Formation of Quartzite

Metamorphic in Origin

Quartzite is aptly named due to its high quartz content and resulting hardness. Comprising of greater than 90% quartz, quartzite is the largest and purest concentration of silica in the earths crust. Commonly regarded as metamorphic in origin, quartzite is formed when Chert or Quartz Rich Sandstone has been exposed to high temperatures and pressures (metamorphism). These conditions fuse the quartz grains together forming a dense, hard, equigranular metamorphic rock that must contain at least 80% quartz by volume to be classified as a quartzite. The process of metamorphism combined with minerals within its environment will determine the quartzite appearance. All though outcrops are found all over the earth, there are several Quartzite deposits in the United Kingdom also. Harder and more resistant than granite, uses are commonly in construction and more increasingly as quartzite worktops.

Silica

Chemical formula SiO2 'Silicon dioxide', also known as silica, is an oxide of silicon, and is poorly soluble. Most commonly found as the mineral quartz and in living organisms. silica is the major constituent of sand and is one of the most complex and most abundant materials. Uses of silica are wide and varied and include the following: Aerogels, Concrete, Casting, Glass, Optical Fibres, Ceramics, Food, Cosmetics, Pharmaceutical, Semiconductors & Dielectrics, and as a Fining agent (clarifying) Wine, Beer and juice (E number E551). All aspects of working with silica can release hazardous levels of crystalline silica dust particles into the air. As a recognized human carcinogen it may lead to diseases of the lungs such as silicosis and pulmonary fibrosis. It is essential during fabrication of quartzite worktops that water is used to prevent dust during each cut. We also use the water during polishing the cut quartzite worktops, by machine and by stonemason hands, where masks are also worn as a precaution.

Diatomaceous Earth

Diatomaceous earth is a naturally occurring, soft, siliceous sedimentary rock crumbled into a whitish abrasive textured powder. Oven-dried diatomaceous earth is 80–90% silica, 2–4% alumina, and trace iron oxide and consists of fossilized remains of hard-shelled diatoms. Each deposit of diatomaceous earth differs by the concentrations of other natural clays and minerals, the diatom may species differ and in each deposit contain varying amounts of silica which depends on the sedimentation conditions. Common uses include: Filtration Aid, Metal Polishes, Toothpastes, Insecticides, Absorbents, Matting Agents, Fillers, Cat litter, Activators, Stabilizers, insulators and Soil.

Living Organisms

Despite its poor solubility, silica occurs in many living organisms. Plant materials with high silica phytolith content (Rigid microscopic structures made of silica) appear to be of importance to grazing animals. it certainly does occur in the living body, although exact reasoning is uncertain, it is thought that it is important to the growth, strength, and management of many connective tissues including bone, tooth and cell structures. Silicification also occurs in bacteria and single-celled organisms such as shells diatoms, Radiolaria and testate amoebae.

Chert - Quartz Rich Sandstone

Mainly of biological origin from Biogenic Sediment, but can also occur as a precipitate of chemicals or Diagenesis. The boundary between diagenesis and metamorphism is not defined but metamorphism occurs at higher temperatures and pressures. The colour of Chert depends wholly on the trace elements and minerals from the environment it is formed, for example red and green are most often related to traces of iron oxides.

Biogenic Sediment

Biogenic sediment is an ocean sediment that contains more than 30% skeletal debris. Debris include micro-organisms, petrified skeletal microfossil remains of diatoms, silicoflagellates, radiolarians, cyanobacteria, and occasional macrofossils that are found in the siliceous and biogenic sediments covering deep ocean floors.

Carbonate Sediments

Carbonate Sediment (Skeletal Material) is of calcium carbonate in the form of the mineral calcite or 'aragonite'. The crystal form of calclium carbonate covers around half of the worlds seafloor above 4500 metres, most dominantly near the equator.

Siliceous Sediments

Siliceous Sediment are most prominent in the ocean around antarctica where approximately 75% of all oceans silica supply is deposited. Siliceous skeletons dissolve quickly in seawater, therefore, only robust skeletal remains are found in the siliceous sediments.

Diagenesis

Erosion of rock (deposition) settles into sedimentary layers, microbial activity and minerals then condensate and precipate from solution (dissolution). Further deposition causes compaction whilst expelling fluids (lithification), gradually forming solid sedimentary rock. The continuous process of depsoition causes it to be carried deeper into the earth. Increased pressure and temperature transforms the organic content into kerogens and bitumens, or they are replaced by other minerals such as calcite, siderite and pyrite.

Metamorphism

High temperature and pressure causes metamorphism when applied to sedimentary rock. Individual quartz grains in combination with former cementing materials recrystallize during the cooling of the original sedimentary structures of the sandstone. The recrystallized quartz grains are roughly equal in size, forming what is called a granoblastic texture (course crystals of equal sizes). They interlock so tightly that when broken, the rock will fracture through the grains forming an irregular or conchoidal fracture. The key difference between an orthoquartzite and a metaquartzite is the onset of recrystallization of existing grains.

Metaquartzite

Pure quartz sedimentary sandstone, converted by metamorphism into Metaquartzite through heat and pressure (tectonic compression) into a hard, non-foliated metamorphic quartzite, which is sometimes called metaquartzite to emphasize its origins. Metaquartzite contains approximately 90% quartz with the remaining materials being of minerals.

Orthoquartzite

It is likely that the sediments that form Orthoquartzite would of took longer to settle having been purified by ocean currents. Orthoquartzite is a quartzite of sedimentary origin, or pure quartz sandstone, with very low percentage of Chert grains with optical and geometric crystal lattice structures (crystallographic). Contains approximately 99% SiO2, lacks fossils and has scarce amounts of iron oxide and trace resistant minerals such as zircon, tourmaline, rutile and magnetite. Orthoquartzite is so highly cemented that it will fracture across grains, not around them.

Appearance

Composed predominantly of an interlocking mosaic of quartz crystals, it has a grainy/sugary appearance and glassy lustre. Although pure quartzite is usually opaque white to grey, other colours are a consequence of impurities being incorporated during metamorphism. The migration of former cementing materials during recrystallization and metamorphosis causes streaks and lenses to form within the quartzite. the former cementing materials include iron oxide, silica, carbonate and clay. Quartzite can sometimes appear similar to marble, however, a metal blade will not scratch a piece of quartzite and will not fizz on contact with dilute hydrochloric acid unlike marble.

Uses

Quartzite is harder and more resistant to stains than granite. Used in slab form for flooring, walls, steps and tiles, and more increasingly as countertops and as very hard wearing quartzite worktops. It is heavily used as crushed stone for road construction and as a source of silica for production of silicon and compounds.

Quartzite in the UK

Quartzite is very resistant to weathering and often forms ridges, crags and hilltops. The high silica content provides little material for soil so quartzite outcrops are often bare or covered only with a very thin layer of soil and void of anything more than grassess and heathers. A few examples of the most prominent quartzite outcrops found within the united kingdom.

Stiperstones, South of Shrewsbury

A craggy ridge of quartzite runs parallel with the Pontesford-Linley fault.

The Wrekin, Telford

5 miles west of Telford with an elevation of 407m it sits on the border of Shropshire.

Hartshill, Nuneaton

Large abandoned and flooded quarry to the South East of Hartshill village.

Holyhead Mountain, Wales

Northern and western Anglesey and the adjacent Holy Island in North Wales.

Foinaven, Arkle, Scottish Highlands

Far north-west running in a narrow band from Loch Eriboll south-west toward Skye.

Errigal in Donegal, Ireland

Across the northwest, with Errigal in Donegal as the most prominent outcrop.