TI{E AMERICAN MINERALOGIST, VOL 4J, NOVEMBER.DECEMBER, 1958
PHOSPHAI'EIIINERALS OF THE BORBORET{APEG}IATITES: II BOQUEIRAO Josnlu \Iunnocn, Uniaersityof CatiJorniaal. Los Angeles.
ABSTRACT
The "alto" Boqueirlo is a tl,pical zoned pegmatite of the heterogeneoustype, which carries some unusual minerals besides those of the normal ber1,1-tantaliteassemblage. Among them are minor arnounts of uraninitc, altered in part to uranophane and other secondary compoundsl native bismuth. Lithiophilite and manl' secondary phosphates appear as usual in the general inner portion of the pegmatite near the cluartz core. Among these phosphates appear a pink variscite (carrying minor ber)'jlium); metastrengite as sometimes showy blue coatings of minute crystals; huriaulite and minor tavorite as coat- ings on fractures of lithiophilitel stewartite, massive or in rare crystals; mitridatite (a hydrous calcium iron phosphate), in deep red blades, or very fine grained massesyerlowish to greenish in color. A new mineral, chavesite is described. rt is a hydrous calcium man- ganese phosphate, occurring as thin platy crystals coating lithiophilite, and associated rvithhur6auliteandtavorite. Triclinic, ooS 49bol3.O7,coS79;a97"lgl;,,p 109.3,,t99"44,. The powder pattern resemblesthat of monetite, ivith which it may be isostructural. The stronger powder lines have the follou'ing spacings and intensities: :.35 A-lo, 3.0% A-2, 2.945i\-3,2.74 A-2.
IxrnoouctroN This is the secondin a projectedseries of reportson the phosphatesand associatedminerals from the complex, lithium-bearing beryl-tantalite pegmatitesof NortheasternBrazil. Their generalposition is shown on the key map (Fig. 1). The first report wason the ,,alto',patrimonio, \,Iurdoch (1955) and the present one describesthe occurrenceat the ,,alto,' Boqueirao. The general geologicsetting, describedin some detail by Johnston (1945)and others,is the same for both deposits:complex peg- matites intruded into a pre-Cambriancomplex and exposedby erosion of the rock cover to form the Borborema,'planal16,'-a more or lesswell developedpeneplane of possibly Cretaceousage, standing now at an elevation of 500 to 550 metersabove sealevel. rn the Statesof paraiba and Rio Grandedo Norte, later erosionhas loweredthe earliersurface so that the pegmatitesstand up in relief on it, hencethe name ,,alto,,given to their outcrops.Even in Cear6,where there has been no suchdifierential erosion,the name is still applied. The pre-Cambrian rocks are largeiy gneissesand mica schists(the Serid6schist), which showsno pronounced regional trend, striking sometimes northwesterly, sometimes north- easterly.
Boquernao The "alto" Boqueirlo lies at the northern summit of a ridge reaching an eievation of about 300 meters above the surrounding territory, some 1148 PI]OSPHAT'EMINEIIAI,S, BORBORE]TAPEGMA'TITE 1149 three kilometers southeastof the town of Pareihas,in the State of Rio Grande do Norte (seeindex map, Fig.2) . The following descriptionis taken largelyfrom Johnston (1945).Exposed cont/acts are nearly every- wherepractically vertical. The outcrop is some 240by 25 meters,pinch- ing out to the west, and cut off sharply by the steepslope of the ridge to the east. Johnston'szone If (coarsetextured quartz-feidspar)with con- siderablebiack tourmaline. composedabout half the outcrop. His zone
4ll4rv rtc
)
(--.--
P E R N AIMEUCO
Frc. 1. General map of area.
