Building stone - Geomaterials: Under the Microscope - page 28

compounds around the quartz grains. Figure 45 shows
'Caithness Stone' which is a durable paving stone of dark
grey colour. It comprises bands of fine sandstone and
mudstone with matrix/cement of calcite and
phyllosilicate minerals.

There are two widely used classification systems for
limestones. In Dunham's classification (1962) rocks are
assigned names according to their depositional texture,
which is primarily related to the energy of the depositional
environment (Table 8 ). The Folk (1959, 1962) limestone
classification assigns names primarily according to the
proportions of different allochems and the nature of the
cement ( Table 9 ). Allochems are grains or particles found
in most limestones such as bioclasts, ooliths, peloids, or
intraclasts. The spaces between allochems may be wholly
or partially filled by a matrix/cement of micrite (carbonate
mud, <4 μm grain size), microsparite (fine calcite crystals,
4-30 μm size) or sparite (coarse calcite crystals, >30 μm
size). The presence of macropores (>5 μm size) and
micropores (<5 μm size) is an important factor which
greatly influences durability of limestone used for building.
Macropores typically occur as interparticle pores between
allochems, while micropores typically occur as intraparticle

L IMESTONE
Limestones are made largely of calcium carbonate
(CaCO 3 ), usually in the form of calcite. They are of many
kinds and they exhibit a wide range of strength and
durability. Organic limestones are formed from fragments
of calcareous animal remains such as shells and coral.
Chemical limestones are formed by precipitation of
CaCO 3 in seawater or at hot springs on land. Clastic
limestones are formed from eroded and redeposited
fragments of pre-existing limestones. Dolomitic
limestones contain a >10% proportion of the mineral
dolomite, CaMg(CO 3 ) 2 .

45

45 'Caithness Stone' (Scotland) comprising layers of
fine sandstone with quartz grains (grey) and calcite
cement (pink) interbedded with mudstone (brown);
XPT, ×35.

Table 8 Dunham (1962) scheme for classification of limestone

Original components not organically bound together during deposition Components

organically bound
during deposition

Contains carbonate mud No carbonate

mud

Mud-supported Grain-supported

<10% allochems >10% allochems

Mudstone Wackestone Packstone Grainstone Boundstone