light. Scale bar equals 0.05 mm

Feldspar	Detrital and authigenic
	Euhedral feldspar crystal showing oscillatory zoning. In cross-polarised light, such oscillatory zoned crystals will show concentric shells of varying extinction. Oscillatory zoning usually reflects varying water vapour pressures in the original melt. Such crystals usually have a volcanic origin, The thin envelope of glass around this particular crystal shows it is derived from pyroclastics. The other grains in the field are mainly volcanic glass. Pleistocene, Tyrrhenian Sea, Mediterranean Sea Plane-polarised light. Scale bar equals 0.05 mm
	Subhedral plagioclase crystal (left) showing polysynthetic twinning on the albite law Pliocene, south of Java Trench, NE Indian Ocean (9º46.53'N, 102º41.95'E) DSDP Leg 22, 211-8-1, 56 cm Cross-polarised light. Scale bar equals 0.05 mm

Feldspar

Detrital and authigenic

Euhedral feldspar crystal showing oscillatory zoning. In cross-polarised light, such oscillatory zoned crystals will show concentric shells of varying extinction. Oscillatory zoning usually reflects varying water vapour pressures in the original melt. Such crystals usually have a volcanic origin, The thin envelope of glass around this particular crystal shows it is derived from pyroclastics. The other grains in the field are mainly volcanic glass.
Pleistocene, Tyrrhenian Sea, Mediterranean Sea
Plane-polarised light. Scale bar equals 0.05 mm

Subhedral plagioclase crystal (left) showing polysynthetic twinning on the albite law
Pliocene, south of Java Trench, NE Indian Ocean (9º46.53'N, 102º41.95'E)
DSDP Leg 22, 211-8-1, 56 cm
Cross-polarised light.
Scale bar equals 0.05 mm

Ferromagnesian minerals	Detrital/volcanic
	Rounded hornblende prism (right) pleochroic in greens Maastrichtian, SE flank of the Nicaragua Rise, Caribbean Sea (15º52.72'N, 74º36.47'E) DSDP Leg 15, 152-10-1, 147 cm Plane-polarised light. Scale bar equals 0.05 mm
	Hornblende prism (right) and crystal fragment (top centre) pleochroic in greens Maastrichtian, SE flank of the Nicaragua Rise, Caribbean Sea (15º52.72'N, 74º36.47'E) DSDP Leg 15, 152-10-1, 147 cm Plane-polarised light. Scale bar equals 0.05 mm
	Scattered broken amphibole crystals (green), probably hornblende, within terrigenous silt Pleistocene, Labrador Sea, NW Atlantic (56º47.40'N, 48º19.91'E) DSDP Leg 12, 113-2-3, 7 cm Plane-polarised light. Scale bar equals 0.05 mm
	Subhedral aegirine-augite pyroxene, pleochroic in greenish blues, other grains in the field of view are green and brown biotite mica, broken feldspar and anhedral accessory minerals Pleistocene, Tyrrhenian Sea, Mediterranean Plane-polarised light. Scale bar equals 0.05 mm

Glauconite	Authigenic/detrital
Irregular and smooth glauconite grains, bright green in colour with clay and detrital grains Lower Palaeocene, lower continental rise, west of Cape Agulhas, South Africa (35º03.97'S, 15º26.91'E) DSDP Leg 40, 361-10-2, 129 cm Plane-polarised light. Scale bar equals 0.05 mm

	Large smooth rounded and irregular glauconite grains. The characteristic green colour of these thick grains can be discerned at grain edges Quaternary, Feni Drift, Rockall Trough, NE Atlantic (53º13.46'N, 18º53.69'W) DSDP Leg 94, 610-4-5, 139-140 cm (picked fraction) Plane-polarised light. Scale bar equals 0.05 mm

Mica	Detrital Hydrous potassium aluminium silicates
Terrigenous silt containing abundant green and brown biotite mica flakes Pleistocene, Bengal Fan, Indian Ocean (8º00.42'N, 86º16.97'E) DSDP Leg 22, 218-3-2, 130 cm Plane-polarised light. Scale bar equals 0.05 mm

	Large biotite mica flake, successive {001} cleavage surfaces exposed through breakage are clearly visible. Note the conspicuously large size of the flake compared with the associated material, a reflection of mica's low hydraulic equivalence Pliocene, south of Java Trench, NE Indian Ocean (9º46.53'S, 102º41.95'E) DSDP Leg 22, 211-8-1, 63 cm Plane-polarised light. Scale bar equals 0.05 mm
	Terrigenous silt containing a conspicuous brown biotite mica flake showing {001} cleavage surfaces (centre, right) Pleistocene, Bengal Fan, Indian Ocean (8º00.42'N, 86º16.97'E) DSDP Leg 22, 218-3-2, 130 cm Plane-polarised light. Scale bar equals 0.05 mm
	Large dark brown biotite mica flake showing {001} cleavage surfaces Pleistocene, Bengal Fan, Indian Ocean (8º00.42'N, 86º16.97'E) DSDP Leg 22, 218-3-2, 130 cm Plane-polarised light. Scale bar equals 0.05 mm

