Luigi Marinoni

Mineralogical and geochemical characteristics (major, minor, trace elements and REE) of detrital and authigenic clay minerals in a Cenozoic sequence from Ross Sea, Antarctica

Abstract The mineralogy and geochemistry of the clay fraction of Victoria Land Basin (Ross Sea, Antarctica) sediments was investigated, to determine the origin of clay minerals and the features of authigenic smectite. The investigated core (CRP-3) is ~800 m long, mostly of Oligocene age. The clay fraction of the upper sequence consists of mica, chlorite and detrital smectite, while that of the central and lower part is largely made up of authigenic smectite. Authigenic smectites are ditrioctahedral, with a composition close to saponite, while detrital smectites such as Al-Fe beidellites are dioctahedral. Authigenic smectites have no illite mixed layers, show a higher degree of crystallization, higher MgO, Fe2O3, V, Cr, Co, Ni and Sc contents and lower SiO2, Al2 O3, K2O, TiO2, Ba, Rb and Zr contents with respect to detrital clay minerals, and a clear depletion of LREE with respect to HREE. Authigenic smectite formed from the alteration of volcanic materials and clay minerals.

XRPD patterns of opals: A brief review and new results from recent studies

A new classification of opals through X-ray powder diffraction (XRPD) methodology, by analysing 75 new samples of opal came from different worldwide areas, is introduced. A brief historical summary of the application of XRPD analysis on opals and the most important XRPD results reported in literature were compared with the newly obtained XRPD data. A simple method for the classification of opals on the basis of their degrees of structural order-disorder calculated from the diffraction data is proposed. In addition, a clear boundary, which has not been previously described by others in literature, related to the presence (or absence) of two-peak characteristic of the cristobalite phase is identified. This boundary allows for a discrimination of opals C from CT.

Clay minerals in late Quaternary sediments from the south Chilean margin as indicators of provenance and palaeoclimate

Abstract An investigation of bulk-rock and clay-fraction compositions of two sedimentary cores from southern Chile was performed to evaluate the record of temporal climatic changes during the late Quaternary (11 ky and 30 ky BP). The bulk mineralogy shows an abundance of feldspars, mica and quartz, with lesser chlorite, amphibole and pyroxene, and variable amounts of carbonates. The clay fraction consists of illite, chlorite and scarce smectite. Smectite shows platy morphology, an Al-Fe beidellite chemical composition, and is detrital. Smectite, together with biogenic carbonate, increases in levels diagnostic of warmer phases. Increases in smectite are attributed either to the beginning of chemical weathering, allowed by the glacial retreat, or to ice extension and sea-level variations. Warmer climates also favoured the increase of carbonate productivity. Levels diagnostic of colder phases show a large decrease in carbonate, small amounts of smectite and large amounts of chlorite and mica, as the abundance of glaciers reduced the productivity and prevented chemical weathering.

Clay mineral assemblages as indicators of hydrothermalism in the basal part of the CRP-3 core (Victoria Land Basin, Antarctica)

Abstract The CRP-3 drilling project collected sediments from 3 to 939 mbsf (metres below sea floor) in the Victoria Land Basin in Antarctica.The upper sequence (down to ~790 m bsf) is of Cenozoic age and made up of detrital glaciogenic sediments; the characteristics of clay minerals in this part have been reported elsewhere.Here, the compositional features of clay minerals in the lower sequence such as conglomerates, Devonian sandstones and dolerites are described and genetic processes clarified.Clay minerals in the deepest part of the sequence derive from the alteration of different lithologies that mostly make up the sedimentary basin. Two clay mineral assemblages were characterized through analysis by X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM).From 790 to 823 mbsf, samples consist of authigenic smectite of variable chemical composition forming imbricated texture of plates or flakes.The smectites probably result from hydrothermal/diagenetic transformation of earlier minerals.The primary smectite cement underwent reorganization during shearing and cataclasis.The lowest part of the sequence (below 823 mbsf) is characterized by an assemblage of kaolinite, mixed-layer illite-smectite, Fe oxyhydroxide, sporadic smectite and poorly crystallized illite.It reflects a stronger alteration process than that recorded in the upper units of core CRP-3, related to hydrothermalism connected with the intrusion of an igneous body.Both assemblages show clear differences in particle morphology, texture and smectite composition to the clay assemblages found in the Cenozoic glaciomarine sediments in the upper sequence.The different phases of alteration appear related to the processes of rifting, exhumation and faulting that characterized this region since the Mesozoic.

Grain size, mineralogy and geochemistry in Late Quaternary sediments from the Western Ross Sea outer slope as proxies for climate changes.

Textural, mineralogical and geochemical investigations of three sedimentary sequences from the Ross Sea continental slope allow to give some important indications on climatic and environmental changes occurred during the Late Quaternary. The cores show cyclical changes in several proxies (grain size, mineralogical and geochemical parameters) which are in phase with glacial/interglacial changes (MIS 1–8). Such fluctuations are supposed to be driven by changes in transport mechanisms, reworking and provenance of the material, as well as by changes in direction and strength of marine currents induced by variations in the ice coverage.

