Martti Lehtinen

The Impact Cratering Record of Fennoscandia — A Close Look at the Database

Currently thirty meteorite impact structures are identified in Fennoscandia, which corresponds to ~19 % of the known global record. Eleven of them have been discovered or confirmed during the last decade. This high number is due to intensive, determined research, including deep drilling, and highresolution geophysical regional mapping. The ages and diameters of Fennoscandian impact structures vary considerably, but many are relatively small (<10 km) and mostly of early Paleozoic age. The latter is probably an effect of the regional geological evolution. The majority of craters were formed in complex targets composed of crystalline basement, a sedimentary cover and frequently a water pile on top. Only those that were excavated in the crystalline shield contain bodies of coherent impact melt rock or reveal indications of such. Distal impact-related deposits of some structures have locally survived, especially those that formed in platform areas with virtually uninterrupted sedimentation after impact. These deposits are useful stratigraphic markers. Apart from the confirmed structures, there are presently some sixty additional structures for which an impact origin has been suggested.

Cookeite from the Muiane pegmatite, Zambezia, Mozambique

This paper describes a cookeite occurring as large rosettes crystallized in an open vug in the Muiane granite pegmatite in Zambezia, Mozambique. Optical properties, chemical composition, X-ray powder pattern, single crystal data and infra-red absorption are given. The mineral represents a pseudo-hexagonal one-layer semi-random cookeite with noticeable contents of beryllium and boron.

Natural boehmite single crystals from Ceylon

Single crystals of boehmite, up to 0.1 mm in size, were found in open cavities inside a corundum crystal from the Ratnapura area gem gravels in Ceylon. The unit cell parameters are (X-ray powder pattern): a0=3.695 Å b0=12.212 Å, c0=2.867 Å. The crystallographic orientation is based on X-ray single-crystal precession photographs. The crystals show the faces (it010), (001), (101), and (221). Systematic extinctions agree with the space group Amam. Optical orientation: a∥γ, b∥β, c∥α. Refractive indices are given. The mineral is optically positive with a large optic axial angle.

New Evidence for Impact from the Suvasvesi South Structure, Central East Finland

The circular Suvasvesi South structure (diameter about 3.8 km) is located in Central East Finland (62°35.8′N, 28°13′E) and correlates with the Haapaselka open lake area, the southern of the two Suvasvesi lakes. Suvasvesi S was first noticed in satellite images and might form a crater doublet with the proven Suvasvesi N impact structure. We have previously presented evidence, such as presence of fractured target rocks and shatter-cone boulders on the eastern shore of Haapaselka, which suggest that the Suvasvesi South is also an impact structure. During the summer 2002 we carried out a field survey in the area of the Suvasvesi lakes, which led to additional discoveries of shatter cones in boulders. We also discovered impact melt boulders in gravel pits along the roadsides, about 5 km southeast of the structure. Microscopic studies of the thin sections of impact melt and of the shatter cone boulders reveal the presence of well developed and decorated PDFs in quartz grains, maskelynite, fluidal textures between impact melt mineral clasts and kink bands in micas. Consequently, the melt rock is considered to be of impact origin. It is unlikely that the boulders with shatter cones and impact melt blocks were derived from the northern structure, because material transported from it by ice drift would not have passed this area. The outcrops on the islets of the Suvasvesi South area are heavily fractured with subvertical NNW-SSE and ENE-WSW trending; however the fracturing may be related to the Svecofennian tectonic deformations occurring in this area. Also, the outcrops show shatter cone features with a maximum of 50 cm in size. We interpret the shatter cone features to be of impact origin because of their shape and because the orientation of their apices differ from the other deformation systems. However, thin section analysis from outcrop specimens has not shown impact evidence so far. The new findings suggest the presence of an eroded impact melt layer in the southern structure. Bathymetric and airborne magnetic data point to a distinct structure of smaller dimension than the northern one. Preliminary paleomagnetic measurements on the granitic host rocks of Suvasvesi South reveal two components, of which one (steep downwards) is probably either a hard viscous remanence of present age or a Svecofennian age feature. The other one is poorly defined, but has a southwest direction similar to that isolated for the northern structure and could be related to impact.

Acid dissolution of uranophane and carnotite

Abstract Uranium dissolution from rock samples containing uranophane or carnotite was tested in acid mineral salts solutions in the presence and absence of Fe‐ and S‐oxidizing bacteria (Thiobacillus ferrooxidans) and with FeSO4 or S0 as the substrate. The addition of FeSO4 enhanced the solubilization of uranium from uranophane. S0‐amendment was also effective, because uranium solubilization was enhanced at low pH resulting from the bacterial oxidation of S0. Uranium was readily dissolved from carnotite. However, the carnotite‐containing rock sample was predominantly composed of calcite that excessively consumed acid, increasing the pH and thereby reducing the concentration of uranium dissolved in leach solution.

Degree of Al/Si order in potassium feldspars. A combination of X-Ray and infrared data

Unit-cell parameters, c*/b* ratios, Δbc values (Stewart and Ribbe, 1969), Or contents and the positions of two of the characteristic infared absorption bands are presented for 16 K-rich and for 5 Na-rich alkali feldspar samples. A more specific interpretation is given for the characteristic absorption bands. It is shown that the position of one of the infrared absorption bands depends on the degree of Al/Si order whereas the other depends both on the degree of Al/Si order and to a lesser degree on chemical composition.It is recommended that a combination of X-ray and infrared data is useful in studying certain mineralogical and petrogenetic problems associated with the alkali feldspars.

Natural bismuth vanadate from the Mutala pegmatite area, Mozambique

Bismuth vanadate (microprobe test) in varying shades of orange color and in well developed crystals (averaging 0.2 mm in size) occurs in bismutite in the Mutala granite pegmatite area, district of Zambezia, Mozambique. Two modifications of BiVO4were identified. An orthorhombic form is identical with pucherite and shows a0 = 5.336 Å, b0 = 5.053 Å, c0 = 12.021 Å. The crystal habit ranges from platy to stout prismatic. The X-ray powder pattern of the monoclinic form matches that of the synthetic monoclinic Bi-orthovanadate with a0 = 5.205 Å, b0 = 11.718 Å, c0 = 5.098 Å, β = 90° 25′. The crystal habit resembles that of a pyramidal scheelite crystal with the b-axis corresponding to the scheelite c-axis. Multiple twinning is seen on (101), in some instances with a composition plane (010).

A note on the feldspars in the granite xenoliths of the nyiragongo magma

The lavas of the Nyiragongo volcano in Eastern Zaire contain partially fused granite xenoliths. The relictic feldspars found in these xenoliths were studied by microprobe analysis and by X-ray diffraction methods.Some xenoliths represent originally two-feldspar granites, in others only alkali feldspars or those of an anorthoclase composition were detected. All feldspars are homogeneous without perthitic textures detectable under the microscope. In thebc-diagram of Stewart and Wright all feldspars plot on the analbite-high sanidine join. These feldspars have been perfectly disordered through heating in the melilite-nephelinite magma of the volcano.