1343

The Canadian Mineralogist Vol. 37, pp. 1343-1362(1999)

LUDWIGITEFROM THE TYPE LOCALITY, OCNA DE FIER,ROMANIA: NEWDATA AND REVIEW

STEFAN MARINCEA}

Depantnentof Mineralogy,Geological Institute of Romania,I CaransebesStr., RO-78344, Bucharest, Romnnia

AesrRAcr

Ludwigite from the type locality, Ocna de Fier, Banat, Romania, was re-investigated in order to establish undocumented chemical and physical properties. The occurs in boron-bearing magnesianskarns, in a restricted associationthat includes calcite, (fayahteO.92-2.607o, tephroite 0.75-l.26%o), (magnesioferrite 18.4846-37Vo) and (Xp. = O.597o,Xp = 40.9l7o). The host skams are developed at the contact between an intrusive Upper Cretaceous"banatitic" body, which is mainly granodioritic, and metasomatizedMesozoic limestones.Ludwigite from Ocna de Fier is closer to the magnesian end-member than previously reported. The analyzed samples are compositionally variable, with vonsenite ranging from 4.41 to 14.27 mol.Vo,minor azoproite [up to 0 10 mol.7o (Mg,Fe2*)2(Ti4*,Mg)(BO:)Oz], and less than 6.55 mol.Vo(Mg,Fe2+)2Al(BOtO2 in solid solution, and with minor Sn, Sb, Cr, Ni, Co, Mn and Zn. Cell parametersta 9.225(8) - 9.290(6), b 12.233(9)- 12 334(6), c 3.033(4) - 3.057(3) Al are influenced by both Fe2* and Al contents.The mean reflectance is approximately llvo,and the mean index of refraction is 1.95. A strong magnetic anisotropy is observed;the maximum of susceptibility (12511.76.106e.m u ) was measured along a direction coincident with fiber elongation. The Mtjssbauer specffoscopy indicates the superparamagnetic behavior of the mineral. The splitting of internal vibrational modes of BO3 group in the infrared spectrais consistent with a C3, or C" point symmetry of the BO3 group, which is characteristic of magnesianludwigite. Heated in air, the mineral is stable up to 1000'C. The compositional data, combined with information on the experimental synthesisofborates, indicate a temperatureof crystallization at 600-650'C and oxygen fugacities of 10 18- 10-14atm.

Keywords: ludwigite, magnesian skarns, X-ray data, thermal behavior, magnetic behavior, Mijssbauer spectroscopy,infrared- absorption data, crystal chemistry, Ocna de Fier, Romania.

Souuarnr

Une nouvelle dtude de la ludwigite provenant de la localitd-type, Ocna de Fier, Banat, en Roumanie, a dtd entreprise pour mettre en 6vidence quelques caractbreschimiques et physiques ignords jusqu'h pr6sent. Le mindral apparait dans des skarns magn6siensmin6ralisds en bore; il fait partie d'un assemblageresheint, i calcite, forstdrite (fayalite0.92-2.6OVa,tdphroite 0.75- I.26Va),magndtite(magndsioferrite184846.3l7a)etclinohumite(Xp.=0.597o,Xp=40917o)Lesskarnsh6tessontddveloppds au contact d'un massif intrusif d'Age Cr6tac6 supdrieur, de composition granodioritique ("banatitique"), avec des calcaires m6tasomatis6sd'dge mdsozoique. La ludwigite de Ocna de Fier est plus proche du p61e magndsien qu'on a prdtendu antdrieurement.Les 6chantillons analysdsont une composition variable, avec une propoftion du composant vonsdnite entre 4 4l et 142'77o(base molaire), peu d'azoproite (iusqu'A O.lIVo de (Mg,Fe2*)2(Ti4+,MgXBO:)Oz),et jusqu'e 6.55Vode (Mg,Fe)2A1(BO3)O2dans la solution solide, et contiennent des teneursmineures en Sn, Sb, Cr, Ni, Co, Mn et Zn. Les paramdtres de la maille rdticulaie fa9.225(8) -9.290(6), b 12.233(9)- 12.334(6),c 3.033(4) - 3.057(3) A] sont influencds dgalementpar les teneurs en Fez+et Al. Le pouvoir rdflecteur moyen se situe autour 117o,et I'indice moyen de r6fraction, autour de 1.95. Une puissante anisotropie magndtique peut Otremise en dvidence; une valeur maximale de susceptibilitd (12511.76.1O6te.m.) a dtd mesur6ele long de l'allongement des fibres. La specffoscopiede Mijssbauer montre un comportement superparamagndtiquedu mindral La d6gdndrescencedes modes intemes de vibration du groupe BO3 dans les spectresinfrarouges est compatible avec une sym6trie ponctuelle C3, ou C, de ce groupe, ce qui est caractdristiquede la ludwigite magndsienne.Chauffd dans I'air, le mindral est stablejusqu'h 1000'C. Les donndesde composition, combindes avec celles issuesdes synthdsesexp6rimentales des borates, indiquent des temp6raturesde cristallisation de 600-650"C pour des fugacitds d'oxygbne de 10-18- 10-1aatm

Mots-clds: ludwigite, skarnsmagn6siens, donndes de diffraction X, analysethermique, comportement magndtique, spectroscopie de M6ssbauer, spectromdtrie d'absorption infrarouge, chimie cristalline, Ocna de Fier, Roumanie.

3 E-mail acldress:[email protected] 1344 THE CANADIAN MINERALOGIST

INrnooucrrow Dogneceabody seemsto have been emplaced at arela- tively shallow depth, during the third phaseof intrusion Ludwigite is a quite widespread oxyborate mineral of the calc-alkaline magmas in the area (Bocsa 3 with an extended general formula of (Mg,Fe2*,Mn2+, according to Russo-Sandulescuet al. 1986). U-Pb Ni)2(Fe3+,Al,Ti,Sbs+,Sn4+,Cr3*,Mn3*,MgXBO3)02 type. radiometric analyseson zircon indicate 79.6 + 2.5 Ma It was first described by Tschermak (1874) from Ocna for the Bocsa 3 phase, whereas the Ocna de Fier gra- de Fier, Banat, Romania, formerly known asMoravicza, nitic rocks yield U-Pb zircon ages of 75.5 + 1.6 Ma Vaskii or Eisenstein. Tschermak (1874) gave the first (Nicolescu 1998), indicating that the two magmatic chemical and optical data on the new mineral, and Mal- bodies may be coeval. The intrusive body at Ocna de lard (1887) provided crystallographic data. Fier consists mainly of granodiorite and granodiorite In subsequentstudies of ludwigite, authorsmade use porphyry with subordinate qrrartz monzodiorite and of material from the type locality to compare it with melanocratic diorite (Russo-Sandulescuet al. 1986). ludwigite from several other occurrences,resulting in Skarns from Ocna de Fier occupy extensive zones additional chemical (Schaller 1911), optical (Larsen within the contact area. Their protolith consisted of 1921, Capdecomme 1946), X-ray (Shabynin 1955), micritic limestones with intraclastic levels and calcare- infrared (Aleksandrov et al.1965) and structural data ous sandstonesof Upper Jurassic- Lower Cretaceous (Bonazzi & Menchetti 1989) on ludwigite from Ocna age, deposited in the Ezeris - Cdrnecea zone of sedi- de Fier. Two monographic studies carried out by mentation. They form discontinuousbands, irregular or Codarceaet al. (1957) and Koch (1960) provided valu- tabular bodies and metasomatic veins, and display an able information concerning its composition, optical obvious metasomatic zoning, essentially exhibiting an propertiesand mineral associations,and a broader study inner Ca-rich skarn zone with mainly andraditic garnet, by Kissling (1967) added new morphological, optical, and an outer zone with Mg-rich skarns,developed near thermal (DTA), chemical and diffractometric data, and the dolomitized marble. Cioflica & Vlad (19'73) a detailed parageneticanalysis. described these features as typical of the Ocna de Fier In order to complement the existing data, a detailed "skarn type". study was undertakenusing refleclance,electron micro- The magnesium-rich skams are mainly diopside- or probe, inductively coupled plasma - atomic emission tremolite-rich, with lesser amounts of phlogopite and spectrometry (ICP-AES), inductively coupled plasma subordinate forsterite and clinohumite; the latter are - mass spectrometry [CP-MS), X-ray diffraction, sus- generally serpentinized. Dolomitized marbles hosting ceptibility, Mdssbauer,thermal and infrared absorption ludwigite are invariably adjacent to this type of skarn, analyses. The new data are important in view of the being separatedfrom them by a thin zone of serpentine. widespreadoccuffences of ludwigite in magnesianskam Spatially they are disposedaround Danilii Hill, between deposits(Aleksandrov 1982, Pertsev 1991). Terezia quarry (in the north) and Iuliana quarry (in the south; Fig. l). The most important occurrences of GsolocrcaI- Ssrrrxc ludwigite are those in Magnet, Arhangheli, Iuliana, Terezia and Delius quarries, as well as in some old As describedby Kissling (1967),ludwigite at Ocna mines, e.g., Jupiter and Reichenstein (Kissling 1967;. de Fier occurs as lenseshosted by the dolomitic marble Only the first four sites of mining activity are still developed in the outer zone of a skam complex located accessible.The locus tipicus for the mineral approxi- in the western pat of the South Carpathians, approxr- mately correspondsto an old pit that was removed dur- mately 12 km northwest of Resita, the major city in the ing the exploitation of the Magnet quarry. area. A pronounced zonation of the ludwigite-bearing lenses shows that these are in fact small rnetasomatic ANarvrrcer- Mrrnoos bodies consisting of an assemblageof ludwigite, mag- netite, forsterite, clinohumite and pr:oducts of their A brief description of the main analytical methods alteration. used during this study is given below. More detailed Both skarn and marble are developed in the contact information about methods of sample preparation and area of an intrusive body named "the Ocna de Fier - analytical techniquesis available in Marincea (1998). Dogneceagranodioritic pluton" by Russo-Sandulescue/ Electron-microprobe analyses were performed rn al. (1986). This body is generally consideredto repre- wavelength-dispersionmode, on standardpolished thin sent a southward continuation of the Bocsa massif, a sections. The apparatusused was a CAMECA SX-50 major composite pluton of Upper Cretaceous age electron microprobe set at an accelerating voltage of (Russo-Sandulescvet al.1986). In general, such mag- 15 kV (except 5 kV for B), a beam current of l0 nA, matic bodies are described by the collective term and a beam diameter of 5 pm (10 pm for B, on separate "banatite", after von Cotta (1864), who first described spots). Natural diopside (Si, Mg and CaKct), synthetic at the locus tipicus, Banat, a consanguineousseries of hematite (FeKa), natural orthoclase(K and AlKct), natu- compositionally intermediate intrusive rocks of Late ral pyrophanite (Ti and MnKct), synthetic fluorine Cretaceous - Paleogene age. The Ocna de Fier - (FKa) and boron nitride (BKcr) were used as standards. LUDWIGITE FROM OCNA DE FIER. ROMANIA 1345

