Los Archipelago is located 5 km offshore of Conakry, located on Kaloum Peninsula. It is a ring structure formed by peralkaline nepheline syenites and later phonolitic dykes and pegmatites.
Two magmatic suites are known : one is miaskitic, the other is agpaitic. The difference between this two kind of petrographic suite is linked to the different mineralogy (augite, hastingsite, zircon, titanite in the miaskitic one; aegirina, arfvedsonite, Zr-Ti-Nb-REE silicates for the agpaitic suite).
Iles de Los are the type locality for villiaumite and serandite, described by Lacroix in 1908 and 1931 respectively.
Odinite was discovered in sea floor sediments at the mouth of the Koukoure (Konkouré) River near the islands.
References
• Lacroix, A. (1908). Sur l’existence du fluorure de sodium cristallisé comme élément des syénites néphéliniques des îles de Los. C. R. Acad.
• Lacroix, A. (1911). Les syénites néphéliniques de l’archipel de Los et leurs minéraux. Nouv. Arch. Mus. Hist. Nat., 3, 1-132.
• Lacroix, A. (1924). Nouvelles observations sur les syénites néphéliniques des Iles de Los (Guinée). C. r. hedb. Séanc. Acad. Sci., Paris, 178, 1109-1114.
• Lacroix, A. (1931). Les pegmatites de la syénite sodalitique de l’île Rouma (archipel de Los, Guinée française). Description d’un noveau minéral (sérandite) qu’elles renferment. C. R. Acad. Sci. Paris, 192, 189-194.
• Parodi, G. C., & Della Ventura, G. (1987). Steacyite from the Rouma Isle (Los Islands, Republic of Guinea). Neues Jarbuch fur Mineralogie, Mh., Jg. 1987, H. 5, 233-239
• Moreau, C., Ohnenstetter, D., Demaiffe, D., & Robineau, B. (1996). The Los Archipelago nepheline syenite ring-structure: a magmatic marker of the evolution of the central and equatorial Atlantic. Canadian Mineralogist, 34: 281-299.
• Parodi, G. C., & Chevrier, V. (2004). New discoveries in nephelinites from Los island (Republic of Guinea). 5th International Conference "Mineralogy & Museums", Paris, Sept. 5-8th 2004, Bull. de Liaison de la Soc. Franç. de Minéralogie et Cristallographie, 2004, Vol 16, N°2.
• Biagioni, C. (2007). I disilicati di Zr-Ti-Nb-REE delle sieniti a nefelina delle Iles de Los (Guinea). Tesi di laurea inedita, Università di Pisa.
• Biagioni, C., Bonaccorsi, E., Merlino, S., Parodi, G.C., Perchiazzi, N., Chevrier, V. & Bersani, D. (2010). Roumaite, (Ca,Na,)3(Ca,REE,Na)4(Nb,Ti)[Si2O7]2(OH)F3, from Rouma Island, Los Archipelago, Guinea: a new mineral species related to dovyrenite. Canadian Mineralogist, 48, 17-28.
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