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Alforsite, a New Member of the Apatite Group: the Barium Analogue of Chlorapatite1

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Alforsite, a New Member of the Apatite Group: the Barium Analogue of Chlorapatite1 American Mineralogist, Volume 66, pages 1050-1053, 1981 Alforsite, a new member of the apatite group: the barium analogue of chlorapatite1 NANCY G. NEWBERRY, ERIC J. ESSENE, AND DONALD R. PEACOR Department of Geological Sciences University of Michigan Ann Arbor, Michigan 48109 Abstract Alforsite, ideally Ba,(P04)3Cl, is a new member of the apatite group occurring in contact m.etamorphosed evaporitic rocks from Fresno and Mariposa counties, California, associated wlth fluor~patit: and m~ny. other rare barium minerals previously described by Alfors et al. (1965). It IS optically umaxlal, negative, with W,E= 1.70(1). The density is calculated to be 4.83(2) gm/cm3 for the end member composition. It is hexagonal, space group P63/m, with a = 10.25(1) and c = 7.64(2)A. The strongest lines in the powder pattern are 3.06(100), 2.95(30), 2.1~(30), 2.03(30), and 1.928(30). The associated assemblage of witherite, quartz, and s~~ornlte buffers the C02 fugacity at metamorphic pressures and temperatures. The name 1.s.mhono~ of Dr: Jo~ T. Alfors of the California Division of Mines and Geology in recogmtlOn of hlS contnbutlOns to the study of barium minerals. Introduction study. The phase was found to have the structural The sanbornite deposits of eastern Fresno and and physical characteristics of apatite, and deter- Mariposa counties, California, contain many rare mined to be the barium analogue of chlorapatite. As barium minerals, principally silicates (Rogers, 1932). the natural phase is rare and very fine-grained even The metamorphic sanbornite-quartz rock and fo- at the type locality, studies were supplemented by liated quartzite, both of which contain the new min- work on a synthetic end-member barium chlorapatite eral, are described by Matthews and Alfors (1962), supplied by Dr. Charles Prewitt of the State Univer- sity of New York at Stony Brook. Alforsite was sub- Stinson and Alfors (1963, 1964), Alfors and Putman (1964), Alfors and Stinson (1965), and by Alfors et al. sequently found in samples from the Incline locality (1965). The san bornite rocks appear to be metasedi- where it is, as at Big Creek, associated with fluorapa- mentary rocks, presumably meta-evaporites. The re- tite and many barium minerals. gional geology of the area is described by MacDon- We have named this mineral for Dr. John T. Al- ald (1941) and by Krauskopf (1953). All the fors of the California Qivision of Mines and Geology sanbornite localities are within a few hundred meters in recognition of his extensive studies of the type lo- cality of the mineral and his descriptions of the other of granodiorite intrusions and it will be assumed be- low that the rocks last equilibrated in a contact meta- associated rare and new minerals. The name and morphic event producing hornblende- and pyroxene- mineral have been approved by the IMA Commis- hornfels facies conditions at pressures of 1-3 kbar. sion on New Minerals and New Mineral Names. Type material, designated to be that of the Big Creek Alforsite was first discovered in samples from the Big Creek locality during qualitative electron micro- locality, is preserved in the mineral collections of the probe analysis of what appeared in thin-section to be University of Michigan and the National Museum of ordinary apatite. A distinctive reddish-violet cath- Natural History, Smithsonian Institution under cata- logue number NMNH 147511. odoluminescence and EDA observations showing major levels of barium, chlorine, and phosphorus were noticed in several grains selected for further X-ray diffraction Single crystals of alforsite were separated by me- I Contribution No. 366 from the Mineralogical Laboratory, De- partment of Geological Sciences, the University of Michigan, Ann ticulously scratching exceptionally small (approxi- Arbor, Michigan 48109. mately 0.02 mm diameter) crystals from thin sections 0003-004)(/81/0910-1050$02.00 1050 NEWBERRY ET AL.: ALFORSITE 1051 where it had been identified and differentiated from Table 1. Powder diffraction data for synthetic Ba5(P04)3Cl. CuKa the more common fluorapatite using optical and elec- radiation, 114.6 mm diameter camera tron microprobe methods. Therefore, space group d( Obs) d(ca1 c) hk1 d(Obs) I/Io and unit cell parameters were determined by stan- dard Weissenberg and precession X-ray diffraction 5.68 5.79 101 5 1.615 20 methods on the synthetic material. Precession photo- 5.17 5.13 110 5 1.600 20 graphs of the natural material, although yielding 4.41 4.44 200 5 1.566 30 