Etn mffi trLuffi K m m 6 w w I tl r 10

Frc 1. Geological sketch of Ocna de Fier - Dognecea area (redrawn from Ilinca 1998). Symbols in the legend represent: 1 Pre- cambrian metamorphic rocks (Bocsita- Drimoxa Formation) in the Bocsa nappe, 2 Middle Paleozoic metamorphosed rocks in the Moniom nappe and Upper Paleozoic - Mesozoic sedimentaryformations in the Resita nappe, 3 marble,4-6 "banatites" (4 gabbro- diorites, 5 granodiorites, 6 lamprophyres), 7 marble and hornfels, 8 skarn bodies, 9 Pliocene - Quaternary sedimentary cover, 10 occurrencesof ludwisite 1346 THE CANADIAN MINERALOGIST

Counting time was 20 s per element.Data reduction was apparatus,the heating was stoppedbefore the fusion of done using the PAP correction procedure (Pouchou & ludwigite. The material used for analysis was separated Pichoir 1985). The scanningelectron microscopy study from the powders used for chemical analysis, after con- (SEM ) was performed sbparately,using a JEOL JSM- ventional magnetic and heavy-liquid separation, but 840 apparatusset at an acceleration voltage of 15 kV before chemical treatment. and a beam curent of l0 nA. The infrared absorption spectrawere obtained using Wet-chemical analyses of ludwigite samples from a SPECORD M-80 spectrometerand a standardpressed Ocna de Fier were done after a careful separation,which disk technique. Samples were prepared by diluting the used the techniquerecommended by Vlisidis & Schaller ludwigite powders in KBr, in ratios of about l:500, and (1974). Carefully hand-picked and pulverized material compactingunder 2500 N/cm2pressure. was first etched with cold 1:3 nitric acid, then purified using both the Franz magnetic sepa.ratorand heavy liq- Assocrareo MtNeRAI-s uids in separatory funnels. Prior to analysis, separates of ludwigite were decomposedby melting with NazCO: Results of an extensive analysis of the associated in nickel crucibles, then removed with water. Except for , including the carbonatephases, was given by boron, whose concentration was determined by induc- Marincea (1998). The list of these minerals, restricted tively coupled plasma - atomic emission spectrometry, to the speciesoccurring in the ludwigite-bearing zones, the concentrationofthe cations was determinedby stan- includes magnetite, forsterite, clinohumite, szaibelyite, dard methodsof quantitative analysis(gravimetry, colo- calcite, dolomite, ankerite, antigorite, lt'zardite, rimetry and absorption spectrometry) following clinochlore, pyroaurite, goethite, lepidocrocite, pyrite, techniquesrecommended by Maxwell (1968) and Iosof pyrrhotite, arsenopyrite,chalcopyrite, sphalerite, galena, & Neacsu (1980). The Penfield method was used for marcasite and covellite. Because only the first three water determinations. minerals show parageneticrelations with ludwigite, and The values of reflectance were measuredusing an becauseszaibelyite and the seryentinesoccur by direct Opton Microphotometer equipped with a HTV photo- alteration of primary minerals, only these specieswill multiplier and an MSP-21 analogousdigital converter. be investigated below. Conditions used were: 25W at 6 V as power supply for Magnetite is the only spinel phase found in the the source(a WFG illuminator), 50 x magnification and samples investigated. Franklinite, mentioned by Koch 0.8 numerical apefture for the objective, lamp apefture (1960), was not found in any of them. At Ocna de Fier, of 0.5, 60 pm diameter for the diaphragm. The mea- magnetite may occur as monomineralic bands of opti- surements were made against a SiC reflectance standard. cally unzoned grains or as isolated grains interstitial to Mcissbauerspectra of ludwigite samples were silicates (i.e., forsterite, clinohumite) or to ludwigite. obtained using a constant acceleration, mechanically The cell parameter calculated for three different samples driven, M

TABLE I RESIJ'LTS OF ELECTRON.MICROPROBE ANALYSES OF SELECTED TABLE 2 RESI]LTS OF RFRESENIATIVE ELECTRON.MICROPROBE SAMPLES OF MAGNETITE ASSOCIATED WITH LUDMGITE* ANAIYSES OF SILICATES ASSOCIATED WTII{ LUDWGITE{

SAMPLE 1168 1799 tt67 t975 2073 2218 2219 2221 1168 TYP,F mgtl pgtl ngtl mgtl mgtl mgtl hgtl mgtl ngttr No.'"735344583 Tio, 0010 0015 0016 0028 0000 0044 0071 0027 0030 sio 42056 42.24143222 42554 42.09437436 38401 38335 41291 Ato, 0_04? 0005 0154 0017 030r 0068 0079 0024 0045 '70729 TiO, nd 0001 00E3 0040 0014 0079 ad ad nd Fezo,a) 1t2tt i1,stl i22s6 70786 73466 7tr22 i1326 69950 AlzO' 0017 nd 00ll trd od nd 3 195 1644 1534 F{@ 2376't 25272 2t',t42 2oM9 t69tt 24547 23872 23251 273t5 B0l !a Da oa !a n,a nd nd 1253 ud MnO 0605 0642 1467 r4l0 031E 0t33 04tM 0E46 0593 l'{sO 54539 55298 54.931 54400 55449 55579 3t517 39304 37704 MSO 4259 3298 50t0 6190 3998 8591 4400 4491 l98l F€O@ 2 596 | 169 0 906 | 753 | 47t 0 586 2 E95 2 631 3 205 CaO 0094 0039 0023 0030 0050 0087 0012 0029 O0E6 MtrO 0767 | r90 0t33 1233 0.929 0295 0093 00q) 0051 TOTAL(3) 10000 1o0oo lo0oo l0oo0 t00oo looo0 loooo rooo0 loooo CaO 0 025 0 l0l 0 014 0 020 0,036 0 032 0 0t5 0022 0 193 NIJMBER OF IONS ON TI.IE BASIS OF 24 CATIONS F a,a na na tra na 2467 ad nd nd Ti 0002 0003 0003 000,5 0000 0010 0016 0005 0007 HrOG) n.a aa tra na !a l6tt nc, nc tro Al 00r5 0002 0054 0@6 0106 0023 002E 0008 0016 Total 10000 10000 10000 100.00 1m.00 98153 E3 116 t32t5 E3,9t8 Fe'* 15 980 15 992 15 940 15 983 15 894 15 957 15 940 15 9E0 t5 970 NUMBER OF IONS ON THE BASIS OF N" OXYGEN ATOMS FeF 5926 6350 5 385 4 933 6'!26 4ogz 5946 sTgt 6g3l si 1000 1000 1018 t00r 0997 4017 3776 3734 40n.2 MD 0153 0163 0368 0351 0080 0204 0ttl 0213 0152 Ti 0000 0000 0001 0001 0000 0006 Mg 189? 1417 2243 Z7l2 l7'1E 3697 r954 1993 0t96 Al 0000 0000 0000 0000 0000 0000 0370 0 lt9 0 175 Ca 0030 00r3 0007 0009 0016 002'1 0004 0009 0028 B - 00m 0000 02ll 0000 * MS t.932 1.950 1.927 r.92r l-957 8 E93 5 6,16 J 70t 5,14E R6ult expGsed in ft % Fe'?* 0.052 0.023 0.018 0.035 0.029 0 053 0 23E 02t5 02fi (l) Dmbs of poilt ualyses Mn 0015 0024 0017 0025 0019 0 02? 0 00t 0007 0005 (2) m deduced from FeOr, aftq the adjusm@t of ore fenic/fenms rdios in order to Ce 0001 0003 0000 0001 0001 0004 0002 0 002 0 020 ftlfill iie charge balance 0 837 (3)els l@l@laledb 100 % | 209

