Arroja dita, um novo mineral do grupo da Wagnerita

Djalma Guimarães

Resumo


Arrojadite occurs in pegmatites in the gneisses of Serra Branca, near Pedra Lavrada, Picui, Paraiba do Norte. A sample of the analyzed material presents in cleavable massive forms of deep green colour and natural faces were not found in the studied samples. Hardness over 5. It often encloses minute crystals of hematite and quartz. Inclusions of euhedral crystals of cassiterite are fairly common. It also shows abundant inclusions surrounded by pleochroic haloes. The following analysis of a sample with little amount of inclusions shows the relation between Arrojadite and inclusions: Cassiterite 0,61 per cent Hematite 1,29 Arrojadite 98,10 100,00 per cent Arrojadite alters to a dark red mineral which probably is monoclinic. Its index of refraction and birefringence are higher than that of Arrojadite. The alteration sets in from the exterior of the crystal and from the cleavage craks. This is a common alteration and the product of it is a biaxial mineral, optically positive (+ ), vellow, and grayish when in thin sections. 2V r = 16° The indexes of refraction are moderately high: Y = 1,724 p = 1,703 ( ± 0,001), a = 1,703 ( ± 0,001) Birefringence o Y-a = 0,021, Y-p = 0,0205, fl-a = 0,0004 In thin sections it is yellow with m a rk e d pleochroism Z = d a rk red, Y = orange yellow, X = brilliant yellow The maximum extinction angle in sections parallel to the cleavage planes is about 2° 15’ The crystallographic characters of arrojadite are: monoclinic, cleavage parallel to (110) is generally highly developped, and the plane of the optic axes is parallel to the cleavage planes. The relations between cleavage planes and crystallographic axes are shown in fig. 1. Optically, A rrojadite is negative (— ) Axial plane parallel to (110); 2 Vr = 82° The index of refraction is moderately high y = 1,657. Double refraction low: Y-« = 0,0079, Y - P — 0,0034, P - a = 0,0045. In thin section colourless, or faintly pleochroic: Z = pale green Y = incolour X = incolour Regarding chemical composition Arrojadite seems to be the same kind from Black Hills, South Dakota, which was described by W.P Headden in the American Journal of Science (1). This mineral and Arrojadite show the same megascopic features. The analyses shown at the pag. 8 give us an idea about the resemblance of these two minerals. The presence of F e 20 3 in the I and 11 analyses is due the that the samples analyzed h ad 2 to 3 per cent of hematite. The higher percentage of H20 is due to the alteration of the Arrojadite. Beside, the author gives a detailed discussion on the question of the priority of the name Arrojadite based, chiefly, on B. Masons works published in the Geologiska Forenniges I Stockholm Forhandlinger, of March-April 1941. This latter author ends by giving the priority to the name Arrojadite over the name Headdenita of Quensel. The author proposed the name Arrojadite to the inoxidized m ineral, which represents, when pure, the Headden’s phosphate from Black Hills, South Dakota, U.S.A

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DOI: http://dx.doi.org/10.11606/bmffclusp.v0i5.121334

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