CRYSTALLOGRAPHICSTUDIESINTHEVIVIANITEGROUP Tou. F. W. Banru, MineralogiskInstitutt, Oslo' CoNrBNrs 325 Acknowledgment 326 Measurernents with the Two-Circle Goniometer. Erythrite... 326 328 Cabrerite Annabergite and K<ittigite 328 329 X-ray measurements s29 Vivianite. Erythrite.. JJI 332 Cabrerite. Bobierrite..... JJJ 334 Unit CelI and Crystallographic Orientation. 336 Space Group Determination . JJO Optical Studies 340 . oulru[ary. 34r References.. AcruowlnpcMENT By courtesy of Professor Charles Palache the present writer' when still a regular stafi member of the Geophysical Laboratory, was enabled to spenJabout five weeks of the summer of 1935 in the laboratories of the Mineralogical Museum at Harvard University' The chief object was to study a rock seriesfrom Iceland and the Faeroe Islands; but a visit to Harvard, this centre of crystallographic research, afiorded also unique opportunities for studies in classical crystallography' When the writer expressed a desire to do some research work in this field, Dr. Palache tittaty suggesteda re-study of erythrite; he supplied the neces- sary material fromlhe collections of the Museum, and himself selected some of the best crystals for goniometer measurements' Dr. M. A. Peacock kindly ofiered to teach the writer the new methods and schemesof presentation worked out for the new edition of Dana's Mineralogy. For the hospitality and the stimulating influence he received at the Mineralogical Museum the writer wishes to expresshis sincere apprecia- tion. All r-ray work was done at the Geophysical Laboratory' As usual Dr' W. F. Foshag kindly supplied the necessary mineral specimens from the U.S. National Museum. Thanks are also due to Mr. C. J. Ksanda for having taken many of the r-ray photographs. The experimental part of the present paper was done while the writer was connectedwith the GeophysicalLaboratory of the Carnegie Institu- tion of Washington. A preliminary report on some of the results was given at the Sixteenth Annual Meeting of the Mineralogical Society paper if SSS);but owing to various circumstancesthe writing of the final has been delayed for one year. 325 326 TH E A M EMCA N M IN ERA LOGI ST MnasunnuBNTS \4/rrn rnp Two-CrncrB GoNToMETER Erythrite, Cobalt bloom, CosAszOa.SHzO.The crystals are prismatic, show few faces, and are always striated vertically, usually also parallel to [101]. For these reasonsfew good goniometer measurementsexist and the crystallographic constants are not known with any great accu- racy. The results of measurementsby Des Cloizeaux(1908) and Green (1910) are given below. Later studies by Lincio (Igl4) and Shannon (1926) did not lead to new values for the geometricalelements. New measurements were carried out on erythrite crvstals supplied by Prof. Palache from the Karabacek Collection in the Mineralogical Museum of Harvard University. Table 1 gives the crystal elements de- rived from the best measurements and the mean measured and calcu- lated angles for the accepted forms. The orientation of v. Goldschmidt for vivianite has been retained. Tesln 1. Envrnnrrp lnolr ScuNnptrnc, S,q.xoNv:Two-Crncr,e Mn,lsunrurnrs oN Ercnr Cnvsr,q.r,s Monoclinic; a i b i c : 0 . 7648 i | : 0 . 7122 ; 0 : 105"02' ps:qn:rn:O.9372:0 .6878:t; p:74" 58' po' :0.9642, qo' : 0.7722, ro' :0.2686 Measured Calculated No. Forms of Faces c 001 90"00' 15"02' 6 010 lo 000 90 00 a 100 I 90 00 90 00 m l7O I uniquely, but there are broad chains of reflections 53 33 90 00 n 610 ) very roughly corresponding to the positions of 100, 8259 90 00 110and 610 u T0l 6* -85 48 to 93 02 34 50 -90 00 34 49+ p rrr 6 61 16 to 59 50 J+ J/; 59 59 54 55 a 117 .) -44 17 to -44 25 44 53 -44 19 44s2+ q 616 zl -80 16 to -82 27 3501 -80 19 3srzi n 101 I .)I U/ 90 00 5U.)/ x Vicinals. f Vicinals to (T01), indications oI faces were seen throughout the zone from (101) to (616). These figures and their relative accuracy will be discussedin a later section after the presentation of the e-ray studies. Fig. la depicts the typical crystal habit, the large D-faceis character- istic, the relative size of the upper terminal faces is however subject to some variations, but very regularly the same combination of faces is JOURNAL MINERALOGICAL SOCIETY OF AMERICA 327 Frc. 1. (a) Erythrite from Schneeberg,Saxony. (b) "Cabrerite" (: annabergite) from Laurion, Greece. TEE A MEMCAN MINEK4LOGIST present. Vertical striation is universal and afiects all faces of the zone [001]; striation parallel to [101] is also practically always present and affects especiallythe faces6(010), and tll(101). This is why good measure- ments are so difficult to obtain. The pinacoidal cleavage (010) in erythrite is well known; in addition to it a prismatic cleavageor slipping was found parallel to r(ll2). Cabrerite,supposed