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Ludwigite from the Type Locality, Ocna De Fier, Banat, Romania, Was Re-Investigated in Order to Establish Undocumented Chemical and Physical Properties

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Ludwigite from the Type Locality, Ocna De Fier, Banat, Romania, Was Re-Investigated in Order to Establish Undocumented Chemical and Physical Properties 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 mineral occurs in boron-bearing magnesianskarns, in a restricted associationthat includes calcite, forsterite (fayahteO.92-2.607o, tephroite 0.75-l.26%o), magnetite (magnesioferrite 18.4846-37Vo) and clinohumite (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
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