ALBERT

Description: Strontiohurlbutite, ideally SrBe 2 (PO 4 ) 2 , is a new member of hurlbutite group discovered in the Nanping No. 31 pegmatite, Fujian province, southeastern China. Crystals are mainly found in zones I, II, and IV; they are platy, subhedral-to-anhedral, with a length from 5 μm to 1.5 mm. Associated minerals mainly include quartz, muscovite, beryl, hurlbutite, hydroxylherderite, apatite-group minerals, and phenakite. Strontiohurlbutite crystals are light blue, translucent-to-transparent, and have vitreous luster. The Mohs hardness is about 6, and the tenacity is brittle. Optically, strontiohurlbutite is biaxial (–), α = 1.563(3), β = 1.569(2), = 1.572(3) (white light), 2 V meas = 68.5(5)°, and exhibits weak dispersion, r 〉 v. The optical orientation is X = b , Y c . Electron-microprobe and SIMS analyses (average of 16) give SrO 29.30, P 2 O 5 51.05, CaO 0.91, BaO 0.64, and BeO 17.71 wt%; total 99.61 wt%. The empirical formula, based on 8 O apfu, is (Sr 0.81 Ca 0.05 Ba 0.01 ) 0.87 Be 2.02 P 2.05 O 8 . The stronger eight lines of the measured X-ray powder-diffraction pattern [ d in Å( I )( hkl )] are: 3.554(100)(121); 3.355(51)(211); 3.073(38)(022); 2.542(67)(113); 2.230(42)(213); 2.215(87)(32); 2.046(54)(223); 1.714(32)(143). Strontiohurlbutite is monoclinic, space group P 2 1 / c ; unit-cell parameters refined from single-crystal X-ray diffraction data are: a = 7.997(3), b = 8.979(2), c = 8.420(7) Å, β = 90.18(6)°, V = 604.7(1) Å 3 ( Z = 4, calculated density = 3.101 g/cm 3 ). The mineral is isostructural with hurlbutite, CaBe 2 (PO 4 ) 2 , and with paracelsian, BaAl 2 Si 2 O 8 . The formation of strontiohurlbutite is related to the hydrothermal alteration of primary beryl by late Sr- and P-rich fluids.

Description: Four beryllophosphates with general formula M 2+ Be 2 P 2 O 8 ( M 2+ = Ca, Sr, Pb, Ba) have been synthesized under hydrothermal conditions at 200 °C. Their crystal structures were solved by direct methods and refined by full-matrix least-squares techniques on the basis of F 2 for all unique reflections to agreement indices R 1 of 2.3, 4.8, 1.5, and 2.1%, respectively. CaBe 2 P 2 O 8 is monoclinic, space group P 2 1 / c , Z = 4, a 7.809(1), b 8.799(1), c 8.309(1) Å, β 90.51(1)°, V 570.98(2) ų. SrBe 2 P 2 O 8 is monoclinic, space group P 2 1 / c , Z = 4, a 8.000(1), b 8.986(1), c 8.418(1) Å, β 90.22(1)°, V 605.10(6) ų. PbBe 2 P 2 O 8 is monoclinic, space group P 2 1 / c , Z = 4, a 8.088(1), b 9.019(1), c 8.391(1) Å, β 90.12(1)°, V 612.22(1) ų. BaBe 2 P 2 O 8 is hexagonal, space group P 6/ mmm , Z = 1, a 5.028(1), c 7.466(1) Å, V 162.51(1) ų. The three first compounds are isostructural and show a paracelsian-type structure composed of a framework of corner-sharing BeO 4 and PO 4 tetrahedra. These tetrahedra are assembled in four- and eight-membered rings showing the typical UUDD and DDUDUUDU patterns, respectively. The M 2+ (Ca, Sr, Pb) cation occurs in a distorted 7+3-coordinated polyhedron located in the eight-membered ring. CaBe 2 P 2 O 8 and SrBe 2 P 2 O 8 are isostructural with the minerals hurlbutite and strontiohurlbutite, respectively. The structure of BaBe 2 P 2 O 8 consists of double layers of tetrahedra, which contain both Be and P in a 1:1 ratio. Inside the layers, the (Be,P)O 4 tetrahedra form six-membered rings by sharing corners. The Ba atoms are located in very regular 12-coordinated polyhedra and connect two successive double layers. This Ba beryllophosphate is the synthetic counterpart of minjiangite, and shows similarities with the mineral dmisteinbergite, CaAl 2 Si 2 O 8 , a hexagonal polymorph of anorthite. Other hydrothermal experiments were performed in order to establish the stability of these M 2+ Be 2 P 2 O 8 compounds as a function of temperature and pH; the results show that synthetic hurlbutite can crystallize in acidic as well as in basic conditions. Furthermore, these experiments show that the four title compounds are stable at high temperatures and pressures (600 °C, 1 kbar).

