http://www.museumwales.ac.uk/mineralogy-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-existing rocks and minerals, and chemical sediments or
precipitates. Sedimentary 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 sequence of sandstone beds deposited as sand-rich
sediment approximately 520 million years age. St. Tudwals
Peninsula, Ll?n, North Wales. Photo J.M. Hork.Minerals in clastic
sedimentsClastic sedimentary rocks are those made up of detritus
that has been eroded by the actions of wind, water and ice from
pre-existing rocks of all types. Such rocks are widespread in
Wales, with vast thicknesses of sandstones, composed dominantly of
quartz grains, and clay mineral-rich mudstones. Whereas the bulk of
minerals making up clastic sedimentary rocks are common
rock-forming species, such as quartz, feldspar and mica, both
common and minor minerals in an eroding rock may end up in the
resulting sediment. Minerals 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 quartz grains
surrounded by brown hematite cement. Long thin detrital grains of
mica are also present. Width of image 2.4 mm. Old Red Sandstone,
Red Wharf Bay, Anglesey. National Museum of Wales.
Red sandstone from the Senni Beds, Old Red Sandstone succession,
Brecon Beacons. National Museum of Wales.Studying 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 extent of ancient river or glacier systems. For
instance, the minerals andalusite, staurolite and cordierite are
not found in metamorphic rocks in Wales but have been described as
detrital grains in glacial sediments, indicating that the sediments
were produced by erosion of such rocks in Scotland or Ireland, then
transported a considerable distance and deposited in Wales.
Two gold nuggets (NMW 99.33G.M.1 & 99.33G.M.2), with maximum
dimensions 24 mm and 29 mm, respectively, from the Afon Mawddach,
Gwynedd. National Museum of Wales.Some 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.
Such deposits are termed placers and are an important source of
gold, tin and many other minerals worldwide. In Wales, placer-gold
deposits, now virtually exhausted, were formerly worked in the
Dolgellau area.Sedimentary rocks formed from Chemical
precipitationChemical 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. Precipitation may occur directly from water
or involve organisms in biochemical processes.The most widespread
limestones in Wales are those of Lower Carboniferous age, which
outcrop across Clwyd, and also in southern Wales around the margins
of the coalfield. Other important limestones include Silurian
reef-deposits in the Welsh Borderland and Lower Jurassic muddy
limestones exposed along the Bristol Channel coast.
Microscope image of fossil fragments in Carboniferous Limestone.
These fragments are dark in appearance as they have been partially
replaced by iron oxide subsequent to deposition and diagenesis.
Rhiwbina Limestone. Specimen NMW 20.361.GR.110b, National Museum of
Wales.
View looking north towards Creigiau Eglwyseg (Eglwyseg Crags),
Llangollen, from Castell Dinas Bran. Carboniferous Limestone
deposited around 330 million years ago. Photo A. Haycock.In some
limestones, magnesium substitutes for the calcium in calcite to
form the mineral dolomite. Such rocks are termed dolomites or
dolomitic limestones. Iron carbonate may also accumulate,
particularly in sedimentary environments where much organic matter
is present. This produces beds of fine-grained, heavy
clay-ironstone which is particularly common in the South Wales
Coalfield, where it was mined as an iron ore.Also 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 quantity to be commercially
mined. Evaporites occur within Triassic rocks exposed along the
South Wales coast near Cardiff, where the beds of gypsum form
conspicuous white layers in the red marls. All of these
precipitated minerals have formed inplace, without transport,and
are referred to as authigenic.
Close up view of alabaster (gypsum) pod eroded from the cliff.
The alabaster can vary in colour from white through pale pink to
salmon pink. The black zones are areas of sediment incorporated
into the evaporite. Note penknife for scale. Penarth, South Wales.
National Museum of Wales.
Cliff section showing layers of red and green marls containing
pods of alabaster (gypsum), formed by the evaporation of highly
saline water. Penarth, South Wales. National Museum of
Wales.Diagenetic minerals consolidation of sediment into
rockAuthigenic 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 squeezing 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. Diagenesis is controlled by increases in temperature
and pressure and by the amount of time over which such changes
occur.In 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.Concretions are
spheroidal or lenticular bodies of mineral-rich rock. In shaly
rocks their greater hardness makes them stand out from eroded
surfaces. Common concretion-forming minerals found in Wales include
siderite (ironstone), calcite, quartz, pyrite, apatite and the
rare-earth phosphate, monazite.Concretions may also contain
internal cracks (septa) within which other minerals have
subsequently crystallized, in which case they are termed septarian
concretions. In Wales the best-known of these occur in the South
Wales Coalfield, where beautifully-crystallized millerite, quartz,
galena and other minerals have been collected by breaking such
concretions open. These have their uses to the geologist too:
trapped fluids within quartz crystals from the concretions have
yielded valuable information regarding the pressures and
temperatures under which diagenesis (including coal formation) took
place.