Ultrabasic rocks

Talc and pyrophylUte are found in metamorphic rocks that are rich in Mg and Al, respectively. Talc is most common in metamorphosed ultrabasic rocks and in metamorphosed siUceous dolomite. PyrophylUte is found in metapeUtes, including metabauxites and metakaoUnites, and in rocks enriched in Al by hydrothermal processes (106). 

O Hara MJ (1968) The bearing of phase equilibria studies in synthetic and natural systems on the origin and evolution of basic and ultrabasic rocks. Earth Sci Rev 4 69-133 O Nions RK, McKenzie D (1993) Estimates of mantle thorium/uranium ratios from Th, U and Pb isotope abundances in basaltic melts. Phil Trans Royal Soc 342 65-77 Oversby V, Gast PW (1968) Lead isotope compositions and uranium decay series disequilibrium in reeent volcanic rocks. Earth Planet Sci Lett 5 199-206... 

Cobalt is the 30th element in the earth s crust with an average of 20-30 mg/kg (Bowen, 1979 Vinogradov, 1959). Ultrabasic rocks and their metamorphized serpentines contain 100-200 mg/kg Co, followed by basic igneous rocks (30-45 mg/kg), while acid rocks have the lowest Co... 

Soil is a mixture of solid materials, air and, usually, water and organic matter. The radium content of soil often reflects that of the rocks from which the solid materials are derived by physical and chemical activity. The observed ranges are from 0 to 20 Bq kg for ultrabasic rocks (dunite) to 1 to 1835 Bq kg for igneous metamorphic rocks (gneiss) (Wollenberg, 1984). While these ranges are broader than those for measured for soils, the mean values for rocks, excluding alkali rocks, is consistent with the means observed for soils. 

Basic and ultrabasic rocks (peridotites, serpentinites), kimberlitic xenolites, rodingites, eclogites... 

Selandian Rb-Sr ages of 59.6 2.8 Ma and 60.3 0.8 Ma were determined for the K1A and K19 bodies, respectively these bodies occur in the southwestern part of the field and are either barren of diamond or have the poorest diamond results within this field (Hood McCandless 2004 Fig 1). Mineralogical (e.g., amphibole, sanidine) and geochemical evidence (e.g., flatter chondrite-normalized REE pattern versus the steep profile of typical kimberlite) enticed Eccles et al. (2008) to conclude that these rocks are better referred to as hybrid kimberlite-ultrabasic rocks. 

Oxidized upper zones of Fe-rich rocks with high concentrations of Fe oxides are found in pyritic (gossan) and ultrabasic rocks. These rocks are often the subject of exploration owing to their content of such non-ferrous metals as Cu, Co, Ni and Zn. These metals are intimately associated with the iron oxides, mainly goethite and he-... 

The fourth and main zone is the Krivoy Rog-Kremenchug, developed 1800-2000 m.y. ago. The accumulation of rocks began here, as in the other zones, with the eruption of metabasic and ultrabasic rocks, which then probably were locally succeeded by rocks of the keratophyre series. Most of the iron cherts, however, are not related to the initial but rather to the middle stage of development of the zone, when the sedimentary cherty iron-formation proper of the Middle suite of the Krivoy Rog group was formed. Maximum development of iron cherts is characteristic of the Saksagan and Kremenchug basins (Fig. 3). 

Den Tex E. (1969) Origin of ultramafic rocks, their tectonic setting and history a contribution to the discussion of the paper The origin of ultramafic and ultrabasic rocks by P. J. Wylhe. Tectonophysics 7, 457-488. 

Jedwab J. (1992) Platinum group minerals in ultrabasic rocks and nickeliferous veins from Zabargad Island (Egypt). Comptes Rendus de I Academie des Sciences, Serie 2 314, 157-163. 

O Hara M. J. (1967b) Garniteferous ultrabasic rocks of orogenic regions. In Ultramafic and Related Rocks (ed. P. J. Wyllie). Wiley, New York, pp. 167-172. 

WyUie P. J. (1969) The origin of ultramafic and ultrabasic rocks. Tectonophysics 7, 437-455. 

O Hara M. J. (1968) The bearing of phase equilibria studies in synthetic and natural systems on the origin and evolution of basic and ultrabasic rocks. Earth Sci. Rev. 4, 69—134. 

World annual production of natural diamonds, the cubic form of carbon, is about 110 million carats (1 carat = 200 mg). Almost all is derived from kimberlite or its weathered remnants, but Australian production is from the Argyle mine, at which the host rock is lamproite. Kimberlites are olivine- and volatUe-rich potassic ultrabasic rocks of variable geological age that typically form near-vertical carrot-shaped pipes intmded into Archean cratons. The volatile-rich component is predominantly CO2 in the carbonate minerals calcite and dolomite, and the texture is characteristically inequigranular, with large grains (macrocrysts), usually of olivine [Mg2Si04], in a fine-grained, olivine-rich matrix. 

Phlogopite is found in metamorphosed magnesium-rich limestones, dolomites, and ultrabasic rocks. Biotite, similar to muscovite, is also widespread. It is usually associated with minerals which were formed under high temperature and pressure. Several elements, other than those included in their typical chemical composition, can be found in these two minerals. These include Na, Rb, Cs, Ba, F, and Ca. The most important difference between phlogopite and biotite is that biotite contains a substantial amount of iron. 

Cobalt toxicity is occasionally found in high-Co soils formed from serpentinite and other ultrabasic rocks. Deficiency is most likely in coarse-textured, acid-leached soils alkaline or calcareous soils and humus-rich soils. Extractability by strong acids can range from very little (< 1%) to a large fraction (>30%) of the total Co, depending on the forms of Co in the soil. 

Clnysotile belongs to the serpentine group of minerals, v arieties of which are found in most of the important mountain ranges and precambrian shields (8). Only a small part of these serpentine occurrences are in the asbestiform clnysotile variety. Chrysotile fibers are found as veins in serpentines or related minerals in serpentinized ultramafic rocks and in serpentinized dolomitic marbles (9). It has been suggested that the ultrabasic rocks (forsterite, Mg-rich pyroxenes, and ampliiboles) are first attacked in an hydrothermal process and transformed in serpentines in a later hydrothermal event, the serpentines are partially redissolved and crystallized as chrysotile fibers (9). (Heady, the genesis of each chrysotile deposit must have involved specific features related to the composition of the precursor minerals, the stress and defomiations in the host matrix, the water content, the temperature cycles, etc. Nonetheless, it is generally observed that the chemical composition of the fibrous phase is closely related to that of the surrounding rock matrix (9). 

Magnesite may also form through the serpentization of ultrabasic rock, under low-temperature and low-pressure conditions, which favor the exosolution of hydrothermal cryptocrystalline magnesite. Reaction (2.6) shows serpentization of olivine, (Mg, Fe)2Si04, to form antigorite, H4(Mg, Fe)2Si209, and magnesite ... 

Unlike its neighbors in the Periodic Table, iron, nickel, and copper, cobalt is not widespread in nature. It has an average abxmdance in Earth s crust of 2 5 parts per million (ppm) in ultrabasic rocks, where cobalt is most common, the average concentration is 110 ppm. Cobalt minerals may be concentrated by a range of geological processes to produce workable ores that typically contain 1,000-2,000 ppm. 

Rendzinae — these are formed at different altitudes and climatic conditions on carbonate and ultrabasic rocks. The main diagnostic horizons ... 

Ultrabasic rock (syn. Ultramafic) igneous rocks containing less than 45% silica, devoid of quartz and feldspar, and composed essentially of ferromagnesian silicates. 

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