NAMBULITE AND NATRONAMBULITE OCCURRENCE IN THE MANGANESE BELT, BISTRIŢA MOUNTAINS,EASTERN CARPATHIANS, ROMANIA

Abstract

Nambulite, (Li,Na)Mn4Si5O14(OH), and natronambulite, (Na,Li)(Mn,Ca)4Si5O14(OH), occur in many deposits of Manganese Belt (MnB) in Bistriţa Mountains, Eastern Carpathians, Romania. The nambulite and natronambulite form small bands which cut the old bands/lens of braunite-bixbyite-hausmannite ore. Nambulite has Li>Na and forms a series with natronambulite. The natronambulite has Na>Li and Mn>Ca. The X-ray analyses of MnB nambulite showed that it crystallizes in triclinic system, space group Pī(2). Its unit cell parameters are: a=7.5300 Å, b=11.7360 Å, c=6.7100 Å, Z=2, V=565.00 Å3, α=92.97°, β=95.23°, and γ=106.27°. The natronambulite has: a=7.6200 Å, b=11.7620 Å, c=6.7370 Å, Z=2, V=574.436 Å3, α=92.81°, β=94.55°, and γ= 106.87°. The cell parameters of natronambulite are very close to those of nambulite. The chemical compositions of nambulite and natronambulite of MnB deposits were determined by SEM and wet chemical analyses. Their Li2O and Na2O were determined by AAS and by wet chemical analyses. The wet chemical analyses of nambulite show variation for Li2O=1.05-1.34 wt.% and Na2O=1.28-0.72 wt.%. The natronambulite has Li2O< Na2O, and higher Na2O and CaO contents than nambulite. In natronambulite the Li2O content varies between 0.45-0.65 wt.% and that of Na2O between 1.10-5.96 wt.%. The main MnO compound for both minerals has variations between 38.20-46.01 wt.%, where Mn cations have minor substitutions with Mg, Ca, Ni and Zn. The mineral textural relations show that the oldest mineral of ore is braunite, closed associated with bixbyite/hausmannite. The braunite is cut/substituted by the new nambulite and natronambulite, which in their turn are substituted by leakeite and holmquistite, respectively. Both minerals are closely associated with aegirine and albite. A wide range of many minerals, some of them unusual and new for Romania, occur as veins through the nambulite/natronabulite associations: jacobsite-Q, namansilite, lithiomarsturite, serandite, hematophanite, fairfieldite, yoshimuraite, imandrite, and others. Also, many common minerals like magnetite, hematite, spessartine, albite, microcline, barium feldspars, barite, nickelian phlogopite/kinoshitalite, and others occur. The mineralogy, geochemistry, and tectonically setting of Mn deposits showed their hydrothermal submarine origin related to subduction processes. The source of Li was not from external supply.