http://www.museumwales.ac.uk/min http://www .museumwales.ac.uk/mineralogy-of-wales/sedimentary-minerals/ eralogy-of-wales/sedimentary-minerals/ Like other rock-types, sedimentary rocks are composed of a variety of minerals. There are two broad groups of sedimentary rocks, clastic (or detrital sediments which accumulate from fragments of pre-e!isting rocks and minerals, and chemical sediments or precipitates. "edimentary rocks may be unconsolidated or consolidated, and the transformation from the former to the latter is known as diagenesis, a process during which further minerals form.
A thick thick sequence of sandstone beds deposited as sand-rich sediment approximately 520 million years age. St. Tudal!s Tudal!s "eninsula# $l%n# &orth 'ales. "hoto ( ).*. +or,k.
#inerals in clastic sediments $lastic sedimentary rocks are those made up of detritus that has been eroded by the actions of wind, water and ice from pre-e!isting rocks of all types. "uch rocks are widespread in %ales, with vast thicknesses of sandstones, comp osed dominantly of &uart' grains, and clay mineral-rich mudstones. %hereas the bulk of minerals
making up clastic sedimentary rocks are common rock-forming species, such as &uart', feldspar and mica, both common and minor minerals in an eroding rock may end up in the resulting sediment. #inerals that have been transported as detrital grains, to end up in a sedimentary rock, are referred to as allogenic.
Thin section of sandstone dominated by angular quart grains surrounded by bron hematite cement. $ong thin detrital grains of mica are also present. 'idth of image 2. mm. /ld ed Sandstone# ed 'harf 1ay# Anglesey. ( &ational *useum of 'ales.
ed sandstone from the Senni 1eds# /ld ed Sandstone succession# 1recon 1eacons. ( &ational *useum of 'ales. "tudying the less common minerals present in sedimentary rocks can provide valuable information regarding the source rocks that were eroded, the direction in which the sediment was transported by ice, water or wind, or the e!tent of ancient river or glacier systems. or instance, the minerals andalusite, staurolite and cordierite are not found in metamorphic rocks in %ales but have been described as detrital grains in glacial sediments, indicating that the sediments were produced by erosion of such rocks in "cotland or )reland, then transported a considerable distance and deposited in %ales.
To gold nuggets &*' 33.44.*.6 7 33.44.*.28# ith maximum dimensions 2 mm and 23 mm# respecti9ely# from the Afon *addach# ynedd. ( &ational *useum of 'ales. "ome detrital minerals are of economic importance, because gravity and water action are very efficient at sorting mineral grains according to their density. The end result is that *heavy* minerals tend to become concentrated, often in layers close to,
or on, the bedrock underlying a river. "uch deposits are termed placers and are an important source of gold, tin and many other minerals worldwide. )n %ales, placergold deposits, now virtually e!hausted, were formerly worked in the +olgellau area.
"edimentary rocks formed from $hemical precipitation $hemical sediments result from precipitation of dissolved substances from sea or lake water. The most common products are limestones, deposited by the precipitation of calcium carbonate as calcite. recipitation may occur directly from water or involve organisms in biochemical processes. The most widespread limestones in %ales are those of Lower $arboniferous age, which outcrop across $lwyd, and also in southern %ales around the margins of the coalfield. ther important limestones include "ilurian reef-deposits in the %elsh orderland and Lower urassic muddy limestones e!posed along the ristol $hannel coast.
*icroscope image of fossil fragments in :arboniferous $imestone. These fragments are dark in appearance as they ha9e been partially replaced by iron oxide subsequent to deposition and diagenesis. hibina $imestone. Specimen &*' 20.4;6..660b# ( &ational *useum of 'ales.
inas 1ran. :arboniferous $imestone deposited around 440 million years ago. "hoto ( A. +aycock. )n some limestones, magnesium substitutes for the calcium in calcite to form the mineral dolomite. "uch rocks are termed dolomites or dolomitic limestones. )ron carbonate may also accumulate, particularly in sedimentary environments where much organic matter is present. This produces beds of fine-grained, heavy clayironstone which is particularly common in the "outh %ales $oalfield, where it was mined as an iron ore. 0lso included within chemical sediments are evaporite deposits. The minerals within evaporites form by precipitation from solutions by evaporation - usually in arid environments such as desert salt-lakes. The most common minerals present are gypsum, anhydrite and halite (rock-salt, sometimes in sufficient &uantity to be commercially mined. 1vaporites occur within Triassic rocks e!posed along the "outh %ales coast near $ardiff, where the beds of gypsum form conspicuous white layers in the red marls. 0ll of these precipitated minerals have formed in place, without transport, and are referred to as authigenic.
:lose up 9ie of alabaster gypsum8 pod eroded from the cliff. The alabaster can 9ary in colour from hite through pale pink to salmon pink. The black ones are areas of sediment incorporated into the e9aporite. &ote penknife for scale. "enarth# South 'ales. ( &ational *useum of 'ales.
:liff section shoing layers of red and green marls containing pods of alabaster gypsum8# formed by the e9aporation of highly saline ater. "enarth# South 'ales. ( &ational *useum of 'ales.
+iagenetic minerals 2 consolidation of sediment into rock
0uthigenic minerals are also formed during diagenesis, a process involving all of the chemical, physical, and biological changes undergone by any sediment following deposition and during its transformation into solid rock. This includes compaction by the pressure of increasingly deep burial, the ensuing s&uee'ing out of much of the trapped pore-water in the sediment and the dissolution and reprecipitation of minerals to form cements to bind mineral and rock grains. +iagenesis is controlled by increases in temperature and pressure and by the amount of time over which such changes occur. )n some sediments, unusually high concentrations of metals and other elements lead to the growth, during diagenesis, of authigenic minerals in crystals (such as pyrite cubes, as fossil replacements and as nodules or concretions. $oncretions are spheroidal or lenticular bodies of mineral-rich rock. )n shaly rocks their greater hardness makes them stand out from eroded surfaces. $ommon concretion-forming minerals found in %ales include siderite (3ironstone4, calcite, &uart', pyrite, apatite and the rare-earth phosphate, mona'ite. $oncretions may also contain internal cracks (3septa4 within which other minerals have subse&uently crystalli'ed, in which case they are termed septarian concretions. )n %ales the best-known of these occur in the "outh %ales $oalfield, where beautifully-crystalli'ed millerite, &uart', galena and other minerals have been collected by breaking such concretions open. These have their uses to the geologist too: trapped fluids within &uart' crystals from the concretions have yielded valuable information regarding the pressures and temperatures under which diagenesis (including coal formation took place.