* R@lts @Hs€d iD fl Yo (l) nmbq of poift aBbtsg (2) aI F€ E @idq€d in divsled rtd€ of qidatio section. A late-stage partial alteration of both genera- (3) s olc{lared io udq to ilfin the charSBbdee tions of magnetite produced goethite and lepidocrocite, (a) a (O) for ftntsne, 16 (O) d 13 c5tioE br clirohumite 14 (O) for srFdtiEs, (5) Fo: fo6t6it€, Chr: cli@bMitc, Lzr liandit€, A4: utigsie which may occur on fractures and fill cavities in the Othq abbEvidici D! : !.* ealyze4 n,c : rct elals,4 d d I not d€tded massesof magnetite. Forsterite was identified in almost all the analyzed samples.It occurs as disseminatedcrystals or, more which indicate an iron-poor [XF"= l00.Fe2+/(Fe2++ Mg; - commonly, in irregular aggregates of euhedral or = 0.591hydroxyl-clinohumite [Xp = 100.F(F + OH) subhedral crystals, enclosed by the massive ludwigite 40.911. and partially seryentinized.Optically, the mineral is bi- Lizardite typically occurs as a rim on forsterite or axial positive with a very high2V angle(86 < 2y< 88"), clinohumite or, more commonly, in pseudomorphtc which indicates a high forsterite content (Trriger 1969). mesh-texturesafter forsterite. Non-pseudomorphictex- The high forsterite content is also supported by the tures of interyenetrating type occur rarely and consist X-ray powder-diffraction data. On the basis of the posi- of antigorite (identified on the basis of X-ray powder tion of the (130) spacing[d( 130) in rhe range 2.t64 to data, using the schemeof Whittaker & Zussman 1956). 2.767 Al and using the relation proposed by Schwab & All the pseudomoryhic textures have mesh-centersof Kustner (1977\: forsterite or "serpophite" (llzardite-lT according to Wicks & Zussmanl9T5). Optically, the mesh-rimshave Fo (mo1.7o)= 100 - 17.522- 14.907(3.020 - negative elongation, which characterizesan o-serpen- d (130\r/21.100 (qo) sensz Wicks & Zussman (1975). Taking into accountthat the ct-mesh-rimsconsist mainly of lizardite- all the investigated samples contain between 97.5 and l7(Wicks & Zussman1975, Wicks & Whittaker 1977, 100.0 mol.7o forsterite. Cressey 1979),lizardite is infened to be present in all Resultsof electron-microprobeanalyses of represen- the samples examined. This inference is supported by tative magnesiansilicates associatedwith ludwigite the X-ray powder-diffraction study, becauseall of the (Table 2) confirm that forsterite from Ocna de Fier has analyzed samples behave as lizardite according to the a low content of Fe and Mn: between 0.92 and 2.60 criteria of Whittaker & Zussman (1956): they show a mol.Vo fayalite, and between 0 75 and 7.26 mol.Vo high-intensity line at a d value ot 2.499-2.503 A and a tephroite are indicated. strongpair of linesat 1.501-1.503 A and 1.534-1.537 Clinohumite in the borate-bearing skarns occurs A, whereasthe strong line of antigorite around 1.563 A most commonly as discrete subhedralto euhedralgrains is absent. Note that the chemical differences between in the matrix of ludwigite and magnetite, but it is also lizardite and antigorite (Table 2) obey the trend pointed found as clusters of crystals where it coexists with out by Whittaker & Wicks (1970). forsterite. Normally, the crystals are polysynthetically Szaibelyite occurs as bunches of small needlelike twinned on (001). The mineral is biaxial positive, with crystals, near the limits of optical resolution. It forms 7l < 2V < 78o and extinction angles c:cr in the range polycrystalline pseudomorphs after being 9-1 1". The weak pleochroism (colorless along 1 and B, generally intergrown with strings of magnetite II. The light yellow along ct) and the relatively low birefrin- averageunit-cell parameters,deduced by least-squares gence are consistentwith the chemical data in Table 2, refinement of three sets of X-ray powder data, are a 1348 THE CANADIAN MINERALOGIST

TABLE 3 RESIJLTS OF ELECTRON-MICROPROBEANALYSES OF identified as [10] and {120}. No terminal faces can SELECTED SAMPLES OF SZAIBELYITE be distinguished. The frequency of subparallel and par- Smole "I),6'1 1168 l4l5 l'799 lt66 allel intergrowths makes a complete moryhological BzOr 41 008 41 101 42096 Al 444 41226 study difficult. Alzor o 111 oo74 ool9 0301 0,034 The SEM and reflected-light studies document a MgO 469'16 46919 46457 45374 46903 of ludwigite crys- MnO 0336 0362 0302 0253 0315 curved appearanceof some bunches Feo(t) o 847 0 815 0 573 2 080 0 852 tals. The individual crystals that compose these aggre- CaO 0 029 0 039 0 022 0 029 0 016 gates are commonly truncated by narrow fractures, HrO(r) ro792 l0 666 9 722 10071 l0 570 F 0 040 0 067 0 058 0 049 0 025 oriented perpendicular to [001]. Fractures of this kind Total 100139 100M3 99 249 99 601 I may be observedin Figure 24. This systemof fractures, NUMBER OF IONS ON TI]E and secondarymagnetite B locally filled with szaibelyite AI 0004 0002 0001 0010 0001 (Fig. 28), progressively changes the orientation of Mg 1 978 | 9'72 1 906 I 891 1 965 bunchesof ludwigite crystals,which appearcurved. The Mn 0 008 0 009 0 007 0 006 0 00t Fe2t 0 020 0 019 0 0ll 0 049 0 020 extensive systemof longitudinal fractures,called "sepa- Ca 0 001 0 001 0 001 0 001 0 000 ration lines" by Koch (1960) and always filled by mag- (orD- 2 022 2 002 | 852 | 920 I 987 F- 0004 0006 0005 0004 0002 netite and szaibelyite, are consideredto be the result of COMPOSITION IN END.MEMBERS (MOI,,%) the unequal expansion in volume causedby the forma- szaibelyite 98 55 98 55 9E91 97 12 9E60 tion of szaibelyite. sssexite 040 045 036 031 040 Fe'2*r(BTO40HXOH)100 095 068 252 100 " sibirskite" 005 000 Oprrcel ANDPHYSICAL PnoPsnrrrs (1) numbq ofpoint analyws (2) total iroD as FeO The study of thin sectionsemphasizes the practically (3) as calculated in order to fulfill tlle chdge balance opaque nature of ludwigite from Ocna de Fier. Only some fine needle-like crystals have a translucent rim, visible where they are embeddedin the carbonatemass; r2.s45(4),b r0.388(9), c 3.135(8) A. B eS.OSrZZr".in these cases,the mineral shows a weak pleochroism Resultsofelectron- microprobe analyses(Table 3) show in dark brown to reddish brown shades.The directions that the szaibelyite is poor in Mn + Fe and contains as of vibration cannot be specified, but may be inferred by usual a very low content of Ca. Up to 0.45 mol.% comparison with the data in the literature (i.e., sussexite (the Mn end-member) and 2.52 mol.Vo Capdecommelg46,Leonardet al. 1962),which specify Fez(BzO+OHXOH)may be deduced,and the "sibirskite" that the dark colors coffespond to ^y.This behavior is component amounts to 0.05 mol.7o. It is worth noting consistent with a low Al content of a magnesianmem- that the ferrous component is always greater than the ber of the ludwigite - vonsenite series.The indices of manganoan one, a feature that appearsto characterize refraction of sample 1168, measuredin immersion and all the szaibelyite pseudomorphsafter ludwigite in the by comparison with standardSovirel glasses,using ob- ^y Romanian deposits (Marincea 1998). lique illumination and sodium light (I = 589 nm) are 2.03(2) and B (= a) 1.90(2). These values are in good Monpnolocv on rse CnvstaLs

At Ocna de Fier, ludwigite generally occurs as rr- TABLE 4 PHYSICAL PARAMETERS OF SELECTED SAMPLES OF DE FIER regular bands of randomly oriented crystals. These LUDWIGITE FROM OCNA bands ("Liesegang bands" according to Kissling 1967), sampte Mt) I^") D, Kc D'..|4 i"tat Kp"' l4KdKc) 3 to 20 cm wide, predominantly consist of ludwigite and tt67 197.832+ 345.537 3801 0.2528 3730 1.961 0.2482 0.018 1168 t9a607* 347,'lt 3180 02516 374a l95r 02495 0008 alternate with magnetite bands having approximately the 1168 2o42tg' 34707A 3906 0-25073761 1979 02414 OO37 same thickness. They exhibit a texture that obviously 1415 tgg3g3' 346670 3819 02498 3775 1.954o24'lo 00ll t4r7 197672* 34644E 3189 02521 i740 1955 02490 0012 results from a metasomatic process. Occasionally, l4l8 187.924+ 345780 37A9 02516 3748 1-953 0-2490 0010 ludwigite aggregatesoccur as nests or veins, usually 1439 200534* 34664 3 841 02509 3'158 1964 02456 0021 1439 lsggtr' 34664 3829 02498 3175 1956 02463 0014 enclosed by the magnetite mass. Generally, ludwigite n9a 198530*346188 3808 0251037s7 1956 02477 0013 in these aggregatesforms fibrous masseswith a silky 1iw lggz0g' 346921 3a13 02523 3738 1962 02474 0019 luster and pitch-black to olive-black color. The compo- 1866 197942' 3462a1 3-796 02s03 3767 rgs0 02445 0007 1867 ]Jajgs' 346345 38ll 02485 3795 1947 0.24'15 0004 nent crystals are fibrous and form parallel, radiating or zzrS 2w.4?9' 346-435 3.842 0.2525 3.735 r.970 0.u5s 0.028 interwoven aggregates,which give to the mass a felt- ' (l) mol@ld ms, * a cal@tatedoa the bsis of wet+hmical malysc, as like appearance.Coarser radiating or fan-like aggregates cal@latedon the basisof electron-midoprobemalyses are scarce;they contain bunches of crystals of up to 35 (2) mit-@il volme (in,F), r deducedfiom lqst-squres teffnment of x-ray (reported up Dowdq data mm long up to 60 mm by Koch 1960) and a3) catculated ac@rding to GladstoneDale lw, by using the mro index of to 2 mm thick. re&action(u = I 943), ud the chemicslmolil refiaotivity (,

