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Eggonite (Kolbeckite, Sterrettite), ScPO4 · 2H2O
- M. H. Hey, Charles Milton, Edward J. Dwornik
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- Journal:
- Mineralogical Magazine / Volume 46 / Issue 341 / December 1982
- Published online by Cambridge University Press:
- 05 July 2018, pp. 493-497
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The curious history of the mineral eggonite is reviewed, and two new occurrences are described. The original specimens, for which Schrauf gave good morphological and optical data in 1879, with a tentative suggestion that it was a cadmium silicate, were fakes; the tiny crystals of the new mineral were glued on to hemimorphite specimens from Altenberg, Belgium. In 1929, Zimanyi edited and published observations by Krenner, who found the mineral on silver ores from Felsöbánya, Hungary, added to Schrauf's physical data, and identified it as an aluminium phosphate. It was not until 1959 that Mrose and Wappner showed that it is scandium phosphate, ScPO4 · 2H2O, and essentially identical with kolbeckite, described by Edelmann in 1926 as a phosphate and silicate of beryllium, aluminium, and calcium from Saxony, and with sterrettite, described by Larsen and Montgomery in 1940 as an aluminium phosphate from Fairfield, Utah.
In 1980 the IMA Commission on New Minerals and Mineral Names, while accepting the identity of the three minerals and rejecting the name sterrettite, were almost equally divided over the names eggonite and kolbeckite, which are thus both acceptable; since eggonite has 47 years priority, we suggest that it should have preference.
The available physical and chemical data on eggonite are summarized and added to, and two new occurrences, at Potash Sulfur Springs, Arkansas, and at Sakpur, Gujarat, India, are described.
Delindeite titanosilicates and lourenswalsite, two new from the Magnet Cove region, Arkansas
- Daniel E. Appleman, Howard T. Evans, Jr., Gordon L. Nord, Edward J. Dwornik, Charles Milton
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- Journal:
- Mineralogical Magazine / Volume 51 / Issue 361 / September 1987
- Published online by Cambridge University Press:
- 05 July 2018, pp. 417-425
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Delindeite and lourenswalsite are two new barium titanosilicate minerals found as microscopic crystals in miarolitic cavities in nepheline syenite in the Diamond Jo quarry, Hot Spring County, Arkansas. Delindeite is found as aggregates of flake-like crystallites in compact spherules, light pinkish grey in colour, with a resinous, pearly lustre. The flakes are biaxial positive with average n ∼ 1.813; the measured density is 3.3 g/cm3. Electron diffraction revealed a monoclinic unit cell in space group C2/m or subgroup, with a = 21.617(13), b = 6.816(5), c = 5.383(3) Å, β = 94.03(5)° (refined from X-ray powder data). The strongest X-ray lines are (hkl, dobs, Irel): (200, 10.80, 100); (311, 3.54, 24); (6̄01, 3.083, 28); (601, 2.888, 31); (2̄21, 2.806, 20); (910, 2.262,18). The crystals are submicroscopically twinned on (100) and also produce additional continuous diffraction streaks parallel to a*, which double the b and c axes. The formula derived from electron and ion probe analyses (H2O by difference), as constrained by density and molar volume data, is approximately (Na,K)2.7(Ba,Ca)4(Ti,Fe,Al)6Si8O26(OH)14, with Na > K, Ba ≫ Ca, Ti ≫ Fe,Al; Z = 1. Lourenswalsite occurs as very thin hexagonal plates in rosettes, silver grey to light brownish grey in colour. The crystals are biaxial negative with very low 2V angle. Indices of refraction are nα = 1.815, nβ ≈ nγ = 1.840; the measured density is 3.17 g/cm = 1.840; the measured density is 3.17 g/cm3. X-ray and electron diffraction show a sharp pseudohexagonal lattice with a = 5.244 Å, but extremely diffuse diffraction streaks normal to the hk0 plane. In these streaks a period of 20.5 Å can be discerned. A hexagonal unit cell with a = 5.244(2) Å, c = 20.49(3) Å can be refined from the powder diffraction data but does not account for some lines, probably because of extreme layer disorder as shown by precession single-crystal patterns. The strong X-ray powder lines are (002, 10.22, 20); (-, 3.93, 20); (111, 2.608, 100); (300, 1.5145, 80); (220, 1.3111, 25). The formula given by microprobe analyses, constrained by density and molar volume data, is approximately (K,Ba)2(Ti,Mg,Ca,Fe)4(Si,Al,Fe)6O14(OH)12 with K > Ba, Ti ≫ (Mg,Ca,Fe), Si > Al > Fe; Z = 1. These minerals are formed under oxidizing weathering conditions, and iron is assumed to be in the Fe3+ state.