The ratio between 12514: 1'8744 :: I : 1'5 The simplest ratio, in whole numbers, is therefore 2 : 3. Hence the formula of the body is Fe2O3. FORMULE OF MINERALS. There exist many groups of minerals of which the individual members are composed of equal numbers of atoms similarly combined, and which possess the same or nearly the same crystalline form; such groups are termed isomorphous groups, and the minerals forming each group are said to be isomorphous with each other. Isomorphous minerals may replace each other without affecting the exterior form of the mineral partially replaced. As the percentage amount of the elements in such minerals is different in each case, mixed minerals do not admit of being directly represented by a simple formula derived as above from the percentage composition. It is the practice to represent minerals of this type by a general formula obtained by considering the particular isomorphous substance present in greatest quantity as the typical substance and reducing the quantities of the replacing bodies to equivalent quantities of the typical body, when the formula may be at once obtained by the ordinary means. When possible, mineral analyses are ordinarily reported as percentages of oxides, and in the calculation these oxides are considered as the isomorphous components to be reduced to terms of that present in greatest amount. As an example we may consider the case of ankerite : an analysis of which by Schrötter yielded the following results : CaO is the oxide present in greatest quantity: we therefore reduce the other oxides to their equivalents of CaO and adding all together we obtain the typical composition of the mineral considered as if it were a Ca compound; thus: : Hence total typical oxide present (M" O) Hence typical composition is M"O.CO2 or M"CO3, where M" represents Ca, Mg, Fe, and Mn. The formula of ankerite may therefore be written (Ca, Mg, Fe, Mn) CO3. 114. Show that a mineral having the following percentage composition is represented by the formula MOCO, when M signifies a metal of the magnesium family: 115. The mineral kerolite gave the following numbers on analysis. Calculate its formula: 116. A specimen of cobalt-bloom was found to have the following composition. Determine its formula : As2 05 Co O Fe O ΙΟΙ H2O 117. Calculate the formula of soda-feldspar from the 118. Calculate the formula of labradorite from the following analysis : SiO 2 Al2O3 Fe2O3 CaO MgO 52°52 30'03 1'72 12.58 0'19 Na2O TO CALCULATE THE PERCENTAGE COMPOSITION OF A COMPOUND FROM ITS FORMULA. FIRST calculate the molecular weight of the compound by adding together the sums of its atomic weights. Thus the formula for water is H2O; its molecular weight is therefore 17.96. In order to calculate from the molecular weight of the compound the percentage amount of its constituentsthat is, to determine how much of each constituent is contained in 100 parts of the compound-multiply the amounts of the several constituents in the compound by 100, and divide each of the products by the molecular weight of the compound. Thus, to calculate the amounts of hydrogen and oxygen contained in 100 parts of water :— Table I. in the Appendix gives the atomic weights of the elements according to the most trustworthy determinations. |