The orange opals of Buriti dos Montes city occur mainly as veins and venules in Serra Grande Group sandstones, where intruded diabases of the Sardinha Formation. Also cementing breccias of the contact zone between host rock and dykes, as secondary deposits type colluvial and alluvial. These opals have homogeneous and massive structure because to filling of the silica spheres interstices by opaline cement. The spheres show an irregular arrangement and the diameters vary of 150 to 500 nm. The solids inclusions observed in the opals were grouped as: bouble (filled, empty and microconcretion), botrioidal (coral reefs, stalactite, stalagmite e column), dendritic, flow structure, fracture (filled, dry, tension, surface and cracking), lamellar, color patch, microcrystal, nodular, web and tube. The mineralogical analysis showed different degree of crystallinity, of opal-A (amorphous) to opal-CT (cristobalite–tridymite). Most inclusions is constituted of kaolinite, quartz and hematite. The associated quartz shows zoning patterns in cathodoluminescence characteristic of hydrothermal origin. The opals present 90,14 % of SiO2, 1,32 % of Al2O3, 0,2 % of Fe2O3 and 8,03 % of water. The high contents of Ba, 195 to 1373 ppm, are related to occurrence of barite in veins within sandstones, a typical mineral of hydrothermal environment, and frequently find in the Parnaíba Basin. Based in the discussed data, a simplified geologic model is proposed to the formation of the orange opals of Buriti dos Montes city, where diabases intrusions were essential as the heat source, heating the hydrothermal solutions, the sandstones contributed with the necessary silica to saturation of the solutions, and the fractures were indispensable to trap fluids which formed the mineralized veins.
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