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In describing a sandstone it is usual to considcr it under the following headings: the grains or particles, matrix, cement and pores.

Grains The most commonly encountered minerał grains are quartz and feldspar. However, many sediments contain grains which are recognizable fragments of the source rock and usually contain morę than one minerał, and so are called rock fragments. These three grain types form the basis of most sandstone classifications (106) although other minerał grains such as micas may be present.

Matrix Matrix is the fine-grained sediment between the principal clastic particles. Much of it is clay-sized materiał too fine to resolve with a light microscope. It nor-mally comprises fine quartz and clay minerałs. Clay minerals are a comp!ex series of hydrous alumino-silicates which mostly form from the Chemical weathering of silicate minerals in the source rock. Matrix is absent from many ‘clean’ sand-stones. If present in smali amounts the sandstone can be said to be ‘muddy’. If morę than 15% of the total rock volume is matrix the sediment is called a wacke or greywacke and classified separately from arenites (107). Clay matrix may be deposited at the same time as the sand particles. On the other hand it may have infiltrated after deposition or be formed diagenetically from the breakdown of chemically unstable rock fragments within the sediment.

Cement Cement is the term used to denote the crystalline materiał precipitated in spaces between the grains. In many sandstones it is ąuartz or calcite. It is entirely post-depositional and may not form until millions of years have elapsed after deposition of the grains. Cement is the principal lithifying materiał which converts a loose sediment into a sedimentary rock.

Pores Any spaces not occupied by grains, matrix or cement are pores. In the sub-surface these will normally be occupied by water or morę rarely by oil or gas. Pores will lose any water during section making and will be filled with air or with an imprcgnating medium in thin section.

Sandstone classification

Q

(Feldspar + granite    (Ali other rock

+ gneiss fragments)    fragments)

106 Folk classification of sandstoncs containing less than 15% matrix.

107

Quartz

107 Classification of sandstones with morę than 15% matrix (greywackes).

109