Ftc. 2. Scanning electron micrographs showing truncated crystals of ludwigite in a felt-like aggregate (A) and a magnetite octahedron disposed on a fissure in ludwigite (B). Scale bar = 10 pm

agreementwith those reported by Palacheet al. (1951) drawn for one of the samplesare given in Figure 3. Each for ludwigite from the type locality: 1 2.02(2) and value in Table 5 represents an average of 1000 mea- B (= ct) 1.85(1).Data in Table 4 show that the mean surements.The mean reflectance values are generally indices of refraction, calculated according to the higher than those reported by Uytenbogaardt & Burke Gladstone-Dale rule using the calculated densities and (1971)(R in the range8-107o), but closelyapproximate the molar refractivities (K6) derived from chemical com- the llVa value. In fact, these values are similar to those positions, fit well with the resulting mean index of documented in some other examples of ludwigite by refraction [n = (ct + B + 1)/3 = 1.9431. Strona ( 1964) (R = 9.4 Vo)or Leonard et al. (1962) (R in Polished sectionsshow that, in plane-polarizedlight, the range9.5-l1.4Vo) ludwigite from Ocna de Fier is distinctly bireflectant in On the basis ofthe measuredvalues ofreflectance. a pinkish gray (ll D to datk greenish gray (r Q tints. calculation of the indices of refraction may be Under crossed nicols, it shows a strong anisotropy in attempted, using the Fresnel equation in the form that yellowish brown (ll ! to bluistr violet gray (r ! colors. assumesan absorption coefficient k = 0. The translu- No intemal reflections were observed. cency of some crystal rims suggeststhat k may be The minimum (R-6) and the maximum (R^u*) val- neglected; it results that n = (l + R + 2J R) / (l - R). ues of reflectance were measuredin air, at wavelengths The indices ofrefraction that were obtained are listed rn of 498, 553, 590 and 650 nm, in sectionsof maximum Table 5. The meanvalues (n-in in the range 1.74-1.83, anisotropybecause ofthe difficulty in obtaining oriented sections, for two different samples. The observed val- ues are listed in Table 5, whereasthe reflectancecurrres

TABLE 5 OPTICAL PARAMETERS OF SELECTED SAMPLES OF LIJDWIGITE FROM OCNA DE FIER R.-

553 590 650 15 79t 795 802 1l 32(12) l0 56(9) 9 83(ll) Refl€tmft R-- 14e6(r3) 7e(r2) 13 R ll r2(ll) R(%) "r. t3 14 1218 tt 47 AR: R*.- Rd, 364 323 329 AR 2'7'10 26 52 28 68 28 86 l0 185 189 194 of 170 174 176 refraction 2 SAMPLE I16E OULIA]\IAOUARRYI Wavclength (m) 488 553 59O 650 Med 823 823 823 R-i 10ts(10) e08(10) 888(11) 925 Refl@tme R* 1278(9') 1r 36(11) 1189(11) rt 75

274 250 Wavelength (nm) Bireflstm@ (A R 1633 2497 28 99 24 t5 Indies I e* l8? 201 1'18 182 Frc. 3. Reflectance curves drawn for a selected sample of ludwigite from Ocna de Fier (sample 1167). 1350 THE CANADIAN MINERALOGIST and ,rmaxin the range 1.91-l .99) are lower that the mea- (1963) found a nearly prolate ellipsoid. The maximum sured values given before, but the maxima compares value of susceptibility (k1) was found to be parallel to well with the data of Palache et al (1951\. the fibers, as reportedby Uyeda et al. (1963). Ludwigite The density of a representativesample of ludwigite thus is (ferro)magnetic and behaves as a sum of mag- (sample 1168), measuredby suspensionin Clerici netic needles; magnetite grains preferentially disposed solution diluted with water. is D. = 3.80(3) g/cm3.It along the "separation lines" described earlier induce compareswell with the calculated densities in Table 4, most probably the weak ferromagnetism as well as the derived from chemical compositions and cell volumes, shape effect of anisotropy. This last hypothesis is sup- assuming Z = 4 (Tak1rchi et al. 1950). The calculated ported by the fact that magnetite was identified by X- densities, as well as the measuredone, are very similar ray diffraction in the powder that resulted from grinding to the values reported by Aleksandrov (1968) for the cube used for measurements. Al-poor ludwigite with up to 25 mol.Vo vonsenite in The data collected here do not allow any conclusion solid solution:3.74 < D < 3.86g/cm3 to be drawn about the magnetic behavior of ludwigite The compatibility indices 1 - (KplKd in Table 4, as itself. A Mcissbauerstudy was undertaken to solve the derived from Gladstone-Dale calculations, using the problem. specific molar refractivities of oxides given by Mandarino (1976) except for a revised one for B2O3 MOsseauen Spncrnoscopv [kszo: = 0.241, as calculatedby Mandarino (1981) for boron oxidel, may all be classified as excellent or supe- Mcissbauerspectra were recorded for two represen- rior (Mandarino 1981).This result is excellentconsid- tative samples of ludwigite from Ocna de Fier. The ering the chemical variability of the analyzed samples, Mcissbauerparameters evaluated from both spectraare for which a constant index of refraction was assumed. listed in Table 6, whereasFigure 4 gives the most com- plex spectrum. The spectraobtained may be described MecNsrrc Susceprrerr-rrv in terms of two or three Zeeman sextuplets(I, II and III in Fig. 4) and of two quadrupole doublets (IV and V in Previous studies concerning ludwigite (i.e., Tllley Fig. 4). 1951,Leonard et al. 1962,Uyedaet aI.1963) led to the The first two Zeeman sextuplets are clearly attribut- conclusionthat the mineral behavesas a magneticphase, able to the admixed magnetite. Their internal magnetic being probably (weakly) ferromagnetic. This property fields (IMF), calculated with respect to the IMF (330 explains the attraction to the hand magnet reported by kOe) of the outemostZeeman splitting of metallic iron, Vlisidis & Schaller(1974),btt may alsobe due to con- are H1 - 480-500 kOe and Htr- 440455 kOe, respec- tamination by magnetite A preliminary measurement tively. They are compatible with the values reported by of the magnetic susceptibility of fine-grained powders Bauminger et al. (1961) for the IMF of ferric (I) iron m preparedby careful hand-picking and heavyJiquid sepa- octahedral (B) and ferrous (II) iron in tetrahedral (A) ration indicates, on the basis ofthe attraction exerled by structural sites of a perfectly ordered magnetite. the Franz isodynamic separator,that the value of sus- The third Zeeman sextuplethas an internal magnetic ceptibilityis alwaysgreater than 6000. 106 e.m.u. This freldof 410420 kOe, which is not compatible with any value indicates a magnetic phase,but the magnetic sus- IMF reported for other possible admixed magnetic ceptibility (MS) must be anisotropic. The study of MS phases,such as amorphous iron oxides (445-508 kOe anisotropy on a cube 2 cm on an edge cut from an ap- according to Murad & Schwertmann 1980), goethite parent monomineralic mass of ludwigite was conse- (380-390 kOe according to Bonnin et al. 1982) or quently undertaken. The instrument used was an maghemite (488-499 kOe according to Shinno & ac-bridge Kl,Y/2; Hrouda (1982) described the tech- nique that was followed. AMS measurementsare approximated by the sec- TABLE 6 MOSSBAUER H\?ERFINE PARAMETERS, RELATI\,E IRON DISTRIBUTIONS AND SITES ASSIGNMENT FITTED TO ond-rank tensor k1;, may .IHE O) which be thought of as a com- LUDWIGITE SPECTRA bination ofthe effects induced by all the magnetic grains within a sample. The three main components of the ki, No. Smple Looation of I (7.) A 6 f tensor give the susceptibility k1 (maximum), k2 iron (mp/s)(t) (mn/s)o (nr/sl3) values ffio- (medium) and k3 (minimum). The values recorded for I 1168 Ml + M2 1380 the measuredsamole (1168) are k' = 12511.76.106 M3 2170 2.O7 l 00 0.36 e.m.u..k2 = 11727.b5.106e.m.u. andk3 = 9060.39.106 M4 46 80 1.09 0 30 0 37 = e.m.u., the mean susceptibility being k 11100.100 2 1798 Ml +M2 000 e.m.u.. The k1ianisotropy obtained on the basis of these M3 19 60 2.15 0 9l O33 values is characterizedby the following factors (nota- M4 1240 134 029 038 = = = tionsafter Uyeda et a|.1963):P k1/ k3 1.1$l;| ftt (1) quarhupole splifiing / k2 =1.967 et F = kz / fu = 1.294.The ellipsoid that (2) isomu shift approximates this effect is clearly oblate;Uyeda et al. (3) width of the over-all lines LUDWIGITE FROM OCNA DE FIER. ROMANIA 1351

COIJNTS 1.050106

1.000106

9.500ld

9.000105

8.500105

VEITCITY(mm/s)

Frc. 4. Room-temperature sTFeMtissbauer spectrum of a selected sample of ludwigite from Ocna de Fier. Zeeman sextupletsand quadrupole doublets are indicated above the spectra.

