George-ericksenite
| George-ericksenite | |
|---|---|
| General | |
| Category | Sulfate mineral |
|
Formula (repeating unit) |
Na6CaMg(IO3)6(CrO4)2(H2O)12 |
| Strunz classification | 4.KD.10 |
| Crystal system | Monoclinic |
| Crystal class | Prismatic (2/m) (same H-M symbol) |
| Space group | C2/c |
| Unit cell | a = 23.645 Å b = 10.918 Å c = 15.768 Å β = 114.42°; Z=4 |
| Identification | |
| Color | Pale yellow (crystals) to bright lemon yellow (aggregates) |
| Crystal habit | Prismatic to acicular along [001] and somewhat flattened on {110} |
| Twinning | None observed megascopically nor during single-crystal study |
| Cleavage | None observed |
| Fracture | Unknown |
| Tenacity | Brittle |
| Mohs scale hardness | 3-4 (estimated) |
| Luster | Vitreous |
| Streak | Pale yellow |
| Diaphaneity | Transparent (cystals) to translucent (aggregates) |
| Density | 3.035 g/cm3 |
| Birefringence | δ = 0.057 |
| Pleochroism | Slight; X 5 very pale yellow, Z 5 distinct yellow-green |
| Solubility | Extreme in cold water |
| References | |
George-ericksenite is a mineral with the chemical formula Na6CaMg(IO3)6(CrO4)2(H2O)12. It is vitreous, pale yellow to bright lemon yellow, brittle, and features a prismatic to acicular crystal habit along [001] and somewhat flattened crystal habit on {110}. It was first encountered in 1984 at the Pinch Mineralogical Museum. One specimen of dietzeite from Oficina Chacabuco, Chile had bright lemon-yellow micronodules on it. These crystals produced an X-ray powder diffraction pattern that did not match any XRD data listed for inorganic compounds. The X-ray diffraction pattern and powder mount were set aside until 1994. By then, the entire mineral collection from the Pinch Mineralogical Museum had been purchased by the Canadian Museum of Nature. The specimen was then retrieved and studied further. This study was successful and the new mineral george-ericksenite was discovered. The mineral was named for George E. Ericksen who was a research economic geologist with the U.S. Geological Survey for fifty years. The mineral and name have been approved by Commission on New Minerals and Mineral Names (IMA). The specimen, polished thin section, and the actual crystal used for the structure determination are kept in the Display Series of the National Mineral Collection of Canada at the Canadian Museum of Nature, Ottawa, Ontario.
George-ericksonite is commonly found as isolated bright lemon-yellow micronodules of crystals that are concentrated on the surface of one part of the mineral specimen. However, in some cases the micronodules occur as groupings instead of isolated occurrences. The average size of these micronodules is approximately 0.2 mm and each consists of numerous individual crystals in random orientation.
This examination was carried out by attaching two acicular crystals to the surface of a disk with epoxy and then examining them with a CAMECA SX-50 electron microprobe. One of the crystals had the (100) surface facing up, and the other crystal had a growth face of the form (110) facing up. The microprobe was operating in wavelength-dispersive mode at 15 kV and ran various currents from 20 nA to 0.5 nA. The CAMECA SX-50 has three spectrometers and the samples were examined in the sequence (Na, Cl, I), then (Mg, S, Ca). When the crystal was exposed to the electron beam for the first 200 seconds, the counts per second on each element varied greatly which indicates that the crystals are extremely unstable in the electron beam. The counts per second for each element were also dependent on the surface of the crystal [(100) or (110)] analyzed. Over shorter counting times (<10 s) at 15 kV and 5 nA, there is a gain in I2O5 and a drop in Na2O relative to ideal values. However, a significant orientation effect exists for SO3 and CaO values for the (100) and (110) surfaces on either side of the ideal values. With increasing exposure to the electron beam, Na2O increases and all other oxides decrease. This behavior is also more rapid on the (100) surface than on the (110) crystal face. The (100) surface is overall more reactive to the electron beam than the (110) surface, but both surfaces seem to approach equilibrium with the beam and give similar oxides weight percentages after 200 seconds.
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