to be (Ni, Mg)3AsrOs'8HrO, but proved to contain no Mg. Like erythrite in habit. Good crystals for measurements have never been found. No reliable figures for the axial ratio exist; those given by Sachs(1906): @:6:d:0.82386:I:0.77677; P:106o29' are at great variance with the present results; his material must have been very poor, therefore, or else his orientation of the crystal was difierent. His paper does not give any information regarding this question, however. Table 2 gives the geometrical elements derived from new measure- ments on cabrerite crystals from the Karabacek Collection in Harvard University. Taslr 2. CestelrrB lnou Leunror, GnnscEiTwo-Crncr,r' MnesuruMeN:rs oN T.rnsnCrvste.r-s Monoclinic: o ib : c : 0 .7 87 4 : | :0 . 7223 ; A: 104"31i' PoiQn:rs:0 -9195 :0. 6993 : 1 ; P: 75"28L' Po' = 0.9498; qo' :o -7224; xo' : o.2591 No. Measured range Calculated Forms of faces c 001 90'00' 14"3r+', b 010 90'00' 000 90 00 o 100 Broad chainsof re- 90 00 90 00 90 00 m7l0 flections 90 00 52 45 90 00 w I0l - 90000'* 34 40 to 34"47' 90 00 34 38 t rrl 5908+ s4s7+ a lll 3 -42 24 to -43055' M35 to4615 -43 43 M59 r Il2 -30 11 to -32 18 2204 to2220 -30 514 22 +9 * Because of the poor development of the prism zone the azimuth of this face was taken as zero. Fig. 1b depicts the characteristic crystal habit; the small faces r(112) are, however frequently missing. Perfect pinacoidal (010)cleavage. Stria- tion parallel to [101]. Annabergite,NirAszOs.SHzO, and Kdttigite, ZnrAszOs 8H2O, are addi- tional minerals belonging to the vivianite family. Much time was spent in search for small crystals suitable for goniometer measurements,but without much success.With the kind assistanceof Dr. Harry Berman of the Mineralogical Museum at Harvard we finally succeededin getting IOURNAL MINERALOGICAL SOCIETY OF AMERICA 329 a few faceted crystals which could be oriented and measured on the goniometer. Within the rather large error of measurement the observed interfa.cial angles were found to correspond to the analogous angles of erythrite and cabrerite. Although the number of faces was insuffi-cient for establishing the geometrical elements,it can be concluded from these measurementsthat the constants must be very similar to those of eryth- rite and cabrerite. X-nav MBesunprunNts By optical goniometry the crystallographic constants of a mineral can be established only if a sufficient number of external facesis developed; if there are few or poorly developedfaces, no values or uncertain values for the constants will result. By using an tr-tay goniometer accurate values can be obtained even though no external faces may be present. The Weissenberg arrangement is ideally suited to this purpose. The instrument used for the present investigation was constructed by Brek- ken (1932) of the Fysisk Institutt of Norges Tekniske Hijiskole at Trond- heim, and was modified at the Geophysical Laboratory so as to render it more readily available for the application of the equi-inclination tech- nique in accordancewith the schemeof Buerger (1934). By this method the lattice spacings and the angles between planes in the crystal were directly measured. As will be seen, the accuracy of the method is very satisfactory. It is believed that greater accuracy can be securedwith the reflection goniometer only if the crystal faces are of a high degree of excellence.It is also obviously possibleto buiLd a Weissenbergapparatus for very high precision of determination of spacings and angles and thereby, of course, the axial ratio. The construction of such an ap- paratus with an accuracy said to exceed that of an optical goniometer was announced by Buerger at the last (seventeenth) annual meeting of the Mineralogical Society (1936). But even with the ordinary Weissen- berg arrangement the results are very satisfactory. By this method the following minerals were studied: vivianite, erythrite, cabrerite, and bobierrite. Unfiltered Cu-radidtion was used throughout, the wave lengths are: Ior:1.5373, tr"e:1.5410; in case of superpositionof the o1 and a2 spot the averagewave length I:1.5388 was used' Other sym- bols used ut"' 6:glancing angle,p':separation of layer linesl o6, be,c6 are the periods along the corresponding crys(allographic axes; dr*r is the spacing between consecutive structure planes of the set (lzftl). Vivianite, monoclinic, FeaPrOa'SHzO. Small needle-like, transparent crystals from Monserrat Mine, Bolivia, kindly supplied by Dr. W' F. Foshagof the U.S. National Museum, Washington, D.C', were used in the present r<intgenographic investigation. The habit of the crystals permitted an easy orientation of the c-axis, and they were therefore 330 THE AMERICAN MINERALOGIST mounted on the Weissenberggoniometer for rotation around this axis.