Maeda 1989).Kan et al. (1984) found such a Zeeman the distribution of cations in the surrounding sites, and sextuplet in the Mtissbauer spectrum of synthetic as the presenceof distorted octahedral sites imply val- vonsenite, taken at liquid nitrogen temperature. This ues of A <2 mrnls (Hawthorne 1988),the recordedA third sextuplet thus seems to belong to the ludwigite. value,2.O7-2.15 mm/s, is consistentwith the iron being Crystal-structure refinements repofted by Takduchi located in the most regular octahedronin the structure, et al. (1950), Mokeyeva (1968), Mokeyeva & Alek- namely M3. The isomer shift 6 recordedfor this doublet sandrov (1969) and Irwin & Peterson(1999) show that, (0.91-1.00 mm/s) is characteristicof ferrous iron. in ludwigite, the octahedrally coordinated iron may oc- The innermost doublet has a lower quadrupole split- cupy three different structural sites, namely M3 (very ting A (1.09-1.34 mm/s) and isomer shift 6 (0.29-O.30 regular octahedra),M4 (irregular or very distorted octa- mm/s), which appearto be consistentwith octahedrally hedra) and Ml-M2 (partially distorted octahedra, in coordinatedferric iron in the major distorted octahedral which iron is subordinateto Mg). As supposedby Kan site, M4. The low values of D as compared with those et al. (1984), and on the basis of peculiarities of the reported by Kurash et al. (1912) and Swinnea & paramagnetic domain of the spectra, the third Zeeman Steinfink (1983) for vonsenite. in which Fe3* occupy sextuplet may be assignedto the iron in Ml-M2 sites. the attenuated eccentric octahedra, are indicative of This assumption raises the problem of the super- ludwigite. paramagnetismof ludwigite. In the Mrjssbauer spectra The values of the hyperfine parameters,as well as reported by Yermakov et al. (1969), Malysheva et al. the sites assignmentfor the central part of the spectra, (1971), Kurash et al. (1972) and Kan et al. (1984), as are closely similar to those found for natural and syn- well as in the second spectrum (Table 4), the sextuplet thetic ludwigite by Yermakov et al. (1969), Malysheva III does not occur. One can expect that the very fine et al. (1911),Ktrash et al. (1972) andKan et al. (1984). particles of ludwigite obtained after fine grinding and etching with nitric acid (the technique used to obtain Cnsurcer Dere the initial powder) will behave superparamagnetically. The outermost quadrupole doublet in the central Critical review of previous data (paramagnetic)part of the spectra (IV in Fig. 4) is widely split in comparison with the other doublet (V rn All the existing chemical data on ludwigite from the Fig. 4). As the quadrupole splitting (A) is sensitive to type locality (Tschermak 1874,Schaller 1911, Codarcea t352 THE CANADIAN MINERALOGIST

TABLE 7. RECALCI.JLATIONOF PREVIOUSLY REPORTEDCIIEMICAL DATA ON LUDWIGITE FROM OCNA DE FIER

Sanple* 1 2 345 ol 89 BzO: 1609 15.06 17.g2tt)17.5gtt) tO.ll t6 82 16.83 17.46 1564 FeoO. 39,92 39.29 3567 3306 41.8s 39.27 36,59 38 20 36.67 FoO 72.46 17.67 15.84 16.39 12.68 to.76 tl 02 10.48 18.45 MgO 31.69 26.91 28.88 28.55 2968 34.76 35 17 33.83 29.18 co, 0.90 SiOz 0.36 - 0.36 o o:n HrO. 0.82 1.50 _r, - 0.01 rLo- 0.51 3.00 Tdal 100.16 98.93 100.00 100.00 1007 101.9 1000 99 97 99.95 0 40

ludwigite 100.16 98 93 90 t7 83.41 99.86 lot 17 99.09 9997 99 85 serpentine 083 siderite 237 szaibelyite 6 63 13.59 0.10 forsterite 0.84 0.77 0 9l

&o, t6.o64 t5.223 t552E 13.607 16.153 16625 16.985 17.46s 15.622 FeuO: 39.857 39.715 39.951 41 938 41.908 38.816 36 926 38.211 36.721 FeO 12M0 77.861 16094 79.062 12698 10.636 ll,l2l 10.483 l8 4?6 MsO 31 639 27.201 28.427 25.393 29241 33923 34.968 33.841 29 181 cATroNSON TrrE BASrS OF s (O) B 0.962 0.940 0 950 0 861 0 973 0 981 0 997 1.021 0 957 Fe$ 1.040 I 069 1.065 I 157 1.100 0.998 0.945 0.974 0.981 Fe2o 0.361 0 534 0 477 0.585 0.370 0 304 0.316 0.29',1 0.548 1.636 1.451 I 501 1.388 1.521 1728 1 772 1.544 VONSENITE CONTENT IN TTIE SOLID 1775 27.OO 2445 19.32 24.36 14.67 1495 14.77 32 corrected 18.07 26.90 24.12 29.65 19.57 14.96 15.13 1481 26.20

* Analyses given as follows: 1 and 2 by Tschermak (1874), 3 by Schaller (1911), a by Koch (1960), 5-9 by Codarcea et al. (1957) and Kissling (1967) (l) - deduced by diffenence; (2) - totals recalculated to 100%; (3) - excepting 4 (calculated without subtraction of inpurities), values given by Kissling (re67)

et al. 1957, Koch 1960, Kissling 1967) were obtained Al, Ti4*, sn4*, Cr3*, Mn3+, sbsr but also Mg or Fe2* by wet-chemical analysis. These results indicate a low (where coupled with cations with higher charge). content of vonsenite in solid solution'($p to 3O moI.%o). Ludwigite may contain la"rgequantities of Al; aluminian None of the data, not even those which permitted ludwigite, with 10.97-12.70wt.% AlzOy was described Tschermak (1874) to deduce the correct fomula, are by Schaller & Vlisidis (1961), Pertsev & Aleksandrov stoichiometric, i.e., with a MeO:Me2O3:82O3molar (1964), Kanishchev & Pertsev (1969). It may also ratio of 4:1: L Some of the previously reported compo- contain Ti; Konev et al. (1970) described a sample of sitions (Table 7) contain small quantities of SiO2,H2O+, titanian ludwigite containing 7.35 wt.VoTiOz. and CO2, which are consistentwith the presenceof ad- Subtracting stoichiometric minerals in order to elimi- mixed minerals (e.9., szaibelyite,serpentines, forsterite, nate SiO2 (in forsterite and serpentines), H2O (in carbonates,and hydrated oxides ofiron). As can be seen serpentinesand szaibelyite), CO2 (in siderite), it is pos- in Table 7, none of the existing compositions take into sible to derive chemical compositions correspondingto consideration the presenceof A1, Ti and Sn, for "pure" ludwigite. It is of note, however, that in spite of example. Ludwigite is, however, a chemically complex the lack of deteminations for Al, Sn, Ti, Sb, Mn, many mineral, with a large number of possible substitutions; of the ludwigite samplesin Table 7 are Fe3+-oversatu- a structural formula may be represented by rated; Fe3+exceeds I atom per formula unit(apfu),re- (MI-3)2M4(BO:)Oz, with Ml-3 representing Mg and gardless of whether the formula is calculated with Fe2* but also Ni and Mn2+, and M4 representing Fe3+, respect to cations or a fixed number of oxygen atoms LUDWIGITE FROM OCNA DE FIER, ROMA\IA I 353

