Mineralogy of the Boqueiraozinho pegmatite: Parelhas Rio Grande do Norte Brazil.
The occurrence described here was first found by one of us (RRW) in February of 1988 and noted by Robinson and King (1990). The unusually high quality of the triploidite from this locality makes it of special interest to collectors, and the occurrence of hessite in pods of digenite is of scientific interest, since these minerals normally occur in hydrothermal deposits.
The Boqueiraozinho(*) pegmatite (also known as the Capoeira pegmatite) lies approximately 3 km east of the village of Parelhas, in the state of Rio Grande do Norte, and was last operated for feldspar by the Companhia de Desenvolvimento de Recursos Minerais - CDM. The more famous Boqueiraozinho pegmatite (Johnson, 1945; Murdoch, 1958), known as the type locality for "chavesite" (shown to be monetite by Kampf and Dunn, 1994) lies approximately 1 km to the south. Access is by road via Equador and Parelhas, from National Road 230, 101 km west of Campina Grande. From Parelhas, the deposit is reached by taking a dirt road leading north then east from the village. The pegmatite is conspicuously exposed on the north side of a hill south of the road, approximately 3 km east of Parelhas.
The Boqueiraozinho pegmatite forms an elongated, lenticular body that intrudes metaconglomerate. The contacts are sharp, and the metaconglomerate is frequently epidotized, especially along shear planes. The pegmatite strikes east-west and is exposed for a distance of about 200 meters. It attains a maximum width of about 25 meters. There are three symmetrically disposed sets of bands progressing inward from the contacts for about a half meter; each set consists of fine-grained microcline and quartz with garnet plus-or-minus minor black tourmaline. Beyond this point, the pegmatite becomes very coarse-grained, with crystals of perthitic white microcline up to several meters across in its central-most part. Some of the microcline crystals contain small pockets (up to half a meter) hosting crystals of milky quartz, muscovite and bright blue crystals of fluorapatite (Robinson and King, 1990). In addition to microcline, the central portion of the pegmatite also contains abundant aggregates of muscovite-quartz-microcline up to several meters across, though the individual crystals within these aggregates average only 2-5 cm. No massive quartz core has yet been encountered in the pegmatite, but isolated, discontinuous concentrations of quartz up to several meters do occur in its central portion.
Numerous other minerals also have been observed in the central part of the pegmatite. These include isolated, somewhat brecciated pods of phosphate minerals up to a meter across composed predominantly of lithiophilite and albite with lesser amounts of triploidite, triplite and triphylite. Additional minerals present include zircon, gahnite, minor spessartine, hectorite and coronadite, and secondary phosphates fluorapatite, eosphorite and crandallite. One specimen examined contains a pale green, vitreous mineral that may be a member of the alluaudite group, but this is exceedingly rare. To date, the phosphate pods appear to be concentrated in the western end of the pegmatite.
An additional feature of particular interest is the occurrence of isolated masses of digenite in some of the central microcline. These masses vary in size from a few centimeters up to nearly a meter across, and are conspicuously stained green by malachite and chrysocolla which occur abundantly as alteration products. The segregations occur throughout the unaltered central microcline, suggesting they were formed simultaneously, and therefore are part of the late, but main pegmatite paragenesis. Other minerals observed scattered throughout the central part of the pegmatite, but in small amounts, are listed in Table 2.
Only those mineral species of collector-quality or scientific interest will be described. The identity of all species has been confirmed by X-ray and/or electron microprobe analyses. Table 1 gives microprobe analyses for the lithiophilite, triploidite, triplite and fluorapatite. These analyses were carried out on a Jeol Superprobe 733 using an accelerating voltage of 15 KV and a beam current of 25 nanoamperes. The electron beam was defocussed to [TABULAR DATA FOR TABLE 1 OMITTED] 20 microns to minimize sample decomposition. Standards used were almandine (Mg and Fe), apatite (Ca and P), tephroite (Mn), titanite (Ti) and fluorian riebeckite (F). Total iron is reported as FeO; [Li.sub.2]O and [H.sub.2]O are calculated stoichiometrically.
Digenite occurs in masses approaching a meter across in the central perthitic microcline. Microscopic examination of polished sections shows the digenite is commonly replaced by covellite at its outer margins, which helps explain the blue appearance of some hand specimens.
Lustrous, blue, often tabular crystals of fluorapatite associated with quartz and muscovite occur in pockets in the central perthitic microcline and are certainly the best known specimens from the locality (Robinson and King, 1990). In addition, fluorapatite also forms pale green to white, fine grained, earthy coatings on lithiophilite, which are probably secondary in origin. From the analysis in Table 1, the formula for the pale green fluorapatite, based on 8 cations is ([Ca.sub.4.48][Mn.sub..44][Fe.sub..03][Mg.sub..02])[([P.sub.1.01] [O.sub.3.94]).sub.3][F.sub.1.4 4].
Hessite is a relatively uncommon mineral, found principally with other tellurides in metalliferous hydrothermal veins and sulfide bodies (Ramdohr. 1980: Palache et al., 1994). To our knowledge, the occurrence of hessite outside such environments has not previously been reported. At the Boqueiraozinho pegmatite hessite forms drop-like inclusions up to 80 microns in diameter within some of the digenite pods. Back-scattered electron images reveal the hessite to be rimmed by galena, as shown in Figure 6. Microprobe analyses indicate both the hessite and galena are essentially pure, though a Bi-S mineral (bismuthinite?) is occasionally present in minute amounts along microfractures in some specimens.
The drop-like form of the inclusions suggests the presence of an initially complex Cu-Ag-Te-Pb-Bi-sulfide liquid which segregated from the silicate melt during crystallization. Upon cooling, the liquid further separated into at least three apparently immiscible sulfide liquids which formed the observed minerals.
