Table of Contents
The study of minero-petrographical characteristics and the subsequent textural and structural characterization of an ore mineral represents the first step of each industrial processing whose aim is the beneficiation of each valuable mineral phase present inside the ore. Analytical, mineralogical and petrographical, chemical and multispectral image analyses based investigations, applied to eclogitic ore samples deriving form “in situ” sampling campaigns, were applied. These procedures enabled to point out some textural and structural aspects characteristics of ore litotypes belonging to eclogitic-amphibolitic dikes. Such dikes are characterized by a paragenesis constituted mainly by siliceous gangue and accessory “useful titanium minerals” as rutile [TiO2], “pseudolilmenite”, ilmenite [FeTiO3] and sphene [CaTiSiO5]. In the beneficiation process applied to ore s is of preliminary importance to establish a strategy in order to obtain a recovery of high quality “mono-minerals concentrates” as rutile, “pseudolimenite”, ilmenite and phene.
In the Liguria region, at Piampaludo (Sassello district, Savona), at the foot of Tarme mountain (931 m) in the middle of Orbarina valley, eclogitic rocks containing titanium minerals outcrop.
These rocks are part of a group of outcrops along the East edge of Setri-Voltaggio area where the mesozoic series of Ligurian zone is well represented. It constitutes the internal unit of the Appennines. Even if the the structural characteristics and the stratigraphy are quite similar to those of the Alps, the basal part of this zone is considered, in the literature, representative of the separation betwwen the Alps and the Appennines.
The local outcrops, where the samples were piked up, are constituted by very common lythological types in the ophiolitic areas present in the Western Alps: serpentinites, amphibolites, prasinites, calcic schist with intercalations of granatites lenses. Well developed minerals as pyroxenes, amphiboles, garnets, chlorites, epidotes [Ca2(Al,Fe)3(SiO4)3(OH)] associated with accessory species as rutile [TiO2], “pseudoilmenite”, sphene [CaTiSiO5], magnetite [FeO4, hematite [Fe2O3], pyrite [FeS2] and chalcopyrite [CuFeS2] are hosted inside the lithoclases.
The systematic analytical procedures, as a whole, adopted in this study, to derive information about textural, mineralogical and chemical characteristics, follow the way usually adopted for ores of the same type. These information can be considered as an essential premise for a study, whose aim is an optimization of the recovery of quality mono-mineral concentrates constituted by rutile [TiO2], “pseudoilmenite” and sphene [CaTiSiO5].
The electronic microprobe analyses (EMPA) were carried out by means of a Jeol JSM-50A at 15 kV and with a current of 10 nA and 100 µA, respectively, for the sample and the filament; rutile standard for Ti Ka and olivine for Fe Ka; beam width 1 µm; detection and counting by means of energy dispersion system (EDS).
The digital images were acquired and processed by a multispectral image analysis system (CIP). The main characteristics of the image system is the capability to discriminate the color levels present on a sample image.
The entire system utilized for this study consists of a CCD color camera (Sony) having as output the RGB components of the observed image connected to an optical microscope (Leica Laborlux).
Mineralogical and Petrographical Characterization
The hosting rocks, where the samples were collected, appears macroscopically of a dark green color, very compact, heavy, without porosity or decays, with high mechanical strength, so that to consume, in a very short time during the comminution, the mill liners.
At the microscope they present a relatively diffuse and fine porosity, due to relicts or alterations of previous siliceous phases, amphibole and pyroxene. The structure ranges mainly from granular to crystalloblastic, heteroblastic and strongly anisotropic texture with unconstant idiomorphism and allothiomorphism relationships.
Clinopyroxene is present as big subautomorphous crystals, millimetric, of greenish color, often with a marked prismatic cleavage and with very fine laminar exolutions of “pseudoilmenite” arranged following the crystallographic directions or as granular aggregates of rutile and/or rutile- “pseudoilmenite”. The clynopyroxene presents a particular form of extinction, typical of the Alpine cloromelanites.
Rutile [[TiO2]] is the more abundant accessory constituent. It is present as sparse granules or under “sticks”, but mainly as aggregates and intergrowths associated with “pseudoilmenite” and sphene. These aggregates present variable dimensions ranging from about a hundred of microns to few millimeters.
“Pseudoilmenite” is always present with the rutile: as granular intergrowth and as very fine laminar exolutions, sometime submicroscopics. At transmitted light (optical microscopy) it is present as grains or skeletonized amoeboids.
Mineral-Chemical Analyses by Electronic Microprobe
In the rutile [TiO2] the only accessory element detected is the iron. The iron variations (content) inside the grains of different samples are not significative.
The “pseudoilmenite” presents different chemical characteristics in comparison with those of the rutile; even if, contrary at what appears at optical microscopy, it cannot be considered as an ilmenite, but as a rutile impoverished in Ti and enriched in Fe, Al and V. More specific investigations were developed focusing the attention on the grains (“pseudoilmneite”), previously identified at the optical microscopy, inside the rutile crystals. The analytical results, strictly related with the morphological aspects, enable to affirm as: i) generally, moving away the beam from the laminar aggregates of “pseudoilmenite” (contained inside the rutile) at a distance of 20-30 µm the FeOtot returns to assume constant values corresponding to those of the TiO2; ii) inside the laminar intergrowths the FeOtot content of the “pseudoilmenites” is quite constant ranging from a minimum of 3.23% and a maximun of 3.65%.
Each image sample, as a result of the digitization procedure, is discretized in a finite number of elements (pixels): discrete values are thus associated to each pixel. For B&W images it is only one value: the gray level, for RGB color images it is a short list of three values (one for each component: R-red, G-green and B-blue), function, as previously reported, of the spectral characteristics of the light source, of the “superficial” characteristics of the sample and of the spectral response of the detector.
Rutile (TiO2) – The trends both in the gray level hystogram (B&W image) and in the RGB components (color image) are well defined and distinct from the trends of the other mineral species. For the color image the characteristics of the RGB components
Ilmenite (FeTiO3) – The trends of the hystograms referred to B&W and RGB color images is, as for the rutile, clean and distinct from the other phases. In this case, as in the previuos, the RGB components present a marked dispersion if compared with the distributions obtainable for the B&W image.
The experiments conducted on this set of samples, adopting CIP techniques enable to foresee the possibilty to avoid, in some cases, the electronic microscopy to reach the requested “correct” morphological characterization.