ON the SYNTHESIS of GREIGITE Masrvurr Ijon, the Insti,Tute Of

MINERALOCTCALNOTES r487

THE AMERICAN MINERALOGIST, VOL 50. SEPTEMBER. 1965

ON THE SYNTHESISOF GREIGITE Masrvurr IJon, The Insti,tuteof physical and,Chemical Research, K omagome,Bunkyo-ku, Tokyo, J apan.

skinner et al., (1964)have discovereda magneticiron surfidewith the formula FeeS+and a spinel structure in a natural sedimentary occurrence, and named it greigite.Prior to the discovery,yamaguchi and Katsurai (1960)have succeededin the hydrothermal synthesisof a magneticiron sulfide with a spinel-likestructure. They have postulated, by analogy with linnaeite co3Sa,that the sulfideis cubic Fe3Sa.Severai attemprs to synthesizea magneticiron sulfidewhich is obviously greigite have been carriedout:Lepp (1957),Piggott and Wilman (1958),and Berner (1964). However, no one has yet succeededin obtaining greigitewithout accom- panying contaminants. rn relation to the discoveryof well-definedmineral greigite, the svn- thesisof pure Fe3Sawith a spinel structure seemsto be of great interest. From a seriesof experiment,a procedurewas found to obtain satisfactory greigite.The procedurewas a modificationof the method.of yamaguchi and Katsurai (1960).rt was found that the quenchingafter autoclaving is necessarvin order to obtain greigitefree from contaminants.The structure of the autoclaving product was confirmed by means of electron diffraction and r-ray diffraction. rnvestigationsof magnetic property and infrared absorptionspectra were also carried. out. A black, non-magneticiron sulfidewas preparedby mixing 100cc each of 0.15 M Mohr's salt and 0.2 M sodium sulfide.The mixture was shaken and 3 ccof the resultingsuspension was taken out and it wastransferred to a pyrex tube, 10 mm in diameter, 150mm in length and 1 mm in thick- ness'The tube was then evacuatedand sealed.rt wasleft to stand 5 or 6 days. The precipitate settled completely.The contentsof the tube con- sisted of two la1'ers,black compact precipitate and clear upper liquid. The sealedtube was subjectedto autoclavingby immersionin an oil bath at 190oC. for t hour. Quenchingwas then carried out by putting the sealedtube into water. The result of electrondiffraction study is listed in Table 1 in compari- son with the r-ray data of mineral greigite. A difiraction pattern was taken with Hitachi HS-7 electron microscopehaving an accelerating voltageof 50 kv. we seethat the diffractionpattern containsonly the re- flectionsallowed for the crystal with a spinel structure. No other reflec- tions and no anomalousdiffraction intensitiesare observed,indicating that the svnthetic greigiteprepared here with containsno contaminant. 1488 MINERAI,OGICAL NOTES

Tnsln 1. Er-ncrnoN DIrln.nCtrott Derl lOn SvNtnnuC Gnprcrre IN ColtpettsoN lvtnr X-Re,v Dare lon MrNnnll GtrrcrrB (Srrxlwn, Erl alm Gnru,tr-or, 1964)

Calculated d for a cubic cell with a:990 A. Intensities were estimated visually. Elec- tron beam with the wave length 0.0548A was used.

Synthetic Greigite Natural Greigite

J dcrtc dous

l1l 5.72 5.70 3 5.720 8 220 3.50 3.50 5 3.498 32 311 2.98 2.99 10 2.980 100 222 2.86 2.97 2 2.855 A 400 2.48 2.48 8 2.470 JJI 2.27 2.260 1 422 2.02 1.02 1 2.017 o 511,333 I .91 1.90 1.901 29 440 1.75 1.75 0 1.746 77 531 r.o/ I O/ <1 l.o/l 1 620 1.57 t.57 <1 r.5625 4 533 1.51 t..)l 1 1.5058 10 622 1.49 1.4883 2 4M r.43 1.43 3 1.4253 9 711,551 1.39 1.3826 1 642 1.32 <1 t.3204 4 731,553 I.29 3 1.2859 IJ 800 1.238 <1 | 2349 9 73s 1.210 r.2097 <1 644 1.201 L.r97s <1 822,660 1.167 1.1640 <1 1.143 1.145 1,1401 2 840 1.r07 1.105 1.1051 t6 91t,7 53 1.087 1.0844 <1 6& r .u.)5 1.0544 2 931 1.038 <1 1.0351 1 8M 1.010 1.0080 JI

For the sake of confirmation, the crystal structure of the product was examinedalso by meansof r-ray difiraction. For this purposea diffractom- eter with the scanningspeed of goniometer'0-5",20 per minute and un- filtered FeK radiation was used. The result coincidedwith that of the electron diffraction. The sample was prepared as follows. The autoclaved precipitate was separatedfrom liquid phase by filtration, washed several times with water and then dried rapidly in an evacuated chamber. Magnetization and coercivity were measured by means of a magnetic balance.The results: M':24 gauss'cn}f g at room temperature' Satur- ation induction: 1200sauss. The calcuiationwas carried out using the MINI:RALOGTCAL NOTE,S 1489

value of M" mentionedabove and assumingthat the density of the product is 4.1 g/cmr as taken from the r-ray data. No noticeable coercivity was found at room temperature,indicating that the product is magneti- cally soft. rnfrared absorption spectra of the product were studied by perkin- Elmer 521 type spectrometerin the range of wave length 2.5-40pt.Con- tinuous absorptionand no characteristicabsorption peak were observed throughout the range. The result is similar to that of pyrrhotite, but differs from that of pyrite and marcasite (pyrite and marcasite have several characteristicabsorption peaks in the range 23-32p). The author wishes to expresshis cordial thanks to Dr. T. Katsurai for his valuable advice in regard to autoclaving and also for his kind interest in the preparationof this paper.

RelrnnNcns

Bnnxnn, R. A. (1964) Iron sulfides formed from aqueous solution at low temperatures and atmospheric pressure.Jour. Geol,.72,293. Ltrr', H. (1957) The synthesisand probable geologicsignificance of melnikovite. Ecoa. Geol.52, 528. Prcoorr, M. R. eNo H. wrr,nex (1958)The sulphiding of mild steel surface.Acta cryst. u,93. Sr

THE AMERICAN MINERALOGIST, VOL. 50, SEPTEMBER, 1965

PREPARATION OF PURE ARAGONITE AND ITS TRANSFORMATION TO CALCITE

M. Sunla Rao eNo S. R. yocaNARASTMHAN, Departmenl oJ Inorgan'ic and. Physical Chem,istry, Ind,ian Institute of S cience, Bongalore, India.

INtnoouctrolr

By varying the conditions of preparation different polymorphic modifi- cations of calcium carbonate have been obtained in comparatively pure state. The methods suggested for the preparation of aragonite, the orthorhombic modification of caicium carbonate make use of (1) the precipitationof calciumcarbonate at or above60o c., from a solutionof calcium salt by alkali carbonate(Faivre, 7946;de Keyser, 1950;Wray and Daniels, 1957), (2) passageof carbondioxide through calcium hydroxide solution (revins eI at,.,19ss) or (3) removal of carbon dioxidefrom a solution of calcium bicarbonate(Kitano et a1.., 1962).