Palagonite	Palagonite
Fields of palagonitised volcanic glass Abyssal hill, Madeira Abyssal Plain, NE Atlantic Plane-polarised light. Scale bar equals 0.05 mm

Phillipsite	Authigenic zeolite Hydrated potassium calcium sodium aluminosilicate
Field of phillipsite crystals in a zeolitic clay showing broken cruciform twins, partial rosettes and intergrown prisms ?Early Neogene, Kings Trough, NE Atlantic (44º02.80'N, 21º47.03'W) Plane-polarised light. Scale bar equals 0.05 mm

Phillipsite

Authigenic zeolite

Hydrated potassium calcium sodium aluminosilicate

Field of phillipsite crystals in a zeolitic clay showing broken cruciform twins, partial rosettes and intergrown prisms
?Early Neogene, Kings Trough, NE Atlantic (44º02.80'N, 21º47.03'W)
Plane-polarised light.
Scale bar equals 0.05 mm

Pyrite	Authigenic/detrital FeS₂ Iron disulphide
	Siliceous clay containing common scattered pyrite framboids and framboid clusters, originally formed within microfossil tests. Framboidal pyrite is the most common form of pyrite seen in marine sediments Late Pleistocene/Holocene, Angola Basin, SE Atlantic (19º11.26'S, 9º23.17'W) DSDP Leg 75, 530B-2-3, 13 cm Plane-polarised light. Scale bar equals 0.05 mm
Clusters of framboidal pyrite enclosed within, or originating from centric diatom frustules Late Pleistocene/Holocene, Angola Basin, SE Atlantic (19º11.26'S, 9º23.17'W) DSDP Leg 75, 530B-2-3, 13 cm Plane-polarised light. Scale bar equals 0.05 mm

Euhedral pyrite, forms include discrete pyritohedra and octohedra, within zeolitic clay Late Miocene, western edge of the Sigsbee Deep, Gulf of Mexico (23º47.80'N, 94º46.09'W) DSDP Leg 10,90-5-4, 100 cm Plane-polarised light. Scale bar equals 0.05 mm

Clusters of framboidal pyrite formed within centric diatom frustules Late Pleistocene/Holocene, Angola Basin, SE Atlantic (19º11.26'S, 9º23.17'W) Plane-polarised light. Scale bar equals 0.05 mm

Volcanic glass	Volcanic/detrital
	Field of volcanic glass shards. A wide range of shard shapes are present, including semi-regular glass fragments packed with subparallel pipe vesicles, glass fragments containing crystallites and bubble wall shards. A foraminifer can be seen bottom right Pleistocene, Tyrrhenian Sea, Mediterranean Sea Plane-polarised light. Scale bar equals 0.05 mm
	Field of volcanic glass. A wide range of shard shapes are present, including semi-regular glass fragments packed with subparallel pipe vesicles, vesicular glass, and bubble wall shards. Pleistocene, Tyrrhenian Sea, Mediterranean Sea Plane-polarised light. Scale bar equals 0.05 mm

Zircon	Detrital Zirconium silicate ZrSiO₄
	Prismatic subhedral zircon crystal. Note the very high relief which makes the grain prominent in the field of view Pliocene, south of Java Trench, NE Indian Ocean (9º46.53'S, 102º41.95'W) DSDP Leg 22, 211-8-1, 63 cm Plane-polarised light. Scale bar equals 0.05 mm
	For more grain identification photographs, please visit: Reference: ROTHWELL, R.G., 1989. Minerals and Mineraloids in Marine Sediments: An optical identification guide, Elsevier Applied Science, London, 279 pp. (ISBN 1-85166-382-7). Available from Kluwer Academic Publishers or from Amazon.co.uk
	Tool for microscopic indentification

Zircon

Detrital
Zirconium silicate
ZrSiO₄

Prismatic subhedral zircon crystal. Note the very high relief which makes the grain prominent in the field of view
Pliocene, south of Java Trench, NE Indian Ocean (9º46.53'S, 102º41.95'W)
DSDP Leg 22, 211-8-1, 63 cm
Plane-polarised light.
Scale bar equals 0.05 mm

For more grain identification photographs, please visit:

Reference:

ROTHWELL, R.G., 1989. Minerals and Mineraloids in Marine Sediments: An optical identification guide, Elsevier Applied Science, London, 279 pp. (ISBN 1-85166-382-7).
Available from Kluwer Academic Publishers or from Amazon.co.uk