Brazilianite

Brazilianite

Brazilianite from type locality, Conselheiro Pena, Minas Gerais, Brazil
General
Category Phosphate minerals
Formula
(repeating unit)		 NaAl3(PO4)2(OH)4
sodium aluminium phosphate hydroxide
Strunz classification 08.BK.05
Crystal symmetry Monoclinic prismatic
H-M symbol: 2/m
Space group: P21/n
Unit cell a = 11.229 Å, b = 10.142 Å, c = 7.098 Å; β = 97.4°; Z=4
Identification
Color Yellow, green, colorless
Crystal habit Prismatic crystals, may be radially-fibrous or globular druses
Crystal system Monoclinic
Cleavage (010) Distinct to good
Fracture Conchoidal
Mohs scale hardness 5.5
Luster Vitreous
Streak White
Diaphaneity Transparent to translucent
Specific gravity 2.98
Optical properties Biaxial (+)
Refractive index nα = 1.602 nβ = 1.609 nγ = 1.621 - 1.623
Birefringence δ = 0.019 - 0.021
References [1][2]

Brazilianite, whose name derives from its country of origin, Brazil, is a typically yellow-green phosphate mineral, most commonly found in phosphate-rich pegmatites.

It occurs in the form of perfect crystals grouped in druses, in pegmatites, and is often of precious-stone quality. One noted deposit of brazilianite is in the surroundings of Conselheiro Pena, in Minas Gerais, Brazil.

Some of these are found on leaves of muscovite with their strong silvery glitter, ingrown in their parent rock. The crystals, dark greenish-yellow to olive-green, sometimes measure up to 12 cm in length and 8 cm in width. Crystals of similar shape and dimensions have been discovered in another deposit in Minas Gerais, near Mantena, but they lack the perfection of the crystal form. Many brazilianites found in mineral collections originated from the Palermo and the Charles Davis mines in Grafton County, New Hampshire.

Composition

Brazilianite, NaAl3(PO4)2(OH)4 is a hydrous sodium aluminium phosphate that forms through the metasomantic alteration of amblygonite-montebrasite.[3] Amblygonite, LiAlPO4F in combination with quartz goes through an OH-F exchange to make montebrasite, LiAlPO4{F,OH} at temperatures greater than 480 °C.[3] Natromontebrasite, NaAl(PO4)(OH), is formed when montebrasite does though Li-leaching process and there is a Na cation exchange at temperatures less than 450 °C.[3] Brazilianite concludes this process by forming as natromontebrasite combines with fluorapatite, Ca5(PO4)3F.[4] Due to its formation caused by the amblygonite-montebrasite alteration and the presence of tourmaline in the environment where brazilianite forms, different elements are present in the mineral such as P, Al, Fe, Mn, Ba, Sr, Ca, Mg, Na, K, F, and Cl.[5] There are many substitution possibilities in the brazilianite formula.[6] Besides sodium, being replaced by any other element, iron can replace aluminium and vanadates or arsenates can replace the phosphates.[6]

Structure

Brazilianite is composed of chains of edge-sharing Al-O octahedra that are linked by P-O tetrahedra with sodium in the cavity of the framework.[7] The crystal structure of brazilianite is a~11.23 Å, b~10.14 Å, c~7.10 Å, β~97.4° and Z = 4.[8] The Al-octahedra has two types of octahedral coordination: trans-AlO4(OH)2 and trans-AlO3(OH)3.[7] The two phosphorus atoms in brazilianite are coordinated in a tetrahedral with four oxygen atoms each.[7] The sodium atom is located within the P-O and Al-O polyhedral in an irregular cavity.[7] The coordination of the sodium is best described as the uncommon seven-coordination.[7] The presence of hydrogen in the same cavity where sodium is causes a repulsion between the two forcing sodium to one side of the cavity so that is it more coordinated with oxygen than its other side.[7] Gatehouse et al., 1974[7] described the four remaining hydrogen as being in a chain and contributing to the complexity of the structure but Gatta et al., 2013,[5] gives a well define H-bonding scheme and how these hydrogen items confines in OH groups. One of the hydrogen in brazilianite splits to make a fifth hydrogen.[5] The splitting of this hydrogen has not been explained why it happens but it was shown that it can affect the hydrogen bond configuration.[5] Some of the oxygen atoms in the four OH groups in brazilianite act as donors and some as acceptors of the hydrogen bond.[5] One of these oxygen items is both a donor and acceptor to accommodate the hydrogen that split into two.[5]

Physical properties

Brazilianite is a mineral in the monoclinic system that is part of the point group 2/m and belongs to the space group P21/n.[8] The crystals of brazilianite are elongated and prismatic along [100].[9] Most common forms that are measured in brazilianite {010}, {110}, { 111}.[10] It displays a perfect cleavage on (010), it is brittle and has a conchoidal fracture.[6] The mineral has a Mohs hardness of 5.5 and was believed it had a specific gravity of 2.94 which was first determined by Pough and Henderson, 1945. With the second occurrence of the mineral, it was determined that the specific gravity of the mineral was actually 2.98.[9] Brazilianite has a vitreous luster, has a white streak, and the mineral is translucent to transparent.[6] The color of brazilianite ranges from dark yellow-green to a pale yellow.[5] Brazilianite begins to lose its color when heated to 200 °C and becomes colorless when it is heated to 300 °C.[10]