III (very large crystals),has huge microclines,partially kaolinized The qtarfz core is multiple, and the various patches are made up largely of milky quartz, with occasional vugs carrying euhedral quartz crystais up to 200 kilograms in weight. The occasionalphosphates occur, as is normal, in the third zone, and often close to the core. The following minerals are reported: arrojadite, vivianite, spodumene, lepidolite, rhodonite (?), pitchblendeand its alteration products, metallic bismuth with bismutite, and some chalcocite.These are in addition to the usual quartz-feldsparmixture, with beryl, tantalite, muscovite,etc. Of theseno arrojadite, rhodonite or chalcocite were observed by the writer. The "arrojadite" is presumablyrepresented by lithiophilite and its alteration products (sicklerite etc.); the "vivianite" is apparently metastrengite, and the rhodonite (?) is probably a pink variscitewhich was observedin some of the alteration Droducts.Several other minerals were observed 1150 J. MURDOCH by the writer, and are includedin the descriptivelist. The seriesof altera- tion products of lithiophilite form an interesting group, essentiallythe sameas that describedfrom Patrimonio. SpooulroNo is not abundant, and is commonly altered to a fine grainedmica (probably muscovite),associated with fine grainedbotryoi- dal growths of cookeite. LBproorrtn is sparingly present as rather fine grained aggregates,
NORTE 9lcurrocs Novos xlog6o do Andrc' et *1* .,,"; ii'o--..... €*'o ;li;' "***oxiqur xiqul opicui ..a
Corngfbinhas doDinh.iF Jorclhos
Frc. 2. Detailed map of area. occasionallyforming masseswith curved surfaces,and associatedwith platy albite. Small prismatic grains of apatite have been seen in this association. TeNtetrm, apart from the commercially large masses,is occa- sionally found in the quafiz-feldspar aggregateas small crystals of man- gano-tantalite.A few tabular crystals of columbite appear in a similar matrix. Brsuurn, as the native metal, has been found sparingly as grains or occasionaliyfair sizedrough crystalsinterstitial to quartz, and associated with coatingsand patchesof secondarybismutite. UnaNtNrrB. This has been reported as pitchblende, but one rough octahedralcrystal wascollected by the writer. This crystal is encrustedby a seriesof secondaryminerals in thin concentric layers, and showing roughly three distinct zones.The outermost of theseis uranophane,fhq PIIOSPHATEMINERAL,S, BORBOREMA PEGMATITE 1151 next an undetermineduranium mineral, or perhapsa mixture, in a num- ber of thin, slightly varied layers, and the innermost perhaps meta- torbernite, although the powder pattern doesnot match too closely.In addition, a dark red mineral appearsin thin veins in the uraninite, and is possiblysoddyite. An analysisof the uraninite is given in Table I. Other oxidized uranium minerals, mainly meta-torberniteand meta-autunite, were found in minor amount, usually as powdery or poorly crystallized coatineson columbite.
Teeru 1. Cnrurcar- ANalvsns
UrOs 91.40 FeuOs 1.00 FeO 1.97 Pbo 5.92 FeO 13.23 FezO:; ,10.38 CuO .04 Mso 1.11 AlrOr 1.20 Bi2o3 .015 CaO .t1 PiOr 39.18 FeO .026 MnO 29.80 MnzOs Al2o3 .30 PiOs 45.00 HtO- 17.42 Tio, .00.5 Li20 8.68 HzO-| .198 CaO .02 98.96 100.35 NazO .03 SiOz .Jl MnO tr. H:Oi l /O H:O- .54 100.37
1. Uraninile, anal;rsts,Johnson, Mathey & Co. Ltd (Zn, Ni, Co, P2O5,As, Sb, Sn not detected). 2. Lithiophilite, analyst L. C. Pech.3. Metastrengite, analyst E. L. Martin.
VenrscrrB. One group of specimensshows a great variety of min- erals apparently derived from the complete alteration of lithiophilite crystals (as shown by the outlines of the aggregates).These occur in a matrix of mixed albite, amblygonite and quartz. One of theseminerals is variscite,in the form of a transparentcrystalline crust, coatingfracture surfacesand lining cavities. It was identified by optical properties, tr-ray powder pattern, and crystal form. It varies from colorlessthrough pale pink to raspberry red. Spectroscopictests show the presenceof a small amount of beryllium. This is in agreementwith the findingson the pink variscite of the Sapucaiapegmatite (l{rose, priv. commun.) N'Irrnroarrrn. This rare mineral occurs in these same specimens in clear, red bladed grains: also probably fine-grained,massive, yellowish brown; and alsoperhaps in compact,almosl earthy form, khaki colored. (This is somewhatdoubtful, but the powder pattern is at least in moder- ate agreementwith that of mitridatite.) tt52 J. MURDOCI!
