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Petrographic image analysis techniques can be used to discriminate
grey levels in Scanning Electron Microscope (SEM) backscattered images
for quantitative modal analysis. In this example, using false
colours for clarity, porosity (blue) and K-feldspar (yellow) have been
discriminated directly from the SEM backscattered image. Quartz (green)
and plagioclase (red) have similar grey levels, and have been discriminated
by combined BSE and X-ray mapping.
Upper Jurassic sandstone, UKCS.
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SEM backscattered electron image (grey-levels)* |
Porosity |
K-feldspar |
Quartz |
Plagioclase |
*
The grey level (0 to 255) of mineral in a backscattered image is roughly
proportional to the atomic density. However, some minerals (eg quartz,
dolomite, plagioclase) have similar grey levels and may need to be distinguished
using X-ray mapping.
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Sandstone, subarkose, composed of quartz (white), K-feldspar (stained
orange), iron oxides/hydroxides (black). Porosity is shown by blue-stained
resin. Minimal compaction of this Permo-Triassic sandstone suggests
that an early diagenetic cement was originally present preserving the
open-framework texture; the cement has been removed by aggressive pore-waters.
St Bees Sandstone Formation (Triassic), Cumbria, UK.
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Early diagenetic ?biogenic spheroids in a fine- to medium-grained
subarkose, showing variations in internal patterns and typical radial-fibrous
texture.
St Bees Sandstone Formation (Triassic), Cumbria, UK.
White - quartz; yellow - stained K-feldspar; brown - iron oxide; blue
- stained porosity.
Scale bar: 100 microns.
from: STRONG, G. E. & PEARCE, J. M. 1995. Carbonate spheroids
in Permo-Triassic sandstones of the Sellafield area, Cumbria.
Proceedings of the Yorkshire Geological
Society Vol. 50, Pt 3, 209-211.
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Cathodoluminescence micrograph of a medium-grained subarkose, showing
detrital quartz (black/brown), K-feldspar (blue), with early diagenetic
spheroids composed of calcite (dark orange) showing well-defined euhedral
overgrowths and later brightly luminescent calcite cement.
St Bees Sandstone Formation (Triassic), Cumbria, UK.
Scale bar: 200 microns.
from: STRONG, G. E. & PEARCE, J. M. 1995. Carbonate spheroids
in Permo-Triassic sandstones of the Sellafield area, Cumbria. Proceedings
of the Yorkshire Geological Society Vol. 50, Pt 3, 209-211.
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Cathodoluminescence micrograph of calcite spheroid in coarse grained
subarkose. Calcite appears corroded against pores filled by later
kaolinite. Orange clasts (top right) are corroded K-feldspar grains
replaced by calcite. Conical structures seen within the spheroid may be
cyanophyte inclusions.
Calcite - orange yellow; quartz - dark brown/purple; K-feldspar -
light blue to white; kaolinite - dark blue.
Triassic (undifferentiated), East Shetland Basin, Well 211/13-1,
UKCS
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Solid and hollow peloids of dolomicrite in anhydrite cement.
St Bees Evaporites (Permian), Cumbria, UK.
Scale bar: 100 microns.
Plane polarised light (top), cross-polarised
light (bottom)
from: STRONG, G. E., MILODOWSKI, A. E., PEARCE, J. M., KEMP,
S. J., PRIOR, S. V. & MORTON, A. C. 1994. The petrology and diagenesis
of Permo-Triassic rocks of the Sellafield area, Cumbria. Proceedings
of the Yorkshire Geological Society Vol. 50, Pt.1, pp. 77-89.
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As above, cross-polarised light.
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Conglomerate of Borrowdale Volcanic Group (Ordovician) clasts (dark
grey) with hematite (mid-grey) and barite (white) cements.
Brockram (Permian), Cumbria, UK.
Scale bar: 100 microns. Scanning electron microscope backscattered
image.
from: STRONG, G. E., MILODOWSKI, A. E., PEARCE, J. M., KEMP,
S. J., PRIOR, S. V. & MORTON, A. C. 1994. The petrology and diagenesis
of Permo-Triassic rocks of the Sellafield area, Cumbria. Proceedings
of the Yorkshire Geological Society Vol. 50, Pt.1, pp. 77-89.
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Needle-fibre calcite developed between two pebbles in fluvioglacial
gravel (Holocene), West Tanfield, North Yorkshire, UK.
Scale bar: 100 microns. Scanning electron microscope
secondary electron image.
from: STRONG, G. E., GILES, J. R. A. AND WRIGHT, V. P. 1992.
A Holocene calcrete from North Yorkshire: implications for interpreting
palaeoclimates using calcretes. Sedimentology, 39:333-347.
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