Page 24 - Atlas of Sedimentary Rocks Under The Microscope

Terrigenous clastic rocks 36, 37,38
San d s t o n e s- M a t r i x

a n d cement

On deposition. many sandstones contain lillie sediment
matrix between the component grains. Some terrigenous
mud may be deposited with the grains and those sedi­
ments with more than 15% clay matrix are classified as
greywackes (62--67). A few sandstones have a matrix of
carbonate mud. 36 and 37 show a sediment containing
large. rounded quartz grains together with smaller,
'ubangular to subrounded grains in a fine-grained matrix
ha,ing high relief. In the XPL photograph. high-order
uHerference colours. characteristic of calcite, can be seen.
Thts sample is a sandstone with a carbonate mud matrix,
'' hich was probably deposited at the same time as the
grain�. rather than being introduced later as a cement.
Cementation is the principal process leading to poros­
ity reduction in sandstones, the most common cements
bctng quart;. calcite and clay minerals. Clay mineral
coatings on component grain surfaces arc important in
the dtagenesis of sediments. in that they may inhibit the
growth of pore-filling quartz or calcite cements. Such
textures require the usc of the electron microscope for
detailed study.
38 and 39 show a highly porous sandstone with
rounded quart7 grains. The speckled areas which appear
black in the XPL photograph are pores filled with the
mounting medium. Although comprising a loose fabric of
grains. the sandstone is well-cemented by secondary
(authigenic) quartz in the form of overgrowths on the
detrital grains. The surfaces of the original grains arc
picked out by a thin red-brown rim of iron oxide. Since
both the overgrowth and the detrital cores of each grain
show untform interference colours, it is clear that the
overgrowths grew in optical continuity with the grains on
"htch they nucleated. otc that where overgrowths are
wcll-de,eloped. the overall shape of the grains has
changed from rounded to subhedral. A good example of
cuhedral cry5tal terminations can be seen near the top of
the photograph on the right-hand side.
Calcite cements in sandstones arc usually fairly coarse­
grained (sparite p. 34). Occasionally they are so coarse
tlwt one cement crystal envelopes many detrital grains,
resulting in a poikiliric texture. 40 and 41 show a
sandstone in which the detrital grains arc subangular to
subrounded quartz. The cement is calcite of such a grain
si7e that there are only a few crystals in the field of view
shown. Individual cement crystals can be distinguished in
the XPL photograph by their slightly different inter­
ference colours (high-order grey and pink).

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