MoresrnoNGrrE occurs as crusts, finely crystalline, and ranging in color from nearly white through pink to pale blue in this seriesof speci- mens.It alsooccurs in considerableamount, deeppurplish blue,and often quite spectacular,as a late product of alteration of lithiophilite in other specimens.See Table I for analysis of this mineral. BBn,luNrrB. An orange colored mineral occurring as blade-like grains in the variscite specimens,may perhaps be assigned to this specieson the basisof a closelymatching powder pattern. Rocr
Tlsln 2. X-nev Pownpn ParrnnNs or Srnw4nnm Fe Reorerror, Mn Frlrr:R. I:1.9373 A
Boqueirlo Hagendori Palermo
10.04 10.10 9 926 10 10.08 10 8.39 8.35 744 7.47 () . /.t 7 6.75 I 6.751 6 o. ,/.) .5 5.88 3 5.93 2 5 869 3 5.91 L .) (rJ.') 5 503 2 5.02 3 4.877 2 4.85 4.817 4b +.84 4.75 3 4.75 4.65 I 4.6+ 4.63 2 1 4.345 2 3.925 5 3.9+ 3.935 3.96 4 1 1 3.83 2 384 2 1 3.77 2 1 3.59 1 3.59 3.602 3.618 2 I I 3.48 2 3 453 2 3.384 2 340 3.351 3 392 2 3.318 3 3.315 3.313 3 I 1 3.246 2 3 238 3 247 2 3.156 J. I l.) 3 3.117 3.118 2 3 128 3 3.03 4 3 .045 3.O2l 3 3 04.5 .5 296 2 2.963 2 945 2 2.966 3 1 1 2 913 2 2 916 2 2.87 3 2 866 2.860 2 2 870 3 1 1 2.812 2 2 813 2 2.80 I z.ll5 2 1 2.743 1 2.737 2 2.734 2 2.7+0 2.60 + 2 595 2 2.586 3 2.593 1 2..).fo 2 2.489 + 2.491 2 2.484 2 2 492 1 I 2.4+ 2 2.443 2 2 420 2 2 398 I 2.365 1 2 374 2 1.363 2 2 380 1 2 342 2 2.337 2.26 1 2 254 1 2.250 2 2.262 I 2.222 2 2.227 2.182 2.r94 2.148 2.1+8 2.137 2 2 t55 2.103 2.113 2.075 2 080 2 077 2 2.086 2 039 2 039 2.052 2.003 2 005 2.008 984 1.989 1.986 3 1.982 956 1.934 1.923 1 1.933 .904 | 892 1.881 1 1.886 .841 | 844 1.838 1 1.84,1 .826 1 828 1.824 .812 1.804 . /6.) |.776 1.774 I l.)4 J. ]'IURDOCH
able that this sequenceprecedes the formation of hur6aulite, since the latter is not uncommonly found intimately associatedwith the altera- tion products of sickleriteand purpurite as well as in cracksin the rela- tively fresh lithiophilite. Cuavrsmo. One specimen of lithiophilite shows, on fracture sur- faceswith hur6auliteand tavorite, an intermittent thin colorlesscrystal- line coating which sometimesshows poorly formed individual crystals. This appearsto be a new mineral, tentatively named "chavesite" (pro- nounced shav-ez-ite), after Dr. Onofre Chaves, an engineer of the Brazilian DepartamentoNacional da ProducdoI{ineral. It is a hydrated calcium manganesephosphate, but not enough material is availablefor a chemicalanalysis. Hardness is near 3, cleavages,two good,in the prism zone,and nearly perpendicularto each other. Optically, biaxial positive, with 2V large, and indices aI.60, 8I.62, 7I.65. lvlultiple twinning, with twin plane parallel to elongationof crystals.Extinction about 30oto the twin plane. A cleavagesliver which proved to be a singleindividual was used to determine the symmetry and cell-dimensions.Rotation, and Weissenbergequator, first and secondlayer lines about c sholv it to be triclinic with the followingvalues:
