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On Rocks; Their Chemical and Mineral Composition, and Physical Characteristics
Published online by Cambridge University Press: 17 March 2016
Extract
In the definition which I have given, a mineral is described as a body possessed of a definite chemical composition and a regular physical form,—meaning by the latter particularly crystalline structure. But, while the crystalline relations of each species only vary within the narrowest limits, the chemical composition has a much wider latitude. Many individuals of the same mineral species are found, by analysis, to differ most widely in their chemical components, while the crystalline form is quite unaltered. Now, although we are as yet unable to trace the relation between the form of a mineral and its composition, there can still be little doubt that some such relation does exist, regulated by laws yet to be discovered; consequently, when we find the same mineral species differing widely in chemical proportions, we recognise a departùre from regular although unknown laws, and seek for a cause. This we find in the doctrine of Isomorphism. This, simply stated, in the capability of two or more substances, of analogous chemical constitution, to crystallize in similar forms.
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References
page 223 note * J. R. Blum, — Die Pseudomorphosen des Mineralreichs, mit Nachtrag. Stuttgardt, 1843-7. A very complete list of pseudoraorphs, from Blum, is given in Brooke and Miller's edition of “Phillips' Mineralogy.”
page 225 note * When the monoxide happens to be a protoxide, the base may also be called a protoxide base; and similarly when the sesqui-oxide is a peroxide, that base may be called a peroxide base.
page 225 note † Salts are often named from their bases. Thus salts of monoxides or protoxides are called monoxide and protoxide salts; and salts of sesqui- or per-oxiues, sesqui-oxide or peroxide salts. Some have also been named kaloid salte, and consist of (1st) certain alcalies and earths united with the soluble acids (carbonic, sulphuric, and boracic); and of (2d) chlorine and fluorine with their metals (that is, the metals of the alcalies and earths). Of the 1st are—Calcite, magnesite, dolomite, anhydrite, gypsum, baryte and boracite; of the 2d — Rock-salt and fluor. This grouping is useful in considering the rocks made up of these minerals, which form a natural family.
page 228 note * “The vast mass of minerals are made up of compound silicates.” See XIX. p. 57.
page 228 note † That is, whether they exist as Fe, Mn, or Fe, Mn.
page 229 note * L'expression de famille de felspaths est défectueuse. En effet, les espèces qui la constituent n'appartiennent pas au même système crystallin; la composition, différente pour quelques-uns sous le rapport des éléments, est atomiquement différente pour la plupart: en sorte qu'il n'existe de rapprochement entre eux ni par la forme ni par la composition. Les caractères extérieurs sont, il est vrai, tellement analogues, que la reconnaissance de ces espèces est une des plus grandes difficultés de la minéralogie. En outre, les formes quoique différentes sous le rapports des systèmes auquels elles appartiennent sont très-rapprochées par leurs angles. La couleur, l'éclat, la dureté, le poids spécifique, presque les mêmes pour ces minéraux, augmentent leur analogie: leur réunion en un group est donc fondée plutôt sur la difficulté qu'on éprouve à les reconnaître que sur les principes philoeophiquce. Coquand, p. 4.
page 235 note * These obscure minerale are noticeable as being the only silicate of Baryta known in the mineral kingdom.
page 236 note * See Dr. Phipson's remarks in July number of The Geologist (vol. I p. 299) on this acid.