Secondary rocks

This ore occurs both in the primitive and secondary rock it abounds in the Island of Elba, Norway, Sweden, and, Switzerland. Pine specimens ore mot with in the fissures of rooks in the volcanic districts. 

Sources of Phosphates.—The most available and most exploited sources of phosphorus and its compounds at the present day are the phosphatic rocks, or phosphorites, which consist of tribasic calcium phosphate associated with calcium carbonate, alumina, magnesia, etc.1 Phosphates of alumina are also useful. The production of these secondary rocks from the older rocks has already been mentioned (p. 208). Although the apatites themselves, as pure minerals, contain a high proportion of phosphoric anhydride, they are difficult to decompose, and are admixed with other minerals of a still more refractory nature. 

The extent of surface weathering of crystalline rocks or of sedimentary rocks such as shales or carbonates, and thus rock permeability (and yield to wells), decreases rapidly with depth. Also, rock weathering is deeper under valley bottoms than on ridges or hill slopes. This reflects the fact that the weathering, which is facilitated by joints, fractures, and faults, tends to create valley bottoms in the first place. Valley bottoms continue to concentrate runoff (R is then a positive term in the infiltration equation) and so remain the locus of deeper development of secondary rock porosity and permeability and thus of enhanced groundwater storing and transmitting capacity. 

The main emphases in oil and gas production are how to explore sites and how to operate resources for optimal flow, and long term resource management. The rock structure, rock stress field, primary and secondary rock permeabilities, wellbore locations and orientations, and fluid flow are all coupled and hence there are many interactions that require modelling. 

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. 

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