TABLE 8 RESULTSOF WET.CHEMICAL ANALYSES OF SELECTED presence of iron in excess imposes that all other ele- SAMPLESOF LT]DWIGITE FROM OCNA DE FIER+ ments are relatively depleted upon recalculation of the Smple 1167 I 168 l4l7 1416 1439 1798 lgEZ 2037 Sioz 035 043 049 046 060 062 0 038 formula. It is, however, possible to quantify a corec- 't2 Tioz o 67 0 t2 0.05 0.06 0 07 o_22 o o 2l tion, admitting that stoichiometric magnetite may be BzQ t7 21 t7 74 1743 1788 17_74 r1 60 174a 1720 AlzO: 022 050 062 031 061 045 O49 042 subtracted in a straightforward manner, to obtain more FqOr 3E2l 38 18 3900 37aO 3790 3693 3733 3E7'l accurateMeO:Me2O3:B2O3 proportions. A proposed FeO 323 473 317 418 ?'14 4-E8 521 501 given in MeO 3855 3776 3912 3422 35E9 3804 1722 3695 nofinative composition is then for all samples MoO 034 025 033 012 012 o22 0ll 012 Table 8. CaO 009 014 011 008 0t4 015 006 036 in Table 8 representin fact flro' 0 ll o2L 0 t5 o27 ozt 032 0 50 024 The derived compositions Hro 007 033 - 018 008 o21 mean compositions at the scale of a sample. As the TOTAL 9905 lm06 10047 9971 10042 9961 9954 99a7 chemical composition of ludwigite within individual PROPOSEpNORMATTVE COMPOSTTIONS (Wr.7o) ludwigite 9E l'7 982t 9934 97 Ol 9821 9679 95 33 95 33 samplesis rather uniform (seebelow), the compositions magoeute 233 in Table 8 are consideredto fit well with the composi- seryfltine 081 099 1.06 138 t43 217 0E8 tions ofthe individual grains. The correspondingchemi- saibelyite - 079 - 131 083 l2r 196 ll2 HrO 0O7 033 - olt 00E 021 cal-structural formulae were calculated on the basis of DEpUCED COMPOSITTONSOF LrJp\VrGrTE (WT.% O)flpES)*{ five atoms of oxygen. The formulae indicate that: (1) BzO: t7 53 l7 72 1756 t7 87 t7 66 t7 67 1749 1756 Tio, 0 68 o t2 0 0s 0 06 0 07 o 23 0.13 o_22 ludwigite from Ocna de Fier is closer to the magnesian ALOr 022 051 062 032 062 047 0 51 0.44 end-member than previously reported. It contains mo- F%O3 3E92 38 E5 3926 3896 38 59 38 l5 39 16 38 9E FeO 329 481 3 l9 431 121 504 541 450 lar percentages of vonsenite ranging from 4.41 to MCO 38 92 37 60 38 88 38 28 35 53 38 06 31.06 31 79 10.307o.(2) The very low contents of Mn (up to 0.010 013 MnO 035 Ozs 033 Ol2 Ol2 023 012 apfu) andCa(ttp to 0.013 apfu) comparewell with simi- lar data in literature (Aleksandrov 1961,1982, Leonard B 0 996 I 0ll o 99'l I 016 l0l I 003 I Vlisidis 19'14).(3) The composi- Ti 0 017 0 003 0 001 0 001 0 002 0,006 0 003 0 005 et al. 1962, & Schaller AI 0 009 o020 o o24 0 012 o 024 0 018 0 020 0 017 tional variation in the M4 structural position appearsto Fe3* 0 964 0 966 0 972 0 966 0 969 o9 o 979 0 970 be insignificant. The occupancy of thesesites by Fe'* is Fe2* 0 091 0 133 0 08t 0l19 0 203 0 139 o r52 o 124 Mg I 910 I 853 1 907 I 880 1 768 l 875 I 835 I t63 between 97.37 and98.6'lVo,with low contentsof Ti (cfl NIn 0 010 0 007 0 009 0 003 0 003 0 006 0 003 0 004 and Al (cf Mg2AlBO5), up to 1.72 and2.4l 0 0 003 0 005 0 005 0 013 azoproite) mol.Vo,respectively. The numerous possible substitutions within the vorenite 455 670 441 595 1030 690 165 624 investigations of ISOMORPHISMIN M4 SITES ludwigite group necessitatedchemical the concentrations in Sn, Sb, Cr, Ni, Co, and Zn. The MgaAlBo5 o 91 202 2.41 1.23 241 l 85 200 t?l concentration of Sb (cf chestermanite)and Sn was de- 172 030 010 010 020 0 62 0 30 0.51 termined by inductively coupled plasma - mass spec- * reslts expressed in wt Yo trometry, and that of Ni (cl ), Co, Zn and ** tdals r@alolat€d to l00P/o Cr, by inductively coupled plasma - atomic emission spectrometry. Between 3.7 and 12.0 ppm Sn, 68.6 and (5 O in Table 7). The presenceof admixed magnetite or 123.4ppm Sb, 8.7 and 13.3ppm Ni, 23.9 and65.1 ppm hematite-maghemite can explain this finding. For the Co, 190.1and880.1 ppmZn, and 1l.4and60.l ppmCr samplesthat are Fe3'-depleted,the molar :m;tioMe2+Ol were detectedin various samples,which appearsto not B2O3is very high, which also indicates the presenceof exert a significant influence on the mineral chemistry. oxide impurities. The previous analyseswere thus made It is of note, however, that the most distinctive feature on impure material, which supports the idea that the of the Ocna de Fier ludwigite is the higher content in Sb proportions of the vonsenite component reported so far and the lower content in Sn than in other examples of for the ludwigite from Ocna de Fier are inexact. New ludwigite in Romania (Marineea 1998). analyseswere thus deemednecessary for a better char- acterizationof the mineral. Ele ct ron-mic rop robe analys e s

Results of new wet-chemical analyses Electron-microprobeanalyses of selectedsamples of ludwigite from Ocna de Fier were made in order to com- The primary results of new wet-chemical analyses plete the chemical data. Representativeresults are sum- are given in Table 8. They show the persistenceof small marized in Table 9. The variation in composition of quantities of SiO2, H2O and, in some cases, a higher individual crystals in the same specimen being negli- concentration of Fe than necessaryfor the stoichiom- gible, compositions in Table 9 are given as averagere- etry, owing to the presence of impurities such as sults of random spot analyses. It is of note that the forsterite, serpentines,szaibelyite and magnetite. Sub- back-scatter electron detector does not indicate any traction of constituents to compensatefor these miner- chemical zoning of the analyzed grains. The amount of als in order to eliminate SiO2 and H2O is easy, but the Fe2O3has been calculated with the assumptionthat the t354 THE CANADIAN MINERALOGIST

TABLE 9. RESULTSOF ELECTRON-MICROPROBEANALYSES OF SELECTEDSAMPLES OF LUDWIGITEFROM OCNA DEFIER(')

sanple 1168 1415 1439 1799 1866 1867 r89l t975 zo73 22lE 2219 22zl

B'O: 16900 l7 813 17749 17425 17923 17672 r7.775 17771 l7 723 l7 .192 l7 496 l7.6ga Tio, 0034 0077 0014 0026 0017 0.040 0042 0.011 0040 0.092 0.03E 0049 ,dlzo: 0.663 0.343 0.308 0-424 0349 1.652 1.600 | 417 0.457 0.412 o 467 o 699 FezoJ') 3E817 37 sgl 37.656 39.685 37,803 36.64s 36468 36gE4 37.437 39.958 3g2zz i7.lsl Feo(3) 9925 66Lg 7.to2 4s64 4.938 6?t5 6.381 s767 7.319 5562 7.2s3 7.E03 MgO 33430 37.42r 37 03s 37.671 3E703 37.000 37.36E 37 735 36.809 36565 36.302 z6.4sl NInO 0.185 0163 0105 0.184 0242 0276 0.314 0.294 0l8t 0197 O.IO2 0.170 CaO 0046 0034 0.028 0.021 0.025 0000 0052 0021 0,034 0OZ2 0.120 0000 Total 10000 100.00 100.00 100,00 100.00 100.00 100.00 10000 1000O lOO.00 100.00 100.00 CATIONSON TIIE BASISOF 5 (O) B 0.991 1,020 1 019 0997 1.019 I 009 I 013 1.018 0.990 L009 I 017 Ti 0.001 0 000 0.000 0.001 0 000 0 001 0 001 0 000 0 001 0 002 0 001 0,001 AI 0.027 0.013 0.012 0 017 0 014 0.064 0 062 0 055 0 018 0 016 0 018 0.027 Fs'- 0 993 0.939 0.943 0.990 0.937 0912 0,906 0.918 0.938 I 003 0,961 0 930 Fe'* ozg2 0 184 o l9B 0 tz| 0 136 0.186 0.176 0 159 0204 0 155 0.203 0.217 Mg | 694 1.851 1^837 1.861 1.901 I 825 1.839 LS55 r8z7 1819 1 807 1.813 Mn 0.005 0.005 0.003 0 005 0.007 0 00E 0 009 0 00E 0 005 0.006 0 003 0 005 Ca 0 002 0 001 0 001 0.001 0 001 0 000 0-002 0 00t 0.001 0.00i 0 000 IN MI-M3 SITES ludwigxte 8573 9096 9027 93.6t 9332 90.75 91.27 9211 89 96 9215 E990 893r vosqrite 1427 9.04 9.73 6.39 6.68 9.ZS 873 i.gg 1004 785 10.l0 l0 59 ISOMORPHISM IN M4 SITES ludwigite 9726 9863 9874 98.21 98.53 93.35 93 50 94 35 98.01 98.24 98 06 97 08 MgzAlBO: 2.64 1.37 1.26 169 | 47 6.55 6.40 5 65 1.89 1.57 I E4 2.82 azorgite 010 000 000 0.10 0 00 0 l0 0.10 0 00 010 0.20 0.10 0.10

(l) Resultsexpressd ill wt % Samplesfrom Magnet(1415, 1439, 1799,2218,2219,2221), lthaIrl$teli (1367, 1S91, 1975,2073) and Iuliana (l 168, 1866)quanies. (2) number ofpoint analyses (3) as calculated

molar ratio BzO:/(FeO + MgO + MnO + CaO) is equal As observedby Aleksandrov(1968, 1982)for natu- to l/4, after assigning all Mg, Ca and Mn to fill the sites ral samples of ludwigite and by Xie et al. (1985) for Ml-M3. Using this method, most of the analyzed syntheticmaterial, the position of the lines of a ludwigite samplesof ludwigite are found to be Fe3+-depleted, X-ray pattem is strongly dependent on the ratio Fe/+/ which is consistentwith thepresence of Al. Ti4+,Sb5+. (Fe2*+Mg) of the analyzed samples. It is evident that Sn4*,Cr3* at the M4 sites. the advanceof the substitution of Fe2* for Mg causesa In all cases,the ludwigite is very magnesian,rang- significant shift toward higher values of d. As can be ing in composition from 85.7 to93.6Eoof the end mem- seenin Table 10, a direct correlation betweenthe d val- ber. The ludwigite at Ocna de Fier thus shows only a ues and Xp" = Fe2*/(Fe'**tg) may be observed,but rs limited proportion of the vonsenite component. As very poor for the analyzed samples of ludwigite. shown by the wet-chemical data,it contains low levels Another factor, namely Al-for-Fe'* substitution, exerts of the azoproite and Mg2AlBO5 components:up to 0.20 an opposite influence on the position of the reflections. mol.Va and up to 6.55 mol.Vo,respectively. Bonazzi & Menchetti (1989), discussedthe importance of the factor XAr = AU(A1+Fe3+)on the cell parameters. X-Rey Powopn-Dn pnAcrroN DATA Note that the unit-cell parametersin Table 10 are in close agreementwith those already in the literature for X-ray powder-diffraction pattefils were obtained for magnesianludwigite with low contents of A1. The val- several samples of ludwigite from Ocna de Fier. The ues previously reported by Bonazzi &Menchetti (1989) main observed (hkl) reflections, as well as the cell for ludwigite from Ocna de Fier le.g., a 9.252(2), b parameters refined using the least-squares computer 12.315(2),c 3.038(1)A for samplesVA and MOI are program of Appleman & Evans (1973), as revised for also in very good agreement with the values in Table microcomputer use by Benoit (1987), are listed in Table 10.Authors ofprevious studies(i.e., Aleksandrov 1968, 10. Marincea (1998) provided the complete set of 7982, Xie et al. 1985, Bonazzi & Menchetti 1989) X-ray data. showed that chemical parameters,and particularly Xp", LUDWIGITE FROM OCNA DE FIER. ROMAN-IA 1355

