574 



NA TURE 



[April 14, 1898 



M IE R SITE, A CUBIC MODIFICATION OF 

 NATIVE SILVER IODIDE. 



SILVER IODIDE is remarkable in being one of the few 

 substances which undergo a contraction in volume as the 

 temperature increases. This contraction is uniform until about 

 146° C. is reached, when there is a further sudden contraction of 

 considerable amount, after which the substance expands. The 

 sudden contraction at 146° is accompanied by a change in all 

 the physical properties of the substance, the pale yellow, hexa- 

 gonal modification which exists at ordinary temperatures, being 

 then changed into a bright yellow, cubic modification. On 

 cooling the reverse phenomena are observed. 



From this behaviour it would be expected that only the pale 

 yellow, hexagonal modification would be found as a natural 

 mineral, and as a matter of fact the only pure silver iodide so 

 far known is the hexagonal species iodyrite. The existence of 

 a cubic modification has, however, long been suspected from the 

 occurrence of iodine in the cubic mineral iodobromite 

 (2AgCl.2AgBr.AgI). This probably represents the artificial 

 cubic modification which is stable above 146°, in which case the 

 natural crystals of iodobromite should be pseudo-cubic ; in fact, 

 pseudomorphs of the hexagonal modification with the external 

 form of the cubic modification. This would be strictly analogous 

 to the pseudo-cubic leucite and boracite, which become isotropic 

 when heated to a definite temperature. 



The new mineral, miersite, is, however, quite distinct from 

 these, and proves that silver iodide is trimorphous. The principal 

 characters of the three modifications are : — 



Iodyrite 



Hexagonal 



Hemimorphic 



Twin plane, a pyramid 



face 

 Cleavage, perfect basal 



Optically uniaxial 



Sectile 



Miersite 



Iodobromite 



Cubic 



Tetrahedral 



A tetrahedron face 



Perfect dodecahe- 



dral 

 Isotropic 



Brittle 



Cubic 

 Holohedral 

 Not twinned 



Indistinct octahe- 

 dral 



Optically anoma- 

 lous? 



Sectile 



two specimens preserved in the British Museum collection are 

 from the Broken Hill silver mines in New South Wales ; the 

 associated minerals on one specimen are quartz, copper glance, 

 and garnet, and on the other, malachite, wad and anglesite. 

 The small crystals of miersite, which do not exceed 2 mm. in 

 diameter, are scattered over the surface of the matrix ; they are 

 of a pale or bright yellow colour, with an adamantine lustre. 

 The only forms present are the cube and one or both of the 

 tetrahedra, the latter usually differing in size but not in surface 

 characters. In many respects the mineral is strikingly similar 

 to the yellow blende which occurs in the white dolomite of the 

 Binnenthal in Switzerland. The bright yellow streak is some- 

 times deeper in colour than the crystals themselves ; this is 

 strikingly shown by perfectly colourless and transparent crystals 

 of marshite, which also give a bright yellow streak. Exposure 

 to bright sunlight for several days does not aff"ect the colour of 

 the crystals. The silver is in part replaced by copper, and as 

 this increases in amount, there is a gradual passage from mier- 

 site to marshite: "cuproiodargyrite" (AgLCuI) from Chillis 

 possibly an intermediate member of this group. 



The new mineral has been named in honour of Mr. H. A. 

 Miers, F.R.S., Professor of Mineralogy at Oxford, who first 

 correctly determined the crystalline form of marshite, a mineral 

 so closely resembling miersite in appearance that the two 

 species are only to be distinguished by chemical tests. 



L. J. Spencer. 



Between these there is a remarkable crystallographic relation : 

 when a regular octahedron is considered as a rhombohedral 

 crystal, the angle 70° 32' corresponds to the angle 70° 36' be- 

 tween the basal plane and a pyramid of iodyrite ; the tetrahe- 

 drism, twinning, and sometimes the peculiar development of the 

 miersite crystals make this relation still more striking. 



