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목차
1. Spinel의 구조의 정의
2.Normal spinel 구조: AB2O4
3.Inverse Spinel 구조 : B(AB)O4
4. Spinel 구조물의 예
5. Atomic Structure of Spinel
2.Normal spinel 구조: AB2O4
3.Inverse Spinel 구조 : B(AB)O4
4. Spinel 구조물의 예
5. Atomic Structure of Spinel
본문내용
etch - in an olivine spinel the Mg would be in octahedral coordination and the Si in tetrahedral, which is what they are in olivine anyway. Below is the structure of olivine, showing two layers of octahedra.
Shown below is the arrangement in spinel. To make the change from olivine to spinel, the continuous rows of octahedra in the top layer have to swap position with the purple terahedra beneath them. The top layer would then look like the top layer in spinel below, but rotated and offset. If we rotate and slide the top layer so the octahedra fit over the tetrahedral voids between octahedra in the bottom layer, and the upward-pointing tetrahedra sit over the rings of octahedra, we have the spinel structure. (This really happens one atom at a time. The geometric description is just a way to allow you to visualize how one structure can evolve into the other. In fact, there is an intermediate form of olivine, beta-olivine, that forms before the final conversion to spinel.)
In both minerals we have close-packed oxygens, silica with tetrahedral coordination and magnesium with octahedral. So how can we gain any space by rearranging the atoms? In olivine the continuous straight rows of octahedra are slightly distorted, probably because of the SiO4 tetrahedra that share octahedral edges. The spinel version of olivine is about 12% more dense. The conversion occurs at around 300-400 km depth in the mantle. The reason we don't find olivine spinels at the surface is probably that the +4 silicon atoms repel each other, and the olivine lattice allows tetrahedra to be a bit farther apart on the whole than the spinel lattice.
The olivine-spinel transition and an analogous transition between pyroxene and perovskite are probably important in driving plate tectonics. High temperature tends to favor low-density polymorphs because the thermally excited atoms take up more room. Thus, when a slab of lithosphere descends, the transition to dense forms occurs at a shallower depth in the cool slab than in the adjacent hotter mantle. Thus the slab becomes heavier than the adjacent mantle and the extra density helps to pull the plate into the subduction zone, a process called slab-pull.
Shown below is the arrangement in spinel. To make the change from olivine to spinel, the continuous rows of octahedra in the top layer have to swap position with the purple terahedra beneath them. The top layer would then look like the top layer in spinel below, but rotated and offset. If we rotate and slide the top layer so the octahedra fit over the tetrahedral voids between octahedra in the bottom layer, and the upward-pointing tetrahedra sit over the rings of octahedra, we have the spinel structure. (This really happens one atom at a time. The geometric description is just a way to allow you to visualize how one structure can evolve into the other. In fact, there is an intermediate form of olivine, beta-olivine, that forms before the final conversion to spinel.)
In both minerals we have close-packed oxygens, silica with tetrahedral coordination and magnesium with octahedral. So how can we gain any space by rearranging the atoms? In olivine the continuous straight rows of octahedra are slightly distorted, probably because of the SiO4 tetrahedra that share octahedral edges. The spinel version of olivine is about 12% more dense. The conversion occurs at around 300-400 km depth in the mantle. The reason we don't find olivine spinels at the surface is probably that the +4 silicon atoms repel each other, and the olivine lattice allows tetrahedra to be a bit farther apart on the whole than the spinel lattice.
The olivine-spinel transition and an analogous transition between pyroxene and perovskite are probably important in driving plate tectonics. High temperature tends to favor low-density polymorphs because the thermally excited atoms take up more room. Thus, when a slab of lithosphere descends, the transition to dense forms occurs at a shallower depth in the cool slab than in the adjacent hotter mantle. Thus the slab becomes heavier than the adjacent mantle and the extra density helps to pull the plate into the subduction zone, a process called slab-pull.
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- 가격1,000원
- 페이지수9페이지
- 등록일2007.04.30
- 저작시기2005.5
- 파일형식한글(hwp)
- 자료번호#401583
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