Brazilianite

Composition

Brazilianite, NaAl3(PO4)2(OH)4 is a hydrous sodium aluminium phosphate that forms through the metasomatic alteration of amblygonite-montebrasite. Amblygonite, LiAlPO4F in combination with quartz goes through an OH-F exchange to make montebrasite, LiAlPO4{F,OH} at temperatures greater than 480 °C. 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. There are many substitution possibilities in the brazilianite formula. Besides sodium, being replaced by any other element, iron can replace aluminium, and vanadates or arsenates can replace the phosphates.

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. The Al-octahedra has two types of octahedral coordination: trans-AlO4(OH)2 and trans-AlO3(OH)3. The two phosphorus atoms in brazilianite are coordinated in a tetrahedral with four oxygen atoms each. The sodium atom is located within the P-O and Al-O polyhedral in an irregular cavity. The coordination of the sodium is best described as the uncommon seven-coordination. The presence of a hydrogen ion in the same cavity where a sodium ion 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. Gatehouse et al., 1974 described the four remaining hydrogen as being in a chain and contributing to the complexity of the structure but Gatta et al., 2013, 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. The splitting of this hydrogen has not been explained why it happens but it was shown that it can affect the hydrogen bond configuration. Some of the oxygen atoms in the four OH groups in brazilianite act as donors and some as acceptors of the hydrogen bond. One of these oxygen items is both a donor and acceptor to accommodate the hydrogen that split into two.

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. Most common forms that are measured in brazilianite {010}, {110}, { 11}. It displays a perfect cleavage on (010), it is brittle and has a conchoidal fracture. 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. Brazilianite has a vitreous luster, has a white streak, and the mineral is translucent to transparent. The color of brazilianite ranges from dark yellow-green to a pale yellow. Brazilianite begins to lose its color when heated to 200 °C and becomes colorless when it is heated to 300 °C.

Geological occurrence

Brazilianite is typically found in granite pegmatite and it is often found within the cavities within the pegmatite where quartz, beryl and mica are also found. Brazilianite also found with tourmaline and feldspar. The sequence of the mineral formation in the pegmatite in Brazil had not been determined. The sequence of mineral formation in New Hampshire was quartz, brazilianite, apatite, whitlockite, and quartz. 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. Brazilianite has been described from other granite pegmatites in Brazil and the United States. It has also been found in different locations in the world, including Rwanda, Yukon Creek in Canada, Argentina, China, France, and Australia.

Special characteristics

Brazilianite is sometimes used as a gemstone. Brazilianite is relatively new phosphate minerals along with amblygonite, turquoise and apatite that are used as gemstones. Brazilianite is often confused with amblygonite, apatite, chrysoberyl, beryl, and topaz.{{cite web |url-status=dead

References

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References

1. Warr, L.N.. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine.
2. [http://webmineral.com/data/Brazilianite.shtml Brazilianite data on Webmineral]
3. [http://www.mindat.org/min-760.html Brazilianite on Mindat.org]
4. Baldwin, J.R.. (2000). "Exotic aluminium phosphates, natromontebrasite, brazilianite, goyazite, gorceixite and crandallite from rare-element pegmatites in Namibia". Mineralogical Magazine.
5. Scholz, R.. (2008). "Amblygonite-montebrasite from Divino das Laranjeiras Mendes Pimentel pegmatitic swarm, Minas Gerais, Brasil. II. Mineralogy". Romanian Journal of Mineral Deposits.
6. Gatta, G.D.. (2013). "Neutron diffraction in gemology: Single-crystal diffraction study of brazilianite, NaAl₃(PO₄)₂(OH)₄". American Mineralogist.
7. Pough, F.H. (1945). "Brazilianite, a new phosphate mineral". American Mineralogist.
8. Frost, R.L.. (2012). "Molecular structure of the phosphate mineral brazilianite NaAl₃(PO₄)₂(OH)₄-A semi-precious jewel". Journal of Molecular Structure.
9. Gatehouse, B.M.. (1974). "The crystal structure of brazilianite, NaAl₃ (PO₄)₂(OH)₄". Acta Crystallographica Section B.
10. Frondel, C.. (1948). "Second occurrence of brazilianite". American Mineralogist.
11. Čobić, A.. (2011). "Crystal morphology and xrd peculiarities of brazilianite from different localities". Natura Croatica.
12. Different habits of brazilianite have been found in different locations. Brazilianite is often found with muscovite. The Corrego Frio pegmatite where brazilianite is found in Brazil is an altered pegmatite dike that had weathered [[biotite]] schist between its walls. In New Hampshire, the pegmatite where the brazilianite was found was made up of 99 percent [[albite]], mica, and quartz.Pecora and Fahey, ''The Corrego Frio Pegmatite, Minas Gerais: Scorzalite and Souzalite, Two New Phosphate Minerals,'' (1949) American Mineralogist: 34: 83
13. Firefly Guide to Gems By Cally Oldershaw