Grain Identification

GRAIN IDENTIFICATION GUIDE

Click on the photographs below to view larger image

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)

Back to top

 

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

Back to top

 

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.

Back to top

 

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

Back to top

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

Back to top

 

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

Back to top

 

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

Back to top

 

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

 

 

 

 

 

 

 

 

 

Back to top

 

Palagonite

Palagonite

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

Back to top

 

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

Back to top

 

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

Back to top

 

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

Back to top

 

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

Back to top