Physical and chemical properties of some italian opals

The physical and compositional properties of some opals from different parts of Italy have been investigated through several methodologies like optical analysis, specific gravity, refractive indices, xrpd, ir spectroscopy, la-icp-ms. The opals show different colors: white, white brownish, white yellowish, white yellowish greenish and greyish. Black and metallic inclusions, consisting of todorokite, are sometimes present. Play of color have not been observed but some opals show small iridescence zones; opals are inactive to the long and short wavelength uv radiation (366 - 254 nm) with the exception of one sample and also phosphorescence is absent. Refractive index and specific gravity values are n = 1.43 - 1.44 and G = 2.07 - 2.33 g/cm 3 in agreement with literature. xrpd analyses highlighted Italian opals are A, CT and C types, but most of them can be classified as CT opals. IR spectroscopy data confirmed the opal classification. The most abundant elements are Mg (between 400 and 900 ppm), Fe (35-400 ppm), Ca (70-96 ppm) and Ni (20-70 ppm). Similarly to what observed in other opals worldwide, Fe appears to be the principal factor that determines the white brownish color and of the yellowish shade. Chromophore elements like V, Cr, Cu, Ti, Co and Ni are present in very low contents and do not influence the physical properties of the Italian opals. Mn is clearly detected (42 ppm) only in the sample n. 2 and is related to the presence of dendrites. Ca and Mg (non chromophore elements) are probably related to the matrix. On the whole the investigated Italian opals show a rather homogeneous trace element composition that appear well differentiated from that of other opals worldwide.

Investigation on the gemological, physical and compositional properties of some opals from Slovakia ("Hungarian" opals)

The physical and compositional properties of some “Hungarian” opals have been investigated through several methodologies such as optical analysis, specific gravity, refractive indices, AvaSpec spectroscopy, XRPD, IR and Raman spectroscopy, LA-ICP-MS. The “Hungarian” opal deposits, now in Slovakia, represented the largest and most significant gem opal deposits in Europe from the Roman Times to XIXth century. The analysed opals include brown semiopals, white opals and hyalite. In general, chromophore elements like V, Cr, Cu, Ti, Co are present in very low concentrations and do not influence the physical properties of the opals. Elements like Na, Mg, Al, K, Ca, Sc, Cr and Fe have been detected, in variable amounts, in all the investigated samples. The semiopals can be used as ornamental and semiprecious material, are mostly made up by CT opal close to pure tridymite and lower amounts of goethite, show high trace elements contents while the REE pattern show a clear Eu negative anomaly. These opals probably formed during an hydrothermal, or late magmatic stage. The white opals are made up by amorphous opal (A), show appreciable play of color and iridescence and are surely the most relevant for gemological purposes. The most pure white opals show low contents of trace elements and a rather homogeneous composition, that can be used as geographical marker. Conversely some opals affected by contamination from host rocks show much higher trace element contents. White opals probably formed during a low temperature tectonic event. The samples of hyalite are yellowish, opaque and greasy. They are made up by CT opal show relevant content of B and As while the other trace element are quite low, and are probably of volcanic origin.

Gemological, physical and chemical properties of prase opals from Hanety Hill (Tanzania)

Chemical, physical and gemological properties of some green “prase” opals from Hanety Hill in Central Tanzania were investigated. The color of the opals ranges from clear green to apple green, diaphaneity from translucent to semitransparent, luster is vitreous, and all result inert to UV lamp radiations. Specific gravity values are between 2.11-2.13, refraction indices between 1.439 and 1.458, comparable with literature data. XRD and FTIR analyses show the opals belong to the CT type, with tridymite higher than cristobalite. SEM observations revealed a lepispheric and mammillary structure formed by spherules with a diameter around 10-12 μm, on their turn composed of amorphous silica microspheres. The opals show a homogeneous chemical composition with very high contents of Ni and lower of Mg, Ca, Fe and transition elements like Zn, Cr and Co. Al and K are nearly absent. The chemical composition of the opals reflects that of serpentine rocks. Because of its abundance, the cromophore element responsible for the green color is Ni. Prase opals may have formed through the process of low temperature metamorphism, which generated the serpentinite from a magmatic ultrafemic rock.

AVASPEC 2048: an innovative spectroscopic methodology to differentiate the natural emeralds from the synthetic ones

New and sophisticated synthetic gems are constantly introduced in the market making the identification of them very difficult, if one uses only the microscope. In this paper we present the results obtained from a new methodology, the AvaSpec-2048 spectrometer, that can help to identify the natural emeralds from the synthetic ones. The AvaSpec-2048 spectrometer is an original instrument that can acquire electromagnetic spectra in a range between 400 nm and 1000 nm (VIS-NIR), and can be used to identify gemological materials allowing an accurate discrimination of natural products from the artificial ones. The physical and optical properties, microscopic features and absorption spectra of natural and synthetic emeralds have been also investigated through others traditional methodologies such as optical analysis, specific gravities, refractive indices and Vis-NIR spectroscopy. The comparison of the absorption spectra obtained with the AvaSpec-248 spectrometer allows identifying and distinguishing the origin of the analyzed gems.