Description: In order to assess the stability of Fe-rich alluaudites in pegmatites, we performed hydrothermal experiments between 400 and 700 °C (1 kbar), in the Na–Fe 2+ –Fe 3+ (+PO 4 ) ternary system. These experiments produced several new phosphate minerals, among which was Na 2 Fe 3+ (HPO 4 )(PO 4 )·H 2 O [ a 15.5004(8), b 7.1465(5), c 29.239(2)Å, V 3238.9(3)Å 3 , Pnma ], which shows a crystal structure based on chains of corner-sharing FeO 6 octahedra, similar to those occurring in the jahnsite-group minerals. Alluaudite-type phosphate minerals occupy a large stability field in the center of the Na–Fe 2+ –Fe 3+ (+PO 4 ) system; this stability field covers between 5.8 (500 °C) and 21.1% (700 °C) of the diagram surface. A comparison of the chemical composition of natural Fe-rich alluaudite-type phosphate minerals with the experimental data obtained herein indicates that these minerals crystallized below ca . 450 °C in pegmatites. This temperature value is in good agreement with the secondary Na-metasomatic origin of these phosphate minerals. Moreover, a structural classification of phosphates occurring in the Na–Fe 2+ –Fe 3+ (+PO 4 ) diagram has been established, taking into account the connectivity between FeO 6 octahedra occurring in the crystal structures of the synthesized phases. This classification indicates a variation of the structural complexity of phosphates, characterized by an increasing dilution of FeO 6 octahedra in the structure as the Na content increases.

Description: The Jocão pegmatite (or Cigana) is located in the well-known Eastern Brazilian Pegmatitic Province (EBPP), Minas Gerais, Brazil. According to their macroscopic textures, three different kinds of phosphate mineral masses were collected from the dumps of the pegmatite: association I is composed of dendritic triphylite, forming intergrowths with silicate minerals (spessartine garnet or albite); association II forms blocky nodules of triphylite-ferrisicklerite-heterosite; and association III shows exsolution lamellae of ferrisicklerite-heterosite in massive beusite. The primary textures and the Fe/(Fe+Mn) ratios of primary triphylite make it possible to establish the crystallization sequence of the primary phases; the petrogenesis of primary intergrowths between garnet and triphylite of association I is also discussed. In the three associations, these primary minerals are hydrothermally altered and secondary species are produced. In association I, the first hydrothermal alteration event was a weakly oxidizing hydroxylation stage, during which triphylite was only replaced by hureaulite (rarely associated with barbosalite) along its cleavage planes. The final stage affecting association I corresponds to meteoric processes during which ludlamite and then vivianite progressively replaced triphylite. Association II evolved under more oxidizing conditions and the first alteration stage corresponds to the progressive oxidation of triphylite accompanied by Li-leaching, leading to ferrisicklerite and heterosite. The second hydrothermal stage corresponds to a hydroxylation event, and the secondary species depend on the phosphate mineral that they replace: triphylite is only altered to Fe 2+ -Mn 2+ -bearing hydrated species (colorless hureaulite); ferrisicklerite is altered to Fe 2+ -, Mn 2+ -, and Fe 3+ -bearing phosphate minerals such as jahnsite s.l. , frondelite s.l., and orange hureaulite; heterosite is replaced by Fe 3+ -bearing species such as ferristrunzite. Consequently, it appears that the secondary phosphate minerals, which crystallize during this second hydrothermal stage, strongly depend on the cations which are locally available in the sample zone. The final stage forms the meteoric species: leucophosphite and phosphosiderite directly replace heterosite, whereas vivianite, a ferrous meteoric species, only appears in the triphylite core. In association III, exsolution lamellae are formed at the expense of a high-temperature homogenous Ca-Li-bearing graftonite-beusite-like phase; when the temperature decreased, Li migrated into triphylite and Ca to the larger M1 site of beusite. During the high-temperature hydrothermal alteration processes, triphylite transforms into ferrisicklerite and heterosite, like in association II; however, beusite is not affected by any transformation process at that stage. During the low temperature hydroxylation stage, ferrisicklerite from the core remains almost unaltered, while beusite is replaced by an intimate mixture of pleochroic Ca-rich hureaulite and tavorite. After this hydroxylation stage, the meteoric stage is characterized by an increase in Ca 2+ and H 2 O activities, responsible for the replacement of hureaulite and tavorite by mitridatite-robertsite. At the nodule border, beusite and heterosite are completely replaced by an intimate mixture mainly composed of frondelite and robertsite-mitridatite. Finally, this study shows that the small phosphate nodules from Jocão are more affected by oxidation than large nodules, thus indicating that the diffusion kinetics of hydrothermal fluids is relatively low in these phosphate nodules. As a consequence, large phosphate nodules show a typical zoning with triphylite (core)–ferrisicklerite-heterosite (rim); this zoning, which preserves the crystal structure of the phosphate minerals, was achieved during the high temperature hydrothermal transformations. The phosphate nodules consequently appear as a relatively closed system compared to the silicate matrix.