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The mica pegmatites are localized in the Uluguru Mountains about 200 Km. WSW of Dar er Salaam.
Larger crystals occur also in Makanjiro pegmatite in the extreme SE of the country in Masasi district.
The Lake Maniara deposit is located about 2.4 Km. W of the shore of Lake Maniara, just S of Moji Moto Hot Springs, or about 110 Km WSW of Arusha, from which town it is reached via the Dodoma-Arusha road.
More resistant emerald crystals might be found in the alluvial placers along the flat shores of Lake Maniara.
The geology of the Maniara lake beds, which are of Pleistocene age: The faulting system of the Rift Valley is of highest importance. The Gregory Rift Valley is formed by a westerly-tilted block bounded on the West by the Maniara fault scarp, and on the East by a series of small normal faults thrown down to the East just outside the eastern margin of the sheet.
The irregular NNE line of the Manyara Escarpment results from a combination of arcuate faults swinging between NE and NNW with a series of NE strike faults. Minor blocking-faulting is produced by a combination in the Hassam area. The strike faults have engendered vertical and horizontal displacement of the bands, and the veins traversing small joints intersecting the biotite rocks which normally strike W 260°-270° in all direction, but predominantly in NW-320° trend.
The most ubiquitous rocks round the area are the mica schist, amphibolites with bands of mica, chlorite schist, amphibolite gneisses, kyanite, almandine, amphibolite gneisses and the highly granitoid gneisses. Two types of gneisses are distinctive in this region. The banded gneisses are of quartz-feldspatic metasedimentary rocks.
The base rocks are meso-and catamorphic schists and gneisses containing lenses of biotite in which occur aggregation of emerald, alexandrite, and corundum.
Usually small pegmatite lenses are also present, apparently formed by the remobilisation of mineral matter in the base rocks and recrystallized along bedding planes.
Working are opencuts exposing biotite mica in which occur the emerald crystals and from which rocks are easily separated.
For each mine we calculated the medium value of lattice parameter.
For each mine we calculated the medium value of chemical analyses (analysed 20 point) by EMPA (Cameca CX 827).
Mn, Se, F, Rb are under detection limits except in few cases. Li and Be are not determined.
Chemical analyses have been normalized at 85%.
H2O was deetermined by EA 1108 Carlo Erba ( gas cromatographer).
Reflectance FTIR (Fourier Transform Infrared Spectra) of gemstones have been acquired using a microscope IRscopeII (Bruker) accumulating 200 scans with a resolution of a cm-1 or better, in the range 7000-600 cm-1. Here are given only the skeletal bands which lie in the interval 1500-600 cm-1.
When the samples were suitably cut in direction parallel or normal to the c axis, polarized spectra were also obtained on the pinacoid or on the basis respectively or both.
According to the group theory 6A2u + 16 E1u infrared modes are predicted: the E1u modes are observed in the spectrum when the electric field is perpendicular to the c-axis (ordinary ray) and the A2u modes are observed for the extraordinary ray (E|| c). Therefore, non degenerate bands (A2u) are observed on pinacoids and show intensity changes in polarization, while degenerate bands (E1u) are detected on basis spectra and show only a lowering in intensity in polarized light.
Lake Manjara Basis
Lake Manjara Prisma
Sumbavanga Solid Spectrum
1. Gubelin E. J 1974 "The Emerald Deposit at Lake Maniara, Tanzania" Lapidary Journal, May 1974, p. 338-344, 346-347, 359-360
2. Sinkankas J. 1981 "Emeralds and Other Beryls" Pensylvania, Chilton Book Co. p. 542-548