Iodyrite, in all its crystallographic characters, is practically 

 identical with wurtzite (ZnS), greenockite (CdS), and zincite 

 (ZnO) ; these are all hexagonal and hemimorphic, possess a 

 basal cleavage, and are optically positive, while the axial ratios 

 vary only very slightly [a : c — i : 0-8109 - i : 0-8196). Many 

 other substances may perhaps be included in this series, e.g. ice, 

 magnesium, cadmium iodide, tridymite (SiOj), &c. In the 

 same way the dimorphous cubic modifications miersite, blende 

 (ZnS), and marshite (Cul) form another parallel series, since 

 they are all exactly alike in crystallographic characters. 



It will now be seen that the same relation exists between 

 iodyrite and miersite as exists between wurtzite and blende. 

 This forms, as far as crystallographic characters are concerned, 

 a perfect example of an isodimorphous group, but apparently 

 the only relation existing between zinc sulphide and silver 

 iodide is that their simplest conceivable chemical molecules 

 contain two atoms. 



From these somewhat remarkable relations one is inclined to 

 ask : why should there not be a third modification of zinc 

 sulphide to correspond with iodobromite ? or why should not 

 all these substances {e.g. ice, &c.) be dimorphous or trimorphous 

 to fill up the gaps in these parallel series? Further, if 

 nantokite and marshite are to be represented by the formulae 

 CugCla and Cujlj respectively, then miersite should be Agol2 : 

 these double molecules, however, only depend on the vapour 

 density of cuprous chloride, but the gaseous molecule cannot 

 be the same as the crystal molecule, especially when there are, 

 as in silver iodide, possibly three types of the latter. 



A detailed description of miersite will be published in the 

 Mineralogical Magazine. It may now be mentioned that the 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 

 The Maryland Senate has passed a Bill granting 50,000 

 dollars a year for two years to the Johns Hopkins University. 



Dr. Charles Chree, Superintendent of Kew Observatory, 

 has received the honorary degree of LL. D. from the University 

 of Aberdeen. 



Mr. C. B. Rouss, who gave 25,000 dollars for a physical 

 laboratory building in the University of Virginia, has given an 

 additional 10,000 dollars for the same object. 



Mr. Chester W. Kingsley, of Cambridge, Mass., has 

 given several large gifts to various benevolent purposes, including 

 the sum of 25,000 dollars each to the Newton Theological 

 Seminary, Andover Academy, and Colby University ; the two 

 first named being situated in Massachusetts, and the last in 

 Maine. 



A COURSE of eight Yates lectures in Archeology will be com- 

 menced at University College, London, on May 4, by Mr. J. 

 Romilly Allen. The subjects of the first two lectures are the 

 origins of primitive art and the evolution of decorative art, and 

 the general object of the course is to trace the developments of 

 Celtic art. 



The first school of forestry in America has just been created 

 by the legislature of the State of New York, to be connected with 

 Cornell University, and the sum of 10,000 dollars has been 

 granted to cover the expenses of the first year. The school is 

 authorised to purchase forest lands to the extent of 30,000 acres 

 in the Adirondack region. 



The University of Paris has (says the Times) arranged for a 

 loan of 1,700,000 francs from the Credit Foncier, repayable in 

 50 annual instalments, for the erection of new buildings in Pans 

 and at Fontainebleau. The Faculty of Science is also about to 

 order the construction at a cost of 25,000 francs of an equatorial, 

 which, after figuring in the Exhibition of 1900, will be placed 

 in the tower of the new Sorbonne. 



At the graduation ceremony of the Glasgow University on 

 April 12, the honorary degree of Doctor of Laws (LL.D.) 

 was conferred upon Mr. Alexander Duncan, Secretary and 

 Librarian to the Faculty of Physicians and Surgeons, Glasgow ; 

 Mr. Douglas Dunlop, Secretary-General to the Department 

 of Public Instruction, Cairo, Egypt ; Mr. John Inglis, formerly 

 president of the Institution of Engineers and Shipbuilders in 

 Scotland, president-elect of the Institution of Marine Engineers, 

 London ; Dr. Elie van Rijckevorsel, of the Batavian Society 

 of Experimental Philosophy, Rotterdam ; and Prof. J. M 

 Thomson, F.R.S., professor of Chemistry in Kings College 

 London. 



The excursions of the London Geological Field Class wi: 

 begin on Saturday, April 23, with a visit to Dorking, Box Hill 



NO. 1485, VOL. 57] 



