Saprolite - Citizendia

Saprolite (from Greek σαπρος =putrid, + lite) is the name for a chemically weathered rock. Greek (el ελληνική γλώσσα or simply el ελληνικά — "Hellenic" is an Indo-European language, spoken today by 15-22 million people mainly Weathering is the decomposition of earth rocks, Soils and their Minerals through direct contact with the planet's Atmosphere. It is mostly soft or friable and commonly retains the structure of the parent rock since it is not transported, but autochthonously formed in place.

Besides resistant relic minerals of the parent rock, saprolites contain predominantly quartz and a high percentage of kaolinite with other clay minerals which are formed by chemical decomposition of primary minerals, mainly feldspars. Quartz (from German) is the most abundant Mineral in the Earth 's Continental crust (although Feldspar is more common in Kaolinite is a Clay mineral with the chemical composition Al 2 Si 2 O 5( OH)4 Clay is a naturally occurring material composed primarily of fine-grained Minerals which show plasticity through a variable range of Water content, and Feldspar is the name of a group of rock-forming Minerals which make up as much as 60% of the Earth 's crust. More intense weathering conditions, exceeding the saprolite stage, give rise to a continuous transition to laterite soils. Laterite (from the Latin word "later" meaning brick or tile is a surface formation in hot and wet tropical areas which is enriched in Iron and Aluminium

1 Terminology
2 Formation
3 Other saprolite morphologies
- 3.1 Mottled zones
- 3.2 Collapse zones
4 See also

Terminology

The saprolite is the part of the regolith which is oxidised and hydrated, the original minerals in the rock weathered and altered by water, oxygen, carbon dioxide and organic acids such as tannins. Regolith ( Greek: "blanket rock" is a layer of loose Heterogeneous material covering solid rock. Redox (shorthand for reduction-oxidation reaction describes all Chemical reactions in which atoms have their Oxidation number ( Oxidation state Weathering is the decomposition of earth rocks, Soils and their Minerals through direct contact with the planet's Atmosphere. Tannins are Astringent, bitter plant Polyphenols that either bind and Precipitate or shrink Proteins The astringency from the tannins is what

The saprolite is divided into three main zones; the upper saprolite, lower saprolite and saprock.

Upper Saprolite

The upper saprolite zone is distinguished by the general absence of fresh rock and the predominance of clay species. The upper saprolite has varigated colors, typically yellow, ochre, orange, tan or light brown in colour. In extreme cases of prolonged weathering, or in particularly iron-poor rocks, the upper saprolite can be bleached pure white.

The upper saprolite is chemically oxidised to the point where few reduced chemical species exist. For instance, iron is present as Fe³⁺, sulfur is present predominately as sulfates, manganese is present as manganese oxide, silicates are present as clays. The silicate minerals make up the largest and most important class of rock-forming Minerals They are classified based on the structure of their silicate Ion group

The upper saprolite is structurally and texturally mostly intact and such structures as occur in the fresh rock can usually be traced into the upper saprolite. Structural geology is the study of the three dimensional distribution of rock bodies and their planar or folded surfaces and their internal fabrics Rock microstructure includes the texture of a rock and the small scale rock structures

The upper saprolite zone may exist down to 300 metres below surface in extreme cases, and throughout large regions of the tropics, may exceed 50 metres. Within the ancient regolith of the Yilgarn Craton, saprolite typically exceeds 100m depth.

Lower Saprolite

The lower saprolite zone is distinguished from the upper saprolite by the generally lower degree of oxidation and the colour. Typically, lower saprolite is greenish in colour and friable, though much more indurated than the upper saprolite.

The lower saprolite sees an oxidation front between chemically oxidised minerals and chemicals above, and chemically reduced minerals and chemicals existing below. This manifests as a change from Fe³⁺ which lends the red or orange suite of colors to the upper saprolite, to Fe²⁺ which tends to colour the rock mass green or green-brown.

The oxidation front is a prime marker and an important horizon for ore deposits, especially uranium deposits. An ore is a volume of rock containing components or Minerals in a mode of occurrence that renders it valuable for mining Uranium (jʊˈreɪniəm is a silvery-gray Metallic Chemical element in the The oxidation front and change from upper to lower saprolite sees a change in redox potential and ionic state of most metals, which may prompt downward-travelling oxidised species to leave solution.