4.25 4.26 111 10 1.511 2 very weak reflections, duplicated those of the syn- 3.85 3.84 201 5 1.477 2 [ 002 1.453 2 thetic material, confirming that a1forsite has the apa- 3.82 tite structure. Due to the rarity of a1forsite grains, 3.52 3.51 102 5 1.423 2 3.37 3.36 120 20 1.398 10 powder diffraction data were also obtained using the 3.06 3.07 121 1 00 1.366 20 [ 3.06 112 1.336 30 synthetic barium ch10rapatite. These data were used 2.95 2.96 300 30 1.286 2 to refine unit cell parameters by least-squares meth- 2.45 [2.46 130 1.262 2 ods. Powder diffraction data for the synthetic phase 2.45 103 1.242 1 are listed in Table 1. Alforsite is hexagonal, with 2.36 [2.34 131 1.227 2 2.34 302 1.207 2 space group P63/m, analogous to apatite. Unit cell 2.13 2.13 401 40 1.181 2 [ parameters are a = 10.25(1) and c = 7.64(2)A. 2.13 222 1.144 2 Extremely long X-ray exposures of Weissenberg 2.07 2.07 132 1 0 1.125 1 2.03 2.04 230 30 1.107 20 and precession photographs of the synthetic apatite [ 2.03 123 1.077 30 failed to reveal the superstructure reflections which 1.971 1.968 231 20 ".067 30 1.928 1 .931 303 30 1 . 042 10 occur in X-ray photographs of pure or nearly pure 1.734 [1.730 501 1 .026 10 ch10rapatites and other members of the apatite 1.728 142 1 . 005 20 group. If present, these reflections would indicate 1.672 [1.674 403 1.667 331 that the true structure is monoclinic, pseudo- 1.638 1.639 241 hexagonal, with one a-translation doubled relative to an hexagonal unit cell (Mackie et al., 1972). The syn- thetic a1forsite shows no evidence of monoclinic Sio.01)OI L96(Clo.99F 005) and (Ba4.05Cao.75Sro.24Pbo.03)(P2. character, but we cannot be absolutely sure about the 94SioOl)OIL9lClo.93FoI4)'respectively. The F + C1 in natural phase. excess of 1.00 and the slight deficiency of Si + P is consistent with a minor C03 . F substitution for P04 Chemical analysis similar to some carbonate-fluorapatites (McConnell, Analyses of alforsite were made with the Univer- 1973) although deviation from the ideal formula is sity of Michigan ARL-EMXelectron microprobe with within the precision of the analysis for P and Cl. The three wavelength-dispersive crystal spectrometers ideal formula is Ba5(P04)3C1 and a1forsite is the bar- and an Ortex current digitizer to compensate for ium analogue of chlorapatite. beam current fluctuations during data collection. The analyses were taken at an excitation potential of 12 Physical properties kV and a specimen current of 0.02 /Lawith LiF, PET Alforsite occurs as isolated small subhedra1 grains, and TAP crystal spectrometers. Standards used were generally less than 0.05 mm in diameter but rarely up fluorapatite for F and Ca; synthetic barium chlorapa- to 0.2 mm. These colorless grains resemble typical tite for Ba, C1, and P; and strontianite for Sr. Analy- fluorapatite, exhibiting low birefringence and high ses for Mn, Si, Pb, and S using synthetic Mn2Si04, relief. This makes it difficult to distinguish alforsite Si02, and PbS standards showed these appeared in from fluorapatite and from many of the associated minor amounts. Careful tests were made to insure high-relief barium minerals except by using the elec- that no damage to or volatilization of the sample oc- tron microprobe. Alforsite is identifiable through an curred during analysis. Drift, atomic number, ab- intense violet fluorescence in the 10-15 kV electron sorption, and fluorescence corrections were made us- beam of a 1uminoscope or electron microprobe. This ing the program EMPADR VII (Ruck1idge and fluorescence is more intense and redder than the vio- Gasparrini, 1969). Analyses of the apatite from the let color exhibited by other apatites, and is very dis- Incline and Big Creek localities are listed in Table 2. tinctive. No fluorescence appears under short- or These analyses result in formulae, calculated on the long-wavelength ultraviolet light. Alforsite is uni- basis of 0 + C1 + F = 13, of (Ba468SrO.19CaO.13)(P2.98axial, negative with extremely low birefringence and 1052 NEWBERRY ET AL.:ALFORSITE Table 2. Electron microprobe analyses of alforsite Occurrence Alforsite occurs as an accessory mineral in samples oxide Bi9 Creek, Inc1 ;ne, from both the Incline, Mariposa County, and Big Fresno, Co. Mar;posa, Co. wt. % Creek, Fresno County, localities as described by Al- S;02 0.1 <O.OS MnO <0.1 <0.1 fors, et ai. (1965). Big Creek samples are from the CaO 4.6 0.7 sites in Section 27, Township 11 S, Range 25 E on SrO 2.7 2.0 their sketch map and were provided by Alfors. The BaO 67.7 >1.3 samples from the Incline locality were obtained from PbO O.B <0.1 the University of Michigan Mineralogical Collection P20S 22.7 21.0 and more specific information on sample sites was C1 3.6 3.
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