ZEOLITE. 



fti aped fragments ; it yields eafily to the knife. The fpecific 

 gravity is from 2.158 to 2.197. Before the blow-pipe it 

 intumefces, and forms a jelly with acids. It may be diftin- 

 guifhed from needle zeolite by its inferior luftre, fibrous 

 ftrufture, and low degree of tranfparency and hardnefs, 

 and alfo by its want of regular cryftallization. 



Needle Zeolite, Mefotype, Haiiy ; and prifmatic mefo- 

 type of fome mineralogifts. Its colours are, greyifh, yel- 

 lowifli, or reddifh-white : it occurs both mafTive and cryf- 

 tallized. The cryftals are acicular-redangular four-fided 

 prifms, terminated by low four-fided pyramids, the faces 

 of which are fet on the lateral planes of the prifm. 

 Sometimes there are only two terminating planes, form- 

 ing an acute bevelment, fet on obhquely. The reAan- 

 gular prifm is fometimes truncated on the edges, form- 

 ing an oftahedron, with four large and four fmall planes. 

 The lateral planes of the cryftals are longitudinally ftriated, 

 but the acuminating planes are fmooth. Sometimes the 

 cryftals are diverging, and fometimes promifcuoufly aggre- 

 gated. The cryftals are externally (hining or fplendent, 

 internally gliftening ; the luftre is vitreous, inclining to 

 pearly. The ftrufture is lamellar, with joints parallel to 

 one of the fides of the prifm ; alfo at right angles to the 

 axis, and parallel to the two diagonals of the prifm. Of 

 thefe, the firft cleavage only is generally vifible. The crofs 

 frafture is imperfeftly conchoidal ; the luftre between vi- 

 treous and pearly. It is tranflucent or tranfparent, with 

 double refraftion. It yields to the knife, but fcratches cal- 

 careous fpar, and is brittle. The fpecific gravity varies 

 from 2.17 to 2.27. This mineral, like the preceding, intu- 

 mefces before the blow-pipe, and gelatinizes with acids. It 

 becomes eleftric by heat ; the extremity of the cryftal, 

 terminated by a pyramid or bevelment, fhews pofitive the 

 bottom of the cryftal negative eleftricity. According to 

 Vauquelin, the conftituent parts of mefotype are. 



Silex 

 Alumine 

 Lime 

 Water 



50.24 



29.50 



9.46 



99.2 



Needle zeolite, or mefotype, is diftinguiftied from radiated 

 zeolite, or ftilbite, by its vitreous luftre, diftinft prifmatic 

 concretions, and greater tranfparency and brittlenefs. The 

 latter has alfo more of a nacry luftre. 



Radiated Zeolite, Stilbite, Haiiy, is generally of a yel- 

 lowifh or greyifti-white colour, and rarely paffes into reddifh- 

 white or red. It occurs maffive in angular pieces, and 

 globular, and alfo cryftallized in broad, reftangular, four- 

 fided prifms, rather acutely terminated by four planes fet 

 on the lateral edges of the prifm. Of thefe planes two 

 adjoining ones are more inclined to the axis of the prifm 

 than the other two. The fummits of the terminating 

 planes are fometimes more or lefs deeply truncated. Some- 

 times the prifm is fo thin as to form a long fix-fided table, 

 bevelled on the fhorter terminal planes. The cryftals are 

 aggregated in diverging radii, and frequently fo clofely 

 joined to each other, that the pyramidal terminations of 

 each cryftal are only vifible. The broader lateral planes of 

 the cryftals are fmooth, and the fmaller longitudinally ftriated. 

 The ftrufture is lamellar, with joints in one direftion, 

 parallel to the axis of the prifm. The furfaces of the 

 broader lateral planes are fplendent and pearly ; internally 

 the luftre is more or lefs Ihining, and is pearly. The 

 cryftals are tranflucent, or femi-tranfparent. The diverging 



Vol. XXXIX. 



radii of the aggregated cryftals are more or lefs brrad, 

 paiTing from fibrous to fohated. It is brittle, and the 

 fragments are wedge-ftiaped and fplintery. Stilbite fcratches 

 calcareous fpar. The fpecific gravity of this mineral is 

 from 2.13 to 2.16. It intumefces before the blow-pipe, 

 yielding a phofphoric light : it becomes white when laid on 

 a glowing coal : it does not gelatinize with acids. The 

 conftituent parts are. 



Silex 



Alumine 



Lime 



Water 



40.98 

 29.09 

 10.95 

 16.50 



97-52 



Foliated Zeolite, Stilbite, Haiiy The colours of this 



mineral are nearly the fame as thofe of the preceding, being 

 chiefly yeUowifh and greyifh-white, and rarely .milk-white, 

 fnow-white, reddifti-white, or red ; it fometimes is yellowifh- 

 grey, and pinchbeck-brown. It occurs both mafCve, dif- 

 feminated, globular, amygdaloidal, and cryftallized. The 

 form of the cryftals is a low, verj' oblique, four-fided 

 prifm ; fometimes truncated on the acute lateral edges, 

 and alfo on the angles of the acute lateral edges. Some- 

 times all the angles are truncated. It occurs alfo in low fix- 

 fided prifms, and equi-angular fix-fided tables ; alfo in eight- 

 fided prifms. The cryftals are generally fmall; the lateral 

 planes are tranfverfely ftriated, and the terminal planes are 

 fmooth. It has a pearly luftre, which is either fhining 

 or fplendent. The pinchbeck-brown variety has a femi- 

 metallic luftre. It has a fohated and flightly curved ftruc- 

 ture, with a fingle cleavage, parallel with the terminal 

 planes of the prifms. Sometimes a conchoidal crofs frac- 

 ture may be obferved. It is brittle, and the fragments are 

 angular and blunt-edged, and fometimes tabular. The 

 mafiive varieties are ftrongly tranflucent ; the cryftals are 

 tranflucent, femi-tranfparent, or tranfparent. It yields to 

 the knife, but fcratches calcareous fpar. The fpecific 

 gravity of this mineral is 2.2 ; and, like the preceding 

 mineral, it intumefces and melts before the blow-pipe, 

 giving out a phofphoric light : it does not form a jelly 

 with acids. According to Meyer, the conftituent parts 



97.6 



All thefe zeolitic fubftances, clafTed as mefotype and ftil- 

 bite by Haiiy, pafs by imperceptible gradations into each 

 other, and occur, as we have before obferved, in bafaltic 

 and volcanic rocks. 



The eafy fufibiUty of zeolites was at one time regarded 



as rendering their occurrence in volcanic rocks a fubjeA of 



difficult explanation ; but the experiments of fir James 



Hall, referred to under Systems of Geology, demonftrate 



y ■ the 