The oxidation front is also the point at which sulfur is reduced enough to exist in a sulfide form and hence, there is typically some form of supergene enrichment in metal sulfide minerals. A sulfide mineral is a Mineral containing Sulfide (S2- as the major Anion. In Ore deposit Geology, supergene processes or enrichment occur relatively near the surface This is an important zone in many ore deposits and an integral part of the ore genesis of Manto ore deposits. The various theories of ore genesis explain how the various types of mineral deposits form within the Earth's crust. Manto orebodies are stratabound irregular to rod shaped Ore occurrences usually horizontal or near horizontal in attitude

Saprock

The saprock is the zone of rock below the lower saprolite where weathering is restricted to failure systems within the fresh rock mass. Within the saprock zone weathering occurs along joints, foliations, faults and other failures, and projects downwards as a network of weathered fingers. In geology the term joint refers to a fracture in rock where there has been no lateral movement in the plane of the fracture (up down or sideways of one side Foliation is any penetrative planar fabric present in rocks. Foliation is common to rocks affected by regional metamorphic compression typical of orogenic In Geology a fault, or fault line, is a planar rock fracture which shows evidence of relative movement

The top of fresh rock, typically, is taken as the point at which no further visible weathering is evident. However, chemically it is usually able to be shown that weathering and oxidation effects may persist a considerable distance into the Earth's crust, especially around large fault systems.

It is also worth noting that in many hydrothermal fields, weathering and oxidation of the rocks is enhanced by the hydrothermal waters, giving rise to an alteration zone of metasomatic origin. Hydrothermal circulation in its most general sense is the circulation of hot water 'hydros' in the Greek meaning water and 'thermos' meaning heat Metasomatism is the chemical alteration of a rock by Hydrothermal and other fluids It is arguable whether or not this counts as saprolite.

Formation

The process of forming saprolite is via addition of water, oxygen, nitrogen, carbon dioxide and organic matter into the rock and via removal of magnesium, iron, silica and metals from the rock. Typically, aluminium is immobile within weathering, although this is not always the case.

Saprolite destruction occurs via physical erosion, predominantly via water movement across the surface. However, in eolian dune systems and arid wind-prone deserts scouring of the land surface via wind-blown particles is an important mechanism for removal of saprolite. Aeolian (or Eolian or Æolian) processes pertain to the activity of the Winds and more specifically to the winds' ability to shape the surface of the Within the ice deserts and some periglacial uplands, wind and freeze-thaw may also remove saprolite. Periglacial is an adjective referring to places in the edges of glacial areas normally those related to past Ice ages rather than those in the modern era

Saprolite is generally not formed in glacial environments as glaciation removes all soft, weathered material readily. "Glacial" and "Glaciation" redirect here For the geological periods see Glacial period. It is for this reason that the majority of the Canadian Shield and the Siberian Craton are devoid of significant saprolite. The Canadian Shield &mdash also called the Laurentian Plateau, or Bouclier Canadien (French &mdash is a large geological shield covered by A craton ( Greek kratos / κρἀτος ( neut. "strength" is an old and stable part of the Continental crust that has survived

Other saprolite morphologies

There are several other specific and restricted saprolite morphologies which are recognised from particular environments, but which do not follow the typical and idealised sequence shown above.

Mottled zones

Within the Yilgarn Craton of Australia and elsewhere (?) there is a widespread development of a mottled zone of saprolite consisting of red and white mottled clay-rich saprolite. The Yilgarn Craton is a large Craton which constitutes the bulk of the Western Australian land mass The mechanism of formation of these mottled zone morphologies was quite contentious, however the general consensus is that these mottled zones are the result of penetration of tree roots into the saprolite during a previous environmental regime which occurred within either the Tertiary or late Cainozoic. The chuprichondira geological time interval covers roughly the time span between the demise of the non- avian Dinosaurs and beginning of the most recent Ice Age, approximately The Cenozoic (also Caenozoic or Cainozoic) Era (ˌsiːnəˈzoʊɪk/ /ˌsɛn- (meaning "new life" ( Greek ( kainos) "new"

The white clay mottles or 'megamottles' tend to form vertically arrayed tubular conduits of bleached white kaolinite within a hematite-rich clay matrix. Kaolinite is a Clay mineral with the chemical composition Al 2 Si 2 O 5( OH)4 Hematite, also spelt hæmatite, is the Mineral form of Iron(III oxide (Fe2O3 one of several Iron oxides In a widely recognised and growing number of sites there have been found fossilised tree roots.

Collapse zones

Within the Yilgarn Craton of Australia and elsewhere that prolonged weathering has occurred, where the rate of erosion is virtually negligible compared to the rate of chemical weathering, it is possible that removal via aqueous solution of the majority of rock components takes place, leaving a laterite profile which consists of highly vesicular hematite-goethite. The Yilgarn Craton is a large Craton which constitutes the bulk of the Western Australian land mass Laterite (from the Latin word "later" meaning brick or tile is a surface formation in hot and wet tropical areas which is enriched in Iron and Aluminium Hematite, also spelt hæmatite, is the Mineral form of Iron(III oxide (Fe2O3 one of several Iron oxides Goethite, named after the German Polymath Johann Wolfgang von Goethe, is an iron bearing oxide mineral found in soil and other low temperature environments If well enough developed, the mass of these boxworks will collapse under its own weight, forming a ferruginous hardcap composed of fragments of weathered residual saprolite and laterite materials.

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