NOTES AND NEWS 879

THE AMERICAN MINERALOGIST, VOL 44, JULY_AUGUST, 1959

AN OCCURRENCEOF GEIKIELITE Wrr.rrarr S. Wrso, JohrcsHopk'ins Uniaersity, Baltimore, Maryland.

Geikielite, the magnesium-analogof ilmenite, has been found'in situ in some highly metamorphosedmagnesian-marbles in the Santa Lucia Mountains, Monterey County, California, and this is the fourth 'in situ occurrenceof geikielitethus far described.Dick (1893)identified bluish- to brownish-black pebbles in the gem sands of Ceylon as geikielite, which he believed was a magnesium analog of perovskite. Crook and Jones (1906) also describedgeikielite from a collection of Ceylon gem sands during a general study of such materials. Kashin (1937)found geikieliteassociated with chloritesin the chrome- spineldeposits of the southernUral Mountains, USSR, whereasMurdoch and Fahey (1949) reported an occurrenceof this mineral in metamor- phosed magnesian-limestonesnear Riverside, California. In the latter occurrencegeikielite is associatedwith calcite and brucite, although spinel and geikielite are concentratedin certain zones.An analysis of this geikieliteshowed 1.4/6 FeO and 31.8/6 MgO. Geikielite with 12.30/6 FeO, 2.80/6Fe2O3, and 21.75/6 nlgO occurs as rare inclusionsin chromian-chloritesin a serpentinizedbody of dunitc (Efremov, 1954)near Mount Jemorakly-Tube,North Caucasus,USSR. OccunnnNce The geology of the Santa Lucia Mountains, California, includes large massifs of metamorphosed rocks of the sillimanite zone. The original rocks were mostly sediments with some limestones and magnesian- limestonesin ranging statesof purity. During metamorphismof the im- pure magnesian-limestones,coarse grained marbles have formed with associatedphlogopite, clinohumite, forsterite, spinei, and occasionally geikielite. Geikieliteoccurs as small (0.01 mm. to 0.10 mm.), black and opaque, irregular to occasionallyrounded grains, closely associatedwith rutile and spinel.If rutile is present,it is alwaysin juxtaposition with geikielite, and powder photographs of apparently pure material show a mixture of the two minerals. Spinel is also closelyassociated with the geikielite. In the associatedferro-magnesian minerals the amount of iron is notably small: The spinel contains5.2/6 ol FeAlzOa,which was determinedthrough (1) chemical analysis,in which FeO:2.a/6; (2) comparisonof the cell edge(oo:8.092 A) with that of pure artifical spinel (oo:8.080 A, Swan- son and Fuyat, 1953);(3) comparisonof the refractiveindex (1.721)with that of pure spinel (1.719, Palache,et al., 1944,p. 690). NOTES AND NEWS

The phlogopite contains approximately 1916 annite, determined through (1) refractive index, r:1.600, indicating 20/6 annite (Wones, 1958a or 1958b); (2) douo:1.537A, indicating L8/6 annrte (Wones, 1958aor 1958b);(3) the intensity ratio of 004/005is 0.49,corresponding to 19/6 of the octahedralpositions filled by iron (Gower, 1957). The clinohumiteis practically the pure magnesiumend-member, since the refractive indices give a mean refractive index of I.646 (Sanama, 1953).The olivine contains fuom 2/6 to 5/6 fayalite, which was deter- mined from 2V*: 97" to 94" (Winchell and Winchell, 1951). Geikielite was also found in a locality in the same marble layer but about 800 yards to the south. Sample ft2 is that from the southern locality, while Sample11 is from the main area studied.

l{nronarocY AND PanecBNBsrs The geikielite can best be separated from the associated minerais, especiallyrutile and spinel, by passing 250-meshmaterial through a Frantz magnetic separatorset at 0.6 amps with 7|" tilt and 15o slope (spinelis separatedat 0.9 amps, leaving the rutile). Powder photographsof the geikielite from both localitieswere taken and are comparedwith the pattern of artificial MgTiO3 (Table I). The cell dimensions,as calculated from the powder patterns, are given in Table II. The principal point of distinction between geikielite and ilmenite patterns is the appearanceof reflectionsfor the planes (003) and (101) in the geikielite patterns. Although the powder patterns appear to be distinctive, geikielitehas at times been mistaken for ilmenite. The d-spacingsand cell dimensionsof Samplefr2 are larger than those in Sampleff|, and thesedata suggestthat the former has a higher amount of iron. This suppositionwas borne out by qualitative density determina- tions. The density of grains from Sample rtl averageabout 4.2, whtle most of the grainsfrom Sample12 would float in Clerici's Solution (den- sity 4.2). Pure geikielitehas a density of 4.05 and ilmenite, 4.79 (Palache, et al., 1944,p. 536). Sincethe material was too scarceand usually too impure for a chemicalanalysis, the amount of iron can only be estimated from the approximate density, eiectromagneticproperties, and cell dimensions.The limits may be set at 25/6 and 40/6 FeTiOe. The abundanceof magnesiumand scarcity of iron in the rock may be the reasonfor developmentof geikieliteinstead of ilmenite, sinceunder- lying the marbles a biotite-plagioclase schist contains bands of calcite, pale-greenpleonaste, and ilmenite with some rutile and sphene. The formation of spinel results from the low amount of silica in the system, and this fact probably accounts for the formation of geikielite in place of NOTES AND NEWS 881

Tlsr,n L Powonn-DrprnecrroN Dlre roR TrrE Sexra Lucre Gn'rrrolrrr

RadiationCuKa:1.5418 A and CuKa,:1.5405A. Cameradiameter 114.59 mm.

