Mineral, accessory secondary

O, H, C, S, and N isotope compositions of mantle-derived rocks are substantially more variable than expected from the small fractionations at high temperatures. The most plausible process that may result in variable isotope ratios in the mantle is the input of subducted oceanic crust, and less frequent of continental crust, into some portions of the mantle. Because different parts of subducted slabs have different isotopic compositions, the released fluids may also differ in the O, H, C, and S isotope composition. In this context, the process of mantle metasomatism is of special significance. Metasomatic fluids rich in Fe +, Ti, K, TREE, P, and other large ion lithophile (LIE) elements tend to react with peridotite mantle and form secondary micas, amphiboles and other accessory minerals. The origin of metasomatic fluids is likely to be either (1) exsolved fluids from an ascending magma or (2) fluids or melts derived from subducted, hydrothermally altered crust and its overlying sediments. 

Cornacya Seamount. This seamount recently was discovered, named and studied by Mascle et al. (2001). It is an about 12 Ma old volcano located SE of the southern Sardinian coast. Samples from Cornacya consist of strongly altered lavas that contain enclaves of mica-rich lamprophyres. The lavas have varied porphyritic textures with ubiquitous phenocrysts of zoned plagioclase and biotite plus some amphibole and clinopyroxene that are surrounded by a glass-rich matrix containing minor Na-rich plagioclase, anorthoclase, biotite and secondary products. Accessory minerals include Fe-Ti oxides, apatite and zircon. The lamprophyric enclaves are por-... 

That the atomic weight of uranium lead is extremely variable has already been shown. In order to interpret this variability its sources must be studied both geologically and mineralogically. On the geologic side of the question the uranium ore can be divided in to three principal classes, which are sharply distinct. The definitely crystallized varieties of uraninite occur in coarse pegmatites, associated with feldspar, quartz, mica, beryl, and other minor accessories. The massive pitchblende is found in metalliferous veins, together with sulphide ores of copper, lead, iron, zinc, and so forth. As for camotite, that is a secondary mineral, found commonly as an incrustation on sandstone, and often, also upon fossil wood. There may be other modes of occurrence, but these are the most distinctive. 

Fig. 3 Schematic representation for release of impurity components from rock-forming and accessory minerals through metamorphism or hydrothermal alteration, resulting in adsorbed metals on exchangeable sites in clays or secondary oxides...

The most abundant carbonate minerals are calcite (CaCOs) and dolomite (CaMg(C03)2). Secondary minerals are anhydrite, chert, and quartz. Accessory minerals are phosphates, glauconite, ankerite, siderite, feldspars, clay minerals, pyrite, etc., depending on the environment of deposition and diage-netic history. 

In geological formations, the content of heavy minerals may vary from 0 to 100%, and there is a tendency for some heavy mineral species to occur preferentially in certain kinds of rock. In any rock the minerals may be divided into primary and secondary rock minerals according to their origin. Primary rock minerals were formed at the same time as the rock containing them, while secondary rock minerals were formed as a result of reactions taking place within the host rock at some time after its formation. Rock-forming minerals may also be classified as essential or accessory. Essential minerals are those implied in the definition of the rock name, while accessory minerals are additional to the above definition, so that their absence would not necessitate a change in the rock name. Heavy minerals as they occur in rocks may be essential or accessory they are usually primary, but occasionally they are secondary. 

---