During the caries process complex reactions involving calcium, phosphate, hydrogen and fluoride ions as main species take place. In this study the precipitation and dissolution reactions occurring in suspensions of enamel, hydroxyapatite (HAP) and fluorapatite (FAP) on addition of fluoride were investigated under well-defined conditions. pH and pF were monitored; calcium and phosphate concentrations were measured at selected times; the solid phases were examined by infra-red, X-ray diffraction and transmission electron microscopy. Precipitation of phosphate-containing calcium fluoride crystals, CaF2(P), can cause severe reduction in the calcium ion concentration and release of hydrogen ions from the precipitated phosphate. These reactions result in considerable dissolution of enamel, HAP and even of FAP. More of the added mineral dissolves with 50 mmol/l fluoride than with 10 mmol/l fluoride, mainly due to the greater reduction in calcium ion concentration. This work shows that phosphate-containing calcium fluoride is most likely an important compound to be considered in the caries process.
A new mineral, eldfellite, was found among fumarolic encrustations collected in 1990 on the Eldfell\ud volcano, Heimaey Island, Iceland. Associated minerals are ralstonite, anhydrite, gypsum, bassanite,\ud hematite, opal and tamarugite, as well as a presumably new mineral with the composition Na3Fe(SO4)3.\ud Along with opal and tamarugite, eldfellite forms soft and fragile aggregates built of thin, platy crystals\ud of micrometre size. The mineral is yellowish-green to greenish-white, with a white streak. The\ud calculated density is 3.062 g/cm3. Eldfellite is monoclinic, C2/m, a 8.043(4) A ˚ , b 5.139(2) A ˚ , c\ud 7.115(4) A ˚ , b 92.13(2)º, Vuc 293.9(2) A ˚ 3, Z = 2 and is isostructural with yavapaiite[KFe (SO4)2]. The\ud strongest lines in the powder diffraction diagram are [d (A˚ ), I (relative to 10)]: 3.72, 8; 3.64, 5; 3.43, 5;\ud 2.77, 10; 2.72, 6; 2.57, 3; 2.370, 6; 1.650, 3. Theche mical analysis and theX-ray diffraction data of\ud eldfellite correspond to those of the synthetic compound NaFe(SO4)2
Hook M, Christoffersen J, Christoffersen MR, Leonardsen ES, Rassing MR, Rostrup E: Effects of aluminum (III) and fluoride on the demineralization of bovine enatnel: a longitudinal mieroradiographie study. Seand J Dent Res 1994; 102: 198-201. © Munksgaard, 1994. Longitudinal microradiography has been used to determine inhibiting effects of aluminum (III), Al, and lluoride on mineral loss from slices of bovine enamel exposed to a demineralizing solution 4 h daily for 35 days. Inhibitor treatment was 5 min four times daily. For the remaining time, the samples were immersed in a neutral calcium phosphate solution which allowed neither remineralization nor demineralization. This study indicates that a 1-mM (27 ppm) solution of Al in a 0.1-M Iactate solution, pH 5, has an inhibitory effect on the in vitro demineralization of bovine enamel. Application of this solution alternating with 20 mM (380 ppm) fluoride gave the same total inhibition as treatment with 20 mM fluoride alone.... -
Heklaite, with the ideal formula KNaSiF6, was found among fumarolic encrustations collected in 1992 on the Hekla volcano, Iceland. Heklaite forms a fine-grained mass of micron- to sub-micron-sized crystals intimately associated with malladrite, hieratite and ralstonite. The mineral is colourless, transparent, non-fluorescent, has a vitreous lustre and a white streak. The calculated density is 2.69 g cm–3. An SEM-EDS quantitative chemical analysis shows the following range of concentrations (wt.%): Na 11.61–12.74 (average 11.98), K 17.02–18.97 (average 18.29), Si 13.48 –14.17 (average 13.91), F 54.88–56.19 (average 55.66). The empirical chemical formula, calculated on the basis of 9 a.p.f.u., is Na1.07K0.96Si1.01F5.97. X-ray powder diffraction indicates that heklaite is orthorhombic, space group Pnma, with the following unit-cell parameters: a = 9.3387(7) Å, b = 5.5032(4) Å, c = 9.7957(8) Å , V = 503.43(7) Å3, Z = 4. The eight strongest reflections in the powder diffraction pattern [d in Å (I/I0) (hkl)] are: 4.33 (53) (102); 4.26 (56) (111); 3.40 (49) (112); 3.37 (47) (202); 3.34 (100) (211); 2.251 (27) (303); 2.050 (52) (123); 2.016 (29) (321). On the basis of chemical analyses and X-ray data, heklaite corresponds to the synthetic compound KNaSiF6. The name is for the type locality, the Hekla volcano, Iceland.
The new mineral jakobssonite, ideally CaAlF5, was first found in crusts collected in 1988 from a fumarole on the Eldfell volcano, Heimaey Island, Iceland. It was subsequently found in similar crusts collected in 1991 from a fumarole on the Hekla volcano, Iceland. It is associated with leonardsenite (IMA2011-059), ralstonite, heklaite, anhydrite, gypsum, jarosite, hematite, opal and several fluoride minerals that have not been fully characterized. Jakobssonite occurs as soft white fragile crusts of acicular crystals <50 μm long. Its calculated density is 2.89 g cm–3. Chemical analyses by energy-dispersive spectrometry on a scanning electron microscope produced a mean elemental composition as follows: Ca, 18.99; Al, 18.55; Mg, 1.33; Na, 0.33; F, 50.20; O, 10.39; total 99.79 wt.%. The empirical chemical formula, calculated on the basis of 7 atoms per formula unit with all of the oxygen as OH, is (Ca0.73Mg0.09Na0.02)Σ0.84Al1.06F4.09(OH)1.01. Jakobssonite is monoclinic, space group C2/c, with a = 8.601(1), b = 6.2903(6), c = 7.2190(7) Å, β = 114.61(1)o, V = 355.09(8) Å3and Z = 4. The crystal structure contains chains of [AlF6] octahedra which run parallel to the c axis. These chains are interconnected by chains of [CaF7] pentagonal bipyramids. Jakobssonite is isostructural with several other CaMIIIF5 compounds. The eight strongest lines in the powder diffraction diagram [d in Å (I) (hkl)] are as follows: 4.91 (18) (110), 3.92 (76) (200), 3.15 (68) (020), 3.13 (100) (11̄2̄), 2.27 (22) (22̄2̄), 1.957 (21) (400), 1.814 (20) (13̄2̄), 1.805 (22) (204̄). The chemical and crystal-structure analyses of jakobssonite are similar to synthetic CaAlF5 with minor substitutions of light elements (e.g. Na) or vacancies for Ca, and OH for F.
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