Composition
Celsian is a barium feldspar with a chemical composition BaAl2Si2O8. It forms part of the feldspar group and belongs to the celsian- hyalophane series and