TABLE 10 DIFFRACTOMETRIC 'AR.9U.S CHEMICAL PARAMETERS OF OCNA DE FIER LUDWIGITE(') tl67* 25347 24951 1 9524 |,7691 5783 | 2498 0 9980 9 12243(9) 3 0s7(3) 345 s37 0.046 0 009 o 11684 2.5470 2 4959 | 9582 | 9143 1 7730 5793 12s15 09980 9263(6) 12317(5) 3042(3) 347078 0 067 0 020 9 550 r4t5' 2s493 24961 1 9s84 I 9178 | 7655 5816 r.2522 09976 9290(6') 12234(8) 30s0(2) 346670 0_090 0 014 9 254 t4174 2 5354 | 9564 I 9131| 7749 5791 12517 09982 9260(4) 12334(6) 3033(4) 346.448 0 044 0 024 9:A24 1418* 2 5368 | 9570 | 9134 | 773s 5792 1.25rs - 9250(7) 12.321(5)3034(s) 345780 0 060 0 012 9,532 1439+ 25376 I 9583 | 9142 | 7732 s7e6 - 0e978 e26O(4\ 12326(5) 3037(2) 346.664 0 103 0.024 9 234 1798* 25470 25342 19558 | 9117 | 7714 5780 rZ5r4 09973 9256(7) 12233(9) 3057(3) 346.18E0 069 0 019 9 519 n99' 25524 25099 I 9168 s773 12sr3 09976 e236(7) 12301(8) 3.053(3) 346e27 o.0u 0.008 9 448 1866' 2.5436 24946 1.9146 s807 r2sr7 09994 9.276(9) 12242(8) 3.049Q) 34628r o 061 0 015 9 495 1867' 25486 24950 19592 t.9166 17657 s801 - 09e7o 9.2s8(e) 122ss(8) 3053(3) 346345 0 093 0 066 9 796 ft72' 2.559724951 19535 19095 s767 12s0s 0ee78 e22s(8) t2.267(e) 3057(3) 34s99r zzrc' 25540 24960 19600 1.9167r77rr 1.s754 1.25100.9975 9.241(6) L2.259(8)3.056Q) 346.43s 0.079 0.016 9.386 (l) Chemicalcomputations wtre ca"rriedout on thd basisofwet-chemical (*) or eleotron-microprobe(') analyses,The (observed)d spacingsare givenin A exert an important influence on the cell dimensions.The thermal analysis of "ludwigite" was reported by parametersa and V depend especially strongly on the Hang (1957) and is mentioned by Smothers & Chiang extent of Mg-for-Fe2+ substitution (Aleksandrov 1968, (1966), but it was obviously made on impure (mainly 1982), and increasewith Xp", but they are also sensitive szaibelyite-bearing) material, as proven by the chemi- to the extent of Al-for- Fe3r substitution (Marincea cal data provided. The admixed szaibelyite is clearly 1998).A discussionof the behaviorof the cell volume responsibleat least for the strong endothermic effect at as a function of chemical variability may be useful in 725'C observedby Hang (1957). understandingthe dispersion of values in Table 10. Taking into account the data reported above, it is Taking into account the differences in ionic radii surprising that Kissling (1967), on the basis of a DTA among vIMg, vIFe2+,v\re3+ and vIAl, it is evident that curve for a sample of ludwigite from Ocna de Fier, the substitution of Fe2* for Mg will increase the cell statedthat the mineral breaks down to kotoite, magnet- volume, whereas the substitution of Al for Fel+ will ite and hematite at a temperaturebelow 1000'C. Com- decreaseit. One may use the comprehensivefactor k as plete setsofthermal curves were consequentlyrecorded an expression of the difference in the steric hindrance for many samples of ludwigite from the type locality, between the cations entering the octahedral sites in the using the sameoperating conditions as Kissling (196'7). structure, as proposed by Bonazzi & Menchetti (1989). DTA and DTG curves are reproduced in Figure 6. The value of ft may be calculated as: The sole effect attributable to the ludwigite itself is the flat exothermic peak centered at about 650'C. 76= {2[r(vlFe2+)- r(vlMg)].(l - XeJ + [r(vlFe3*) - Becauseendothermic effects are not to be expected,the r(vlAl;1.)1o,1.100 (vo). main effects of this type were assignedto different ad- mixed minerals by analogy with data on pure materials Using the ionic radii given by Shannon & Prewitt (1969) for the four cations [r(vlFe2+)=0.770 A, r(vltvtg) =0.720 =0.645 A, 4vIFe3*1 A and rlvlAl; = 0.535 Al, t47.5 the k value may be calculated as: k (Vo)= 10 + 11 Xar - 10 Xp". The values obtained for samplesfrom Ocna de Fier 347.O o are given in Table 10. As shown in Figure 5, and in perfect agreementwith the data of Bonazzi & Menchetti 346.5 (1989), a poor inverse correlation between ft and V is oo o' established,which best explains the variability of the V v(A1 values. A similar correlation may be established be- 345.0 tween k and c, whereas a less regular behavior is ob- a served for a and b parameters. 345.5

Tnenrr,rer-Burevron 345.0 Data concerning the thermal behavior of ludwigite 9.2 9.7 are scarcebecause the mineral is expected to have a high k melting point (up to 1000'C), and no structural changes Flc 5. Correlation between cell volume and tchemical fac- ean be expected below this temperature. Note that a tor, as establishedfor ludwigite from Ocna de Fier. I 356 THE CANADIAN MINERALOGIST

DTG CTIRVES SAMPLE

lt67 670

u6E 570

t4t7 410' 660

l41t 380"

590" 560

DTA CURYES

655' lt67 67d

115E at0

6CO" 74t7

595

$o' l41t 235c 655. soo"

6 22f t439 d ,; 7*'

400 500 t00 ro00 TEMPERATURE CC)

FIG. 6, Thermal curves recorded for selectedsamples of ludwigite from Ocna de Fier: dlfferential thermogravimetric (top) and differential thermal analysis (bottom).

reported for hydrated ferric oxides of gel type (Gheith tion of the broad exothermic effect mentioned before. 1952, Ammou-Chokroum & Pianelli 1970), goethite The most probable structural cause of this exothermic (Kauffman & Dilling 1950, Ammou-Chokroum & effect is the oxidation of the ferrous to ferric iron, but it Pianelli 1970), antigorite (Caillbre 1936), szaibelyite is not possibleto specify if this iron pertainsto ludwigite (Watanabe1953, Shabynin 1955) and calcite (Kauffman or to admixed magnetite. Kissling (1967) mentioned a & Dilling 1950), taking into account the slight shifts in similar effect, but it was not assignedto any structural temperature due to the admixture (Todor 1972). The change. In magnetite, the analogous effect occurs assignment of the main thermal effects, together with between 625 and675"C,and corresponds,according to the losses in weight recorded on the TGA curves, are Gheith (1952) and Schmidt & Vermaas (1955), to the given in Table I L The presenceof the admixed phases change of 1-Fe2O3(maghemite) to ct-Fe2O3@ematite). given in the table, supported by optical and diffrac- A first exothermic effect, marking the superficial oxi- tometric data,raises the problem of the correct assump- dation of magnetite, which produces maghemite ovei LUDWIGITE FROM OCNA DE FIER. ROMANIA 1357

TABLE 11 TIIERMAL DATA FOR ADMI)(ED MINERALS IN IIAND- nized in the powder patterns recorded at 200, 400, 600 PICKED LT]DWIGITE and 800'C. A broadening of peaks occurs at 1000'C, Smple Type of [email protected] of Loss in Rev@led mineml fusion and the amorphization of the effect rhe effect ('C) weight on proving an incipient onDTA onDTG theTGA mineral. Marincea ( 1998) listed the complete set of dif- fraction lines. The assertionofKissling (1967) concern- @ddh€mic 670 660 175* ing the breakdown into kotoite, magnetite and hematite 1 I 68 exothemic 655 @gnetite should be consideredwith caution. mdothmic 6E0 670 1.55* szaib€lyite 1417 ddothemic 235 220 1 20 hydmted fenic oxide mdothmic 420 4 10 2-35 coethite hvrneneo Auar-vsrs qothmic 660 odotimic 695 680 0 30r 14lE @dothmic 220 2lO 0 50 Infrared absorption specffa of selected samples of sdothmic 395 380 155 goethite exothermic 630 magrctite ludwigite from Ocna de Fier were recordedbetween 250 endothmic 665 655 0 35* @ibelyite and 4000 cm-1, before and after heating at 1000'C. In endotl|mic Eoo 79o | 20 calcite high-wavenumber 1439 endothermic 23o 220 125 hydntedfqricoxide the spectra given in Figure 7, the @dothmic 620 590 2 25* mtigorite domain between 1700 and 4000 cm-l is not reproduced qothmic 660 magnetite characterof ludwigite. A com- ododrqmic 670 660 | 20+ saibelyite becauseof the axhydrous €ndothmic 790 780 0 30 cslcite plete list of absorptionbands recorded for many samples Kissling endothemic 230 hydEted feftic oxide is given in Table 12. (1967) exothmic 67E - - rognetite As expected, the most significant absorption_bands ' reduced by the imere in weigh dw to the magnctite oxidation in the mid-wavenumber region (600 to 1350 cm-') may be assignedto vibrations belonging to the (BO3) group. These absorption bands (cm-r) are assignedas follows: the interval 280-375'C, according to Gheith (1952) and 1300-1 335 and 126O-1268: asymmetric stretching; Schmidt & Vermaas (1955), may be inhibited in 920-940: symmetric stretching; 7 04-7 08: out-of-plane sampleswith large particle sizes, such as those used in bending; 622-630: in-plane bending. The latter should this study (Egger 1963). On the other hand, taking into in principle be doubly degenerate,but the band at 560- accountthe thermal behavior of vonsenite,which breaks 570 cm-1 could not be unequivocally a_ssignedbecause down to hematite and amorphousFe2B2O5 at'764"C the theoretical superposition of the Fer+-O stretching. (Galan et al. 1981), one can expect that below this tem- The band assignmentsgiven above are coincident with perature, ludwigite remains unchanged. It thus seems those reported for ludwigite by Aleksandrov et al. that the exothermic effect centered at about 650oC czm (1965), Nekrasov & Brovkin (1966) and Xre et al' be assignedto the oxidation of magnetite. (1985). An analysis of the factor-group splitting show A supplementaryinvestigation of the thermal behav- that the splitting of both asymmetric stretching and in- ior of ludwigite was undertaken by recording the XRD plane bending is consistentwith a C3, or C" point sym- pattern as heating in air progressedup to 1000'C. The metry of BO3 (Duval & Lecompte 1952, Steele & main reflections of this mineral may be clearly recog- Decius 1956), which implies that this molecular group