Lithiophilite is the major constituent of the phosphate pods. It occurs both as massive aggregates and isolated crystals up to 20 cm embedded in albite. The cores of both the masses and crystals are unaltered and pale salmon-pink in color. The outer surfaces of the hthiophilite show varying degrees of alteration, and appear pale to dark brown due to minute films of coronadite and/or hectorite. Microprobe examination of the lithiophilite shows it is generally homogeneous with an average composition of Li([Mn.sub.74][Fe.sub.20][Mg.sub.04])[P.sub.1.01][O.sub.4], based on 4 oxygen atoms and assuming 1 Li. Triphylite, with Fe:Fe + Mn + Mg [greater than] 1.0, has also been observed, but is uncommon.
Triplite occurs sparingly as dark brown, waxy grains and patches associated with lithiophilite and triploidite in some of the pods. It is relatively homogeneous in composition, with an average empirical formula ([Mn.sub.1.26][Mg.sub..45][Fe.sub..21][Ca.sub..03][Ti.sub..02])[P .sub.1.01][O.sub.4.00]([F.sub. .79][(OH).sub..21]), based on (O + F + OH) = 5.
Triploidite, while not a rare mineral, is certainly an uncommon one, especially in euhedral crystals. It is found primarily in phosphate pods in granitic pegmatites, though at least two hydro-thermal occurrences are known (Clark and Couper, 1979; Adam and Gagny, 1986). At the Boqueiraozinho pegmatite triploidite occurs as elongated, subparallel, prismatic, pinkish red crystals up to 8 cm in length within some of the lithiophilite. The crystals have a brilliant luster and the smaller ones are nearly transparent. While individual, terminated crystals have not yet been found, the size and quality of the Boqueiraozinho specimens make them some of the finest known. Regrettably, only a few of the phosphate pods encountered have contained triploidite, and the area from which they came is now under water. Microprobe analyses of the triploidite show it is relatively homogeneous, with an average composition of ([Mn.sub.1.58][Fe.sub..34][Mg.sub..06])([P.sub.1.02][O.sub.4.03]) (OH), based on 3 cations and assuming 1 (OH).
Unknown No. 1
One specimen composed chiefly of lithiophilite and triploidite contains rare, minute grains of a vitreous, green, unidentified mineral. The X-ray powder diffraction pattern of this mineral is somewhat similar to those of the alluaudite group minerals. The strongest lines are (d in [Angstrom], I/[I.sub.o]) 3.16 (10), 6.15 (7), 2.74 (4), 5.51 (3), 2.69 (3), 3.51 (2), 2.55 (2) and 2.51 (2). Unfortunately, the amount of sample currently available precludes further characterization.
Unknown No. 2
An unidentified, powdery, yellow-green, bismuth-bearing mineral occupies tiny fractures in some of the digenite. based on its variable composition and overall heterogeneous appearance, it is most likely a mixture of secondary copper and bismuth carbonates or oxides probably originating by oxidation of a primary bismuth sulfide mineral.
Table 2. List of minerals identified from the Boqneiraozinho pegmatite. Central Pegmatite Sulfide Pods Phosphate Pods Primary Minerals Primary Minerals Primary Minerals Albite Bismuthinite (?) Albite Beryl Covellite Gahnite Fluorapatite Digenite Lithiophilite Gahnite Galena Spessartine Manganocolumbite Hessite Triplite Microcline Triploidite Muscovite Secondary Triphylite Pyrochlore Minerals UK-1 Quartz Chrysocolla Zircon Rutile (niobian) Malachite Schorl UK-2 Secondary Phosphates Spessartine Crandallite Spodumene Eosphorite Uranmicrolite Fluorapatite Uranpyrochlore Zircon Secondary Minerals Coronadite Hectorite
Special thanks are given to the Companhia de Desenvolvimento de Recursos Minerais - CDM, for permitting access to their property. Jerry Van Velthuizen and T. Scott Ercit (Canadian Museum of Nature) assisted with the X-ray and microprobe analyses, and provided many helpful discussions.
* Pronounced "Bo-care-ahn-zeen-yo."
ADAM, D., and GAGNY, C. (1986) L'expression mineralogique du phosphore dans les leucogranites. Apport a la metallogenie de l'etain-tungstene. Cas de la mine de Ribeira (Tras Os Montes, Portugal). Bulletin de Mineralogie, 109, 441-460.
CLARK, A. M, and COUPER, A. G. (1979) End-member triploidite from Cornwall. Mineralogical Magazine, 43, 179-180.
JOHNSON, W. D., Jr. (1945) Beryl-tantalite pegmatites of northeastern Brazil. Geological Society of America Bulletin 56, 1015-1070.
KAMPF, A. R., and DUNN, P. J. (1994) Chavesite discredited. American Mineralogist, 79, 385-386.
MURDOCH, J. (1958) Phosphate minerals of the Borborema pegmatites: II - Boqueirao. American Mineralogist, 43, 1148-1156.
PALACHE, C., BERMAN, H., and FRONDEL, C. (1944) The System of Mineralogy of James Dwight Dana and Edward Salisbury Dana. Seventh Edition, 1, 184-186. John Wiley and Sons, Inc., New York.
RAMDOHR, P. (1980) The Ore Minerals and Their Intergrowths. Second edition, 1, 421-424. Pergamon Press, New York.
ROBINSON, G. W., and KING, V. T. (1990) What's new in minerals? Mineralogical Record, 21, 487-488.
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|Author:||Robinson, George W.; Wegner, Reinhard R.|
|Publication:||The Mineralogical Record|
|Date:||May 1, 1998|
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