SCAPOLITE, a group of rock-forming minerals, which are silicates of aluminium with calcium or sodium but containing more or less chlorine, carbonate or sulphate radicals. The scapolites (Gr. aKii.ros, rod, Mhos, stone) must be regarded as variable isomorphous mixtures of the following five components : Ma =3NaA1Si3Os•NaCl—chloride-marialite MaS=3NaAISi30S.Na2SO4—sulphate-marialite MaC=3NaAlSi308.Na2CO3—carbonate-marialite MeC=3CaAl2Si2Os•CaCO3—carbonate-meionite MeS=3CaAl2Si20s.CaSO4—sulphate-meionite It seems not improbable that in marialite, NaCl, etc., may be replaced to a limited extent by KOH, and in meionite etc., be similarly replaced by CaF,. The close relation which this group of minerals bears to the plagioclase felspars is apparent in the formulae indicated above. The scapolites crystallize in the tetra gonal system, the crystals being prismatic hemihedral. The domi nant cleavage is (I oo). The hardness is 5-6, while the sp.gr. varies with the composition, pure chloride marialite 2.56o carbon ate-meionite 2.772. The colour is usually white or grey. Between the end members, marialite and meionite, there are mixtures which have been given definite names. These include dipyre and miz zonite, and may, as in the case of the plagioclase felspars, be arbitrarily fixed as follows: Marialite Maloo--4Maso Dipyre MasoMe2o---*MasoMeso Mizzonite MaooMeso-4MazoMeso Meionite The scapolites show varying resistance to acids, the meionite rich members being most readily attacked, CO, being released by HCI-HF mixtures from the carbonate meionite, while marialite is only slightly attacked. None of the scapolites have as yet been synthesized. They are destroyed by fusion, the lime-rich members giving plagioclase felspar on cooling. The commonest alteration products are calcite kaolin, white mica and zeolites. The scapo lites are essentially metamorphic minerals, appearing very com monly in metamorphosed limestones.
Well-developed glassy crystals of scapolite occur in the ejected blocks of limestone of Monte Somma, Vesuvius, and large crystals occur in the apatite deposits of Bamle, southern Norway, where they arise from alteration of plagioclase felspar in gabbro. The
mode of occurrence of scapolite is referred to in detail below.
(C. E. T.) Scapolite Rocks.—According to their genesis the scapolite rocks fall naturally into four groups.
I. The scapolite limestones and contact rocks. As silicates rich in lime, it is to be expected that these minerals will be found where impure limestones have been crystallized by contact with an igneous magma. Even marialite (the variety richest in soda) occurs in this association, being principally obtained in small crystals lining cavities in ejected blocks of crystalline limestone at Vesuvius and the craters of the Eifel in Germany. Mizzonite and meionite are far more common at the contacts of limestone with intrusive masses. The minerals which accompany them are calcite, epidote, vesuvianite, garnet, wollastonite, diopside and amphibole. The scapolites are colourless, flesh-coloured, grey or greenish ; occasionally they are nearly black from the presence of very small enclosures of graphitic material. They are not in very perfect crystals, though sometimes incomplete octagonal sections are visible; the tetragonal cleavage, strong double refraction and uniaxial interference figure distinguish them readily from other minerals. Commonly they weather to micaceous aggregates, but sometimes an isotropic substance of unknown nature is seen re placing them. In crystalline limestones and calc-silicate rocks they occur in small and usually inconspicuous grains mingled with the other components of the rock. Large, well-formed crystals are sometimes found in argillaceous rocks (altered calcareous shales) which have suffered thermal metamorphism. In the Pyrenees there are extensive outcrops of limestone which are penetrated by igneous rocks described as ophites (varieties of dolerite) and lherzolites (peridotites). At the contacts scapolite occurs in a great number of places, both in the limestones and in the cal careous shales which accompany them.