Weddellite

Weddellite
General
Category	Organic compounds
Chemical formula	Oxalate(Oxalic acid) (Calcium oxalate dihydrate CaC2O4·2H2O )
Strunz classification	10.AB.40
Identification
Color	Yellowish brown
Crystal habit	eight-face bi-pyramid (typical)
Crystal system	Tetragonal
Twinning	yes
Cleavage	OUI
Fracture	Conchoidal
Mohs scale hardness	4
Luster	Vitreous (Glassy)
Streak	White
Refractive index	1.52-1.54 - DR +0.009 (B-G interval)

Weddellite (CaC₂O₄·2H₂O) is a mineral form of calcium oxalate named for occurrences of millimeter-sized crystals found in bottom sediments of the Weddell Sea, off Antarctica. Occasionally, weddellite partially dehydrates to whewellite, forming excellent pseudomorphs of grainy whewellite after weddellite's short tetragonal dipyramids.

Structural properties

The weddellite or calcium oxalate di-hydrate crystallizes in the tetragonal system. The classic crystal shape is the eight-face bi-pyramid. In bright field microscopy, the weddellite crystals are recognized easily by their shape that reminds a mail envelope. More complex shapes of weddellite are possible. The dumbbell shape is not rare. The former has no precise angles or sides. This form is, in reality, a microcrystalline agglomerate that takes the shape of a biconcave disc. Weddellite crystals are poorly birefringent and do not show any interference pattern under polarized light.

Biological role

Weddellite crystals are usually of little clinical value. Many specimens develop weddellite crystals on standing. Together, whewellite and weddellite are the most common renal calculi.

Scanning Electron Micrograph of the surface of a kidney stone showing crystals of Weddellite (calcium oxalate dihydrate) emerging from the amorphous central part of the stone. Horizontal length of the picture represents 0.5 mm of the figured original.

Varieties and related minerals

Natroxalate, Whewellite, Caoxite, Novgorodovaite, Oxammite, Glushinskite, Humboldtine, Minguzzite, Zhemchuzhnikovite, Stepanovite, Moolooite, Wheatleyite, Coskrenite

References

• "Weddellite". Weddellite: Weddellite mineral data. http://www.mindat.org/min-4254.html. Retrieved June 23, 2005. 
• "Weddellite". MINERALOGY OF KIDNEY STONES. http://www.mindat.org/min-4254.html. Retrieved June 21, 2005.