Geological occurrence

Brazilianite crystals on muscovite, Galilea mine, Minas Gerais, Brazil

Brazilianite is typically found in granite pegmatite and it is often found the cavities within the pegmatite where quartz, beryl and mica are also found.[6] Different habits of brazilianite have been found in different locations. Brazilianite is often found with muscovite.[5] The Corrego Frio pegmatite where brazilianite is found in Brazil is an altered pegmatite dike that had weathered biotite schist between its walls.[6] In New Hampshire, the pegmatite where the brazilianite was found was made up of 99 percent albite, mica, and quartz.[11] Brazilianite also found with tourmaline and feldspar.[11] The sequence of the mineral formation in the pegmatite in Brazil had not been determined.[11] The sequence of mineral formation in New Hampshire was quartz, brazilianite, apatite, whitlockite, and quartz.[9] During the hydrothermal stage, the pegmatite containing the brazilianite is traversed by a late stage low temperature hydrothermal veins where amblygonite-montebrasite is altered to form brazilianite.[3] Brazilianite has been described from other granite pegmatites in Brazil and the United States.[2] It has also been found in different locations in the world, including Rwanda, Yukon Creek in Canada, Argentina, China, France, and Australia.[2]

Special characteristics

Brazilianite is relatively new phosphate minerals along with amblygonite, turquoise and apatite that are used as gemstones.[5] Brazilianite is often confused with amblygonite, apatite, chrysoberyl, beryl, and topaz.[12] Even though it was first described in 1945, its discovery was actually in 1944 but it was believed it was chrysoberyl until analysis was done to the mineral indicating a new mineral.[6] The State of Minas Gerais is the largest producer and exporter of gemstones in Brazil and is accountable for 74 percent of the official production which includes brazilianite.[13] It is soft and fragile causing it not to be a popular stone.[6] When brazilianite is heated, it losses it yellow color and becomes colorless.[10]

References

  1. ↑ Brazilianite data on Webmineral
  2. ↑ 2.0 2.1 2.2 Brazilianite on Mindat.org
  3. ↑ 3.0 3.1 3.2 3.3 Baldwin, J.R.; Hill, P. G., Von Knorring, O., & Oliver, G. J. H. (2000). "Exotic aluminium phosphates, natromontebrasite, brazilianite, goyazite, gorceixite and crandallite from rare-element pegmatites in Namibia". Mineralogical Magazine 64 (6). ISSN 0026-461X.
  4. ↑ Scholz, R.; Karfunkel, J., Bermanec, V., Magela, G., da Costa, A. H. H., Souza, L. A. C., & Bilal, E. (2008). "Amblygonite-montebrasite from Divino das Laranjeiras Mendes Pimentel pegmatitic swarm, Minas Gerais, Brasil. II. Mineralogy". Romanian Journal of Mineral Deposits 83.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 Gatta, G.D; G. D., Vignola, P., Meven, M., & Rinaldi, R (2013). "Neutron diffraction in gemology: Single-crystal diffraction study of brazilianite, NaAl3(PO4)2(OH)4". American Mineralogist 98 (8-9).
  6. ↑ 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 Pough, F.H; Henderson, E. P (1945). "Brazilianite, a new phosphate mineral". American Mineralogist 30.
  7. ↑ 7.0 7.1 7.2 7.3 7.4 7.5 7.6 Gatehouse, B.M.; Miskin, B. K. (1974). "The crystal structure of brazilianite, NaAl3 (PO4)2(OH)4". Acta Crystallographica Section B: Structural Crystallography and Crystal Chemistry 30 (5).
  8. ↑ 8.0 8.1 Frost, R.L.; Xi, Y (2012). "Molecular structure of the phosphate mineral brazilianite NaAl3(PO4)2(OH)4-A semi-precious jewel". Journal of Molecular Structure 1010.
  9. ↑ 9.0 9.1 9.2 Frondel, C.; Lindberg, M.L. (1948). "Second occurrence of brazilianite". American Mineralogist 33.
  10. ↑ 10.0 10.1 10.2 Čobić, A.; Zebec, V., Scholz, R., Bermanec, V., & de Brito Barreto, S. (2011). "Crystal morphology and xrd peculiarities of brazilianite from different localities". Natura Croatica 20 (1).
  11. ↑ 11.0 11.1 11.2 Pecora and Fahey, The Corrego Frio Pegmatite, Minas Gerais: Scorzalite and Souzalite, Two New Phosphate Minerals, (1949) American Mineralogist: 34: 83
  12. ↑ Gemstoneindex.net. /gemstones/brazilianite.html "Brazilianite gem". Retrieved 2013-10-21.
  13. ↑ De Brito Barreto, S.; Bittar, S. M. B. (2010). "The gemstone deposits of Brazil: occurrences, production and economic impact". Boletín de la Sociedad Geológica Mexicana 62 (1).
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