b'Don Emersons best of Exploration GeophysicsFeatureFigure 7.Histograms of SI mass susceptibility and density for plutonic rock types from Finland (after Puranen 1989). Note the unimodal density distribution contrasting with the bimodal susceptibility distribution. The ferromagnetic subpopulation is shown as black; the small proportion of diamagnetic samples is shown hatched. To convert mass susceptibility to SI volume susceptibility, multiply by the density in kg/m3.from this ratio cannot be accommodated by varying theFor example, mature sedimentary rocks, their metamorphic feldspar compositions or relative proportions, but must beequivalents, and granitic rocks derived from partial melting of expressed in the varietal mineralogy. Peraluminous rocks arethe metasediments are peraluminous because of the severing of oversaturated with respect to alumina, i.e. molar Al 2 O 3(A)the nexus between alumina and Na + Ca during the sedimentary exceeds the sum of Na 2 O + K 2 O + CaO (denoted A/CNK1), andcycle. Sodium is partitioned strongly into seawater and calcium are characterised by aluminous minerals, such as corunduminto carbonates, leaving sedimentary rocks with excess alumina.(rarely), andalusite, sillimanite or kyanite, almandine garnetQuartz is a major constituent of many igneous rocks, and or, most commonly, muscovite. Peralkaline rocks, on the otherits presence or absence is a very significant petrological hand, contain insufficient alumina to consume all of thecharacteristic. Many minerals exhibit a clear sympathetic or sodium and potassium in feldspars, i.e. molecular Al 2 O 3 is lessantipathetic association with quartz. Oversaturated rocks contain than Na 2 O + K 2 O (A/NK 1). Such rocks are characterised byfree quartz, together with oversaturated (compatible) minerals minerals of the aegirine, riebeckite, arfvedsonite or aenigmatite(e.g. Al- and Ti-poor pyroxenes, feldspars, amphiboles, micas, classes. Metaluminous rocks are intermediate in aluminafayalitic olivine). Undersaturated rocks contain undersaturated saturation, such that all the alumina, soda and potash can beminerals that are antipathetic to quartz (e.g. nepheline, accommodated in feldspars, with excess calcium appearingmagnesian olivine, sodalite, leucite, Al- and Ti-rich augite).in the norm as diopside and in the mode as calcium-bearing pyroxene, amphibole etc. Peraluminous chemistry may resultThe abundance of Na and K exerts a strong influence on the either from high Al content, or from low levels of Na, K or Ca.silica saturation state. In feldspars, every molecule of soda or 51 PREVIEW APRIL 2020'