TABLE 12 POSITIONSAND ASSIGNMENTSOF THE INFRAXEDBANDS RECORDEDFOR SELECTED SAMPLESOF LUDMGITE FROM OCNA DE FIER*

1460 1,f40 1390 1412 1420 1436 1420 t420 1420 t430 6 ll"o 1300 1320 1305 1305 1300 l3l0 t300 1300 1300 t340 1335 v: BOr t265 1254 1264 12f8 l26t 1266 L26E t26o t26E t268 t 2 vr'BOr 940 938 935 935 935 935 920 935 920 920 vr BO: 't95 * 194 795 79s t00 798 795 7 794 798 B-o- ue '.104 '706 1t1 701 106 7M 704 7M 708 704 7M 704 vz BOr 625 625 620 625 625 630 628 630 622 630 622 vr B0r - 610 62n 620 620 6t5 n-o-tnue 1?; 520 to) 564 5?0 565 5'to 560 560 560 566 570 va'BOr (?) 538 530 520 535 525 - 535 54A 510 510

+ wavenumbqs in cm't (1) as giv€n for a s&mple ofludwigite frofl Ocm de Fiq, ofunkrom @mposition (2) not reorded below 400 orn'. 1358

N SAMPLE 2

q q

q z

F

- t4t7 A

r?001500 r5oo t60o t3001200 lloo tooo 9oo coo r00 600 500 4@ loo

- WAVENUMBER(cm.I)

FIc. 7 Infrared spectraof selectedsamples of ludwigite from Ocna de Fier before (A) and after (B) heating at 1000oC,in air.

is non-planar. As this kind ofbehavior is characteristic occur, however, in the low-wavenumber region (250 to for (magnesian)ludwigite, in opposition with vonsenite, 600 cm-l), and consist in the broadening of the back- in which B03 has a clear D35 (planar) symmetry ground of the continuum of bands in this region. The (Aleksandrovet al.1965, Nekrasov & Brovkin 1966, partial-oxidation of the admixed magnetite must displace Xre et al. 1985), the infrared analysis confirms the low the Fer+-O vibrations toward lower wavenumbers,as at contents in ferrous iron reported earlier. Note that the the transition magnetite - hematite. As observed by effect of compositiondoeJ not appearro be evidenr in Liese (1967), the main absolption band (probably the details of band positions and intensities in the recorded Fe3'-O stretching) is centered near 570 cm I in pure spectra. magnetite, and around 530 cm-1 in pure hematite, The spectra of heated ludwigite (Fig. 7) are similar whereasin mixtures of magnetite and hematite, the cor- in all respects with those of unheated samples, which responding band may be found between 530 and 570 indicates the samebasic structure.Minor differences do cm-r. After heating, the absorption bands correspond- LUDWIGITE FROM OCNA DE FIER, ROMANIA 1359 ing to stretching and bending modes of Fe3*-O bonds Aleksandrov & Pertsev (1968) and Nitsan (1974), the in magnetite should be reduced in intensity, vanish or oxygen fugacity during forsterite and ludwigite crystal- decreasein wavenumber,which should give rise to com- lization is inferred to have been relatively high (p2 - posite, broad bands toward the low-wavenumber part l0-18 to l0-14 atm). The most likely sourcefor constitu- of the spectra. Broadening of the component bands ents other than magnesium is the Ocna de Fier intrusive should normally lead to a decreasein intensity of their body. Initial post-skarnalteration took place at tempera- background, which can be observed in the B series of tures above or near 300oC, with a basic pH required by spectrain Figure 7. the crystallization of szaibelyite and serpentines. The recorded spectra also show that oxidation and hydration occur after heating. In the spectrain Figure 7, AcKNowLEDGEMEi.rrs the definite presence of a band near 1630 cm-t, also reported in spectra of ludwigite by Moenke (1962), Financial support for this study has been provided Plyusnina & Kharitonov (1963), Nekrasov & Brovkin by the French Government, in the form of a doctoral (1966) andFranz et al. (1981),is completelyconsistent fellowship granted to the author. The staff of Ecole with the hydration of the samples. This band, which Nationale Supdrieure des Mines of Saint-Etienne was becomes more intense after heating, may be attributed of invaluable help. The author is deeply indebted to to H-O-H bending, which implies that adsorbed HzO Messrs. Hubert R6my, Michel Fialin and to Mrs. was present in both the initial materials (in which the Claudette Richard (Universit6 Paris VI) for their gener- corresponding stretching mode occurs as a broad band ous assistancein carrying out the electron-microprobe 1) centered at about 3360 cm and in heated ones. In analyses. Special thanks are extended to Dr. Jean heated samples, two intense narrow bands, which ap- Ladridre (INAN Laboratory, Universitd Catholique de 1, pear at about 3240 and 3560 cm indicate that both Louvain) for performing and processingthe Mcissbauer strongly bonded and weakly bonded hydroxyl groups spectra. Thanks are also expressed to Mrs. Gabriela coordinate with iron in hydrated compounds that result Stelea (Geological Institute, Bucharest) for infrared from the breakdown of masnetite. analyses, Mr. Traian Draghiciu (Geological Institute, Bucharest) for thermal records, Mr. Jacques Moutte DrscussroN (Ecole Nationale Sup6rieuredes Mines, Saint-Etienne) for ICP-AES determinations, Mrs. Jacqueline Given the above information, one can make several Vander Auwera-Coppens (Universit6 de Lidge) for statementsconcerning ludwigite in general. (1) Ther- ICP-MS measurements, and Mrs. Doina Necsulescu mal, X-ray powder and infrared data on the mineral (PROSPECTIUNI S.A. Bucharest) for wet-chemical show that the hand-picked material still contain mineral analyses.The quality of this contribution was improved impurities; it seemsworthwhile, therefore, to inquire a after reviews by Drs. Nikolai N. Pertsev and Ronald little more carefully into the question of the vonsenite Peterson. Drs. Robert F. Martin and George W' content in solid solution. (2) The strong effect of shape Robinson are acknowledgedfor editorial assistanceand in the magnetic anisotropy is induced by the presence useful suggestions. of impurities (i.e., magnetite); ludwigite itself behaves superyaramagnetically.(3) The unit-cell dimensions of REFERENcgS ludwigite are not strongly related to Fe2* content, because of the opposite influence of aluminum; even Alsxslmnov, S.M (1961):Some geochemical peculiarities small contents of Al may balance the influence of of the processof szaibelyitizationof magnesium-iron increasing contents in Fe2+.(4) Ludwigite is thermally borates.Geokhimiya 6,493-499 (in Russ.). inert up to 1000"C. (1968): Mineralogy and diagnosis of magnesian- The mineral-composition data presentedhere, com- ferruginous borates in the ludwigite-vonsenite series synthesis of borates, allow a bined with experimental Mineral Syrye 18, 102-115(inRuss.). reconstruction of the T-/(O2) history. As stated by Kravchuk et al. (1966), magnesianludwigite needs the (1982): Geochemistry of Boron and Tin in highest temperaturesof synthesis:about 700'C for the Magnesian Skam Deposits. Nauka, Moscow, Russia (rn magnesian end-member and 500-650'C for ludwigite Russ). withXp" - 0.25. The chemical composition of ludwigite from Ocna de Fier provides information on mineralogi- ArHunNova, MV & K,qnvarrN, A.V (1965): in ludwigite-vonsenite cal constraints, implying a crystallization temperature Investigation of borate minerals series by means of IR spectra.Geokhimiya 9, 1ll4-lll9 of about 600-650'C. It compares well with the mini- (in Russ.). mum temperaturesof 600oC estimated from carbonate (Marincea geothermometry 1998),andthus indicatesthe - & PenrsEV,N.N. (1968): Correlation between com- minimum prograde temperaturenear the contact. In the positions of magnesium-iron borates and minerals associ- same way, on the basis of chemical compositions and ated with them in magnesianskarns. Geokhimiya \1, 1328- taking into account the stability fields established by 1337 (in Russ). 1360 THE CANADIAN MINERALOCIST

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Vlrsrors, A.C. & Scsl'r-r-ER,W.T. (1974): The identity of Received November 28, 1998, revised manuscript accepted paigeite with vonsenite and chemical analysesofvonsenite, Sentember25. 1999.