ao:S 49 bo:13.07 co:S.79 a9l' 78!' B 108'3' t 99' 44' tr 84' 58' p 71"20' v 78"0O' ro':0.325 I'o':0.093 a:b:c:1.4200:1:04438
The powder pattern (seeTable 3) closelyresembles that of monetite, and it has been suggested(n{rose, priv. commun.) that chavesiteand monetite may be isostructural. The r-ray powder pattern can be ade- quately indexedusing the above valuesfor the elements. XftNnner. A. One of the variscite specimensshows an unidentified mineral,chestnut brown in color, occurringin radiating blades,and with one good cleavage.Qualitative testsshow it to be an anhydrous (?) iron phosphate,with index well over 1.74; extinction parallel to elongation; biaxial negative, with 2V large; pleochroicpale to amber brown. The powder pattern gives the following spacings and intensities for the strongerlines: 3.145-10, 2.405-6, 3.57-5, 3.36-5, 4.7 1-4. MrwBnar. B. Associatedwith tavorite are small, spherulitic aggre- gates,greenish white in color, of a calcium manganesephosphate with very high index of refraction It is too fine-grainedto make further optical determinations.The powder pattern does not appear to match that of any publishedmineral. Spacingsand intensitiesof the strongerlines are as follows:4-67-10, 3.42-5,3.01-5, 2.52-2. PIIOSPIT ATE MIN ERALS, BORBOREMA PEGMATITE 1155
Tesln 3. X n,tv PowoBn PnrronN ol Crr.lvesrre
Fe Rrdiation,Mn Filter. ),:1.9373 A
Chavesite Monetitel
I --t-I 6.33 I o.20 5.03 100,011 4.56 Ill,l2o 4.35 030 3.89 t2l 3.69 130 J.IL I 3.54 120 3.48 121 3.17 1 3.35 031,131 3.38 10 3.2+ 111 3.18 040 3.13 l2l J.IJ 3 3 .076? 3 .023 1tl 2.99 9 2.945 130 2.87 131 2.81 1.1t a 1t 2 002,o41 2.76 8 2.72 ) 121 I 2.66 2 2ot,0t2 I 2.63 2 2IO,012 I 2.56 2 220,050 2.489 i 140,l4l 2.50 5 2 .305 1 051,22r 2.32 1 2.23 3 240,141 2.26 () 2.r98 i 220 2.21 2 2.155 151,160 2.16 2 2.r24 060,150 2.087 I 201,221 2.r0 2 2.034 1 250 2.0+ 1 r.978 1 230,061 2.00 2 1.918 2 241 t.93 1.8.5 2 151,003 1.86 6 1?O 2 307,240 1.80 2 t. /.) 071 | 725 3 310,311 1.73 7 1 685 1 330 t.69 3 1 657 340,161 1.65 2
I Comparative list of spacings for monetite from Prien & Frondel (1947), p 949' 1156 J. MURDOCII
AcrNowr-BIGMENTS A grant from the Research Funds of the University of California was of material assistancein allowing the collecting of minerals. Miss Mary Nlrose of the U. S. Geological Survey has been very helpful in the iden- tification of minerals, and in other advice. Manuscriptrecei.teil April, n, 1958. RnronDncrs
Jonnstcrx, W. D., Jn. (1945). Beryl-Tantaiite pegmatites of Northeastern Brazil, Geol,. Soc.Am. BuIl.,56, 1015-1070. Munoour, J. (1955), Phosphate minerals of the Borborema pegmatites: I-Patrimonio, Am. LIineral., 40, 50-63. Pnrrn, E. L. exl FnouoEr,, C. (1947), Studies in Urolithiasis: I. The composition of urinaty calctrli, J our. U r ology, 57, 949-99 l. TbNrvson, Cnn. (1956), Das Stewartit-Problem, Die NaturuissenschaJten, 6, 128-729.