Scrutinyite

Scrutinyite /ˈskruːtɪni.aɪt/ is a rare oxide mineral and is the alpha crystalline form of lead dioxide (α-PbO2), plattnerite being the other, beta form. The mineral was first reported in 1988 and its name reflects the scrutiny and efforts required to identify it from a very limited amount of available sample material.[1][3]

Scrutinyite
General
CategoryOxide mineral
Formula
(repeating unit)		PbO2
Strunz classification4.DB.20
Crystal systemOrthorhombic
Crystal classDipyramidal (mmm)
H–M Symbol: (2/m 2/m 2/m)
Space groupPbcn
Unit cella = 4.91 Å, b = 5.95 Å
c = 5.43 Å; Z = 4
Identification
Formula mass239.20 g/mol
ColorDark reddish brown
Crystal habitCrystalline, platy
Cleavage{100} perfect, {010} imperfect
FractureBrittle
LusterSub-metallic
StreakDark brown
DiaphaneityTransparent to translucent
Specific gravity9.867 (calculated)
Optical propertiesBiaxial
Refractive indexn > 2
Other characteristicsNon-fluorescent, nonmagnetic
References[1][2][3][4]

Identification

The synthetic orthorhombic form of lead dioxide, α-PbO2, was known from 1941. Although natural lead dioxide has been known, as the mineral plattnerite (β-PbO2), since 1845,[5] its alpha form could only be recognized in 1981 and reliably identified in 1988.[3]

The new mineral was spotted in several samples collected at Bingham, New Mexico and Mapimí, Durango, Mexico. It was first thought to be minium (lead tetroxide mineral) because of its high lead content, brown color and association with other lead oxide minerals plattnerite and murdochite. Its holotype specimen consisted of crystalline plates 25–30 micrometers (µm) across and 1–2 µm thick with the total weight below 1 mg. The flakes were collected from a fluorite, quartz, limonite and rosasite matrixes. Identification and characterization of scrutinyite by the standard X-ray diffraction (XRD) technique was hindered by scarcity of material and strong signal interference with plattnerite. The unusual amount of effort required for the analysis resulted in its name derived from the word "scrutiny". The holotype specimen is preserved in the US National Museum (catalog number NMNH 165479).[1][3]

Characterization
Crystal structure

The PbO2 composition of scrutinyite was deduced by energy-dispersive X-ray spectroscopy. Slight oxygen deficiency is generally attributed to the surface effects, especially in thin samples, namely oxygen in the surface layers of PbO2 is usually substituted by the hydroxyl groups.[3]

The crystal structure was deduced by XRD as orthorhombic, space group Pbcn (No. 60), Pearson symbol oP12, lattice constants a = 0.497 nm, b = 0.596 nm, c = 0.544 nm, Z = 4 (four formula units per unit cell) were in reasonable agreement with previous results obtained on synthetic samples.[3]

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References
  1. Barthelmy, Dave. "Scrutinyite Mineral Data". www.webmineral.com.
  2. "Scrutinyite: Scrutinyite mineral information and data". www.mindat.org.
  3. J. E. Taggard Jr.; et al. (1988). "Scrutinyite, natural occurrence of α-PbO2 from Bingham, New Mexico, U.S.A., and Mapimi, Mexico" (PDF). Canadian Mineralogist. 26: 905.
  4. "Handbook of Mineralogy" (PDF). arizona.edu.
  5. Haidinger W (1845) Zweite Klasse: Geogenide. II. Ordnung. Baryte VII. Bleibaryt. Plattnerit., p. 500 in Handbuch der Bestimmenden Mineralogie Bei Braumüller and Seidel Wien pp. 499-506 (in German)
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