Sample 11 Sample12 Itkl, d A (MsTio, d (meas)A d (meas) A

003 4.67 10 4.Ol 10 4.64 101 4.19 5 4.19 .) 4.18 102 3 723 30 3.70 30 3.703 104 2.735 100 2.742 100 2.722 110 2.536 40 2.542 60 2.527 113 2.226 45 2.232 40 2.218 202 2.O d* 2.O90 204 1.859 45 1.861 30 1.852 116 r.714 65 1.718 60 1.708 108 1.627 10 1.6148 214 t.4993 10 1.502 30 1.4938 030 1.4650 10 1 -466 30 .4592 208 t.3628 5 .3606 1.0. 10 1.3280 I -.tJ4 15 .3247 220 r.2695 5 .2634 128 1.204 d .1978 2.0.r0 1.180 5 1.1735 134 1.152s 5 t.1462 226 1.1155 5 1.1093 2.1.r0 1.0715 .) |.0642 324 0 .970 d o.9646 140 0 .958 d 0 .9552 0 920 d 0.889 d

* Diffuse line. sphenein such a calcium-rich environment. Sphenewas formed in less magnesianlayers of the marble associatedwith minor amounts ol quartz and diopside. The existenceof rutile as excessTiOz, may (but not

Tlnm IL Crr.r, DrrrensroNsron MgTiOs, Sasra. Lucrl Grn

doA coA

MgTiO: (Swanson, et ol. 1955) .) . u54 13.898 Sample 11 < o7?) 13.9s01 ^^- "'''- ! +- ooq'-' t 'uu5 Sample 12 s.08oJ 13.89s1 Ilmenite (United Steel Co., Sheffield) 5.079 14.135 882 NOTES AND NEII.S necessarily)result from a deficiencyof magnesiaat the time of the for- mation of geikielite. I am indebted to Dr. C. O. Hutton {or his willine assistanceand helpful criticism. RnrBnnncrs

Cnoor<, T. eNo Joxes, B. M. (1906), Geikielite and the ferrornagnesium titanites: Min. Mag., 14, 160-166. Drcr, A. (1893), On geikielite, a new mineral from Ceylon: Min. Mag,lO,745-147. Elnnuov, N. (1954), Geikielite from Mount Jemorakly-Tube, North Caucasus, USSR: Am. M'inuatr., 39, 395-397. Gowrn, J. A. (1957), X-ray measurement of the iron magnesium ratio in biotites: Am. I. Sei.,255, 142-156. KasnrN, S. A. (1937),Metamorphism of chromspinelids in the Camel Mountains (Southern Urals): Chromites of the USSR: Acad. Sci. USSR. Munoocn, J. ,luo Fenev, I . J .0949) , Geikielite, a new find from California : Am. Mineral. 34, 835-838. P.lr.ncnn, C., BrnnaN, H., eNo Fnoxonr., C. (1944), The Sltstem oJ Mineralogy, Wiley, New York. Sarrana, Tu. G. (1953), Mineralogy of the humite grotp: Annal,es Acad. Scient. Fenn. Ser.A lI. No.31, 50 pp. SweNsoN, H. E., exo Fuv,q.r. R. K. (1953), Standard X-ray difiraction powder patterns: U S. Nat. Bur. Stand,.,Circ. 539, 2, 35. SwaNsoN, H. E., Grllmcn, N. T. enn UcmNrc, G. M. (1955), Standard X-ray difiraction powder patterns: U. S. Nat. Bur. Sland., Circ. 539,5r 43. WoNrs,D.R. (1958a),ThePhlogopite-annitejoin: Carn. Inst.oJWash.Vearbook57,194- 195. WoNns, D. R. (1958b), Phase relations of biotites on the join phlogopite-annite: BUII. G.S ,4.,69, 1665.

THE AMERICAN MINERALOGIST,VOL 44, JULY_AUGUST,1959

CERIANITE, CeOz, FROM POQOS DE CALDAS, BRAZIL

Crrrronl FnoNoBr- AND URSULA B. lVlenvrw, Haraard Llniversity, ond' (Jn'ionCarb'id'e Ore Company, l{ew York.

Cerianite has been identified as a secondarymineral from Nlorro do Ferro on the Pogosde Caldasplateau, Minas Gerais,Brazil. The cerianite occursin soft, weatheredmaterials lying within a radioactive zo:neon the southeasternslope of l\forro do Ferro. Samplesof these materials were collectedin July, 1956,by T. C. Marvin, Geologistfor the Union Carbide Ore Co., during a visit to the locality in company with Helmuth Wedow, of the U. S. GeologicalSurvey, who had made a detailed study of the geologyof the mountain. The samplescontaining cerianite were taken from a small, irregular zone, a few inches in width, of especially high