Peralkaline Rocks

The Kenya Rift Valley contains many ochre sources that are currently used by indigenous peoples for adornment, rituals, and art. Ochre pigments occur in rock art and archaeological sites spanning over 250,000 years. Chemical analysis for provenience of geological sources is the first step in the process of reconstructing provenance of archaeological artifacts for cultural heritage, archaeological, and paleoanthropological research. Development of an ochre source chemical composition database can facilitate reconstruction of social interaction networks and cultural heritage conservation efforts in this region. Techniques such as Laser Ablation-Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) and Instrumental Neutron Activation Analysis (INAA) are often used for compositional analysis and sourcing of ferruginous mineral pigments. Sourcing has proven challenging due to the diverse range of rocks and minerals that are classified as red and yellow ochres, and the diverse processes...

The Kenya Rift Valley contains many ochre sources that are currently used by indigenous peoples for adornment, rituals, and art. Ochre pigments occur in rock art and archaeological sites spanning over 250,000 years. Chemical analysis for provenience of geological sources is the first step in the process of reconstructing provenance of archaeological artifacts for cultural heritage, archaeological, and paleoanthropological research. Development of an ochre source chemical composition database can facilitate reconstruction of social interaction networks and cultural heritage conservation efforts in this region. Techniques such as Laser Ablation-Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) and Instrumental Neutron Activation Analysis (INAA) are often used for compositional analysis and sourcing of ferruginous mineral pigments. Sourcing has proven challenging due to the diverse range of rocks and minerals that are classified as red and yellow ochres, and the diverse processes...

The new mineral umbrianite, ideally K7Na2Ca2[Al3Si10O29]F2Cl2, was discovered as an essential groundmass mineral in melilitolite of the Pian di Celle volcano, Umbria, Italy. It forms rectangular, lamellar or lath-shaped crystals (up to 25 x 30 x 200 μm in size), typically flattened on {010}, and sheaf-like aggregates (up to 200-500 μm across). Umbrianite is mainly associated with kalsilite, leucite, fluorphlogopite, melilite, olivine (Fo>60), diopside, nepheline, Ti-rich magnetite, fluorapatite, cuspidine–hiortdahlite series minerals, götzenite, khibinskite, monticellite-kirschsteinite, westerveldite, various sulfides and peralkaline silicate glass.
The empirical formula (based on Si+Al+Fe3+=13) for the holotype umbrianite (mean of 58 analyses) is (K6.45Na0.35(Sr,Ba)0.01)Σ6.81(Na1.22Ca0.78)Σ2.00(Ca1.85Mg0.13Mn0.01Ti0.01)Σ2.00[(Fe3+0.34Al3.06Si9.60)Σ13.00O29.00]F2.05Cl1.91(OH)0.04. The X-ray diffraction powder-pattern (MoKα -radiation) shows the strongest lines {d [Å](Iobs)} at: 9.65(100), 6.59(97), 3.296(77), 3.118(70), 2.819(53), 2.903(52), 6.91(43). The unit-cell parameters and space group are: a = 7.0618(5), b = 38.420(2), c = 6.5734(4) Å, V = 1783.5(2) Å3, Pmmn, Z = 2. The calculated density is 2.49 g/cm3. The crystal structure of umbrianite has been refined from X-ray single-crystal data to R1 = 0.029 %. R = 0.0941. The strong bands in the Raman spectrum of umbrianite are at: 525, 593, 735 and 1036 cm-1. Umbrianite from Pian di Celle is unstable to postmagmatic alterations and partially or completely replaced by Ba-rich hydrated phases, and one of them is very close to günterblassite. Umbrianite and günterblassite represent a new structural type of phyllosilicates with triple-layer Si-Al tetrahedral blocks. The structural and chemical comparison of these minerals with members of the rhodesite silicate mero-plesiotype series (double-layer block) and mountinite family (single-layer block) is given.