Sphene-a geological chameleon
- Univ. of Vermont, Underhill, VT (United States). Geology Dept.
Synthetic sphene, CaTiO(SiO[sub 4]), is colorless and the natural, rock-forming mineral, yellow-brown-red-black. Sphene is:(Ca, U, Th, Pb, REE)(Ti, Fe[sup 3+], Al, Nb, Cr) (O,F)(SiO[sub 4]). Color appears to change and deepen with geologic age and radiation dosage, based upon study of 100-200 samples concentrated from calc-alkaline and alkaline plutonic rocks. Maximum absorption of light coincides with Z, the slow vibration direction, and high index of refraction, n=1.943-2.110, along a zig-zag or kinked octahedral chain, made up of: O-Ti-O-Fe-O-Ti-O-Nb-O-Ti-O..., where each O is an octahedral corner, and each transition metal, M, and octahedral center. Octahedra in each chain are cross-linked by (SiO[sub 4]) tetrahedral corners, and Ca ions lie in 7-fold holes. Along a chain, O-Ti-O distances alternate between 1.973 and 1.766 [angstrom]. Short O-M distances favor O-M charge transfer and adsorption of UV, blue, and green light, yielding yellow transmission. Energetic U+Th alpha-particle recoil will induce lattice damage over time (t) where dosage =[alpha]t, or alpha/mg/t and may involve 10[sup 15] alpha events in a Precambrian mineral. High dosage promotes a metamict state with partial loss of crystallinity, strain, expansion, hydration, and partial reduction of Fe[sup 3+] [r arrow] Fe[sup 2+]. Yellow-brown sphene has become black over time. Grains and fragments of black, metamict Precambrian sphene from Ontario and S. Africa, placed between graphite electrodes and energized for 3 secs. at T=7800[degree] in a carbon arc, turned bright yellow on cooling, without having fused. Reoxidation of Fe[sup 2+] [r arrow] Fe[sup 3+] in the arc would induce lattice contraction as r[sub Fe[sup 3+]] < r[sub Fe[sup 2+]]., initiate annealing, and explain the chameleon-like behavior of sphene.
- OSTI ID:
- 5768533
- Report Number(s):
- CONF-9303211-; CODEN: GAAPBC
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Vol. 25:2; Conference: 28. annual Geological Society of America (GSA) Northeastern Section meeting, Burlington, VT (United States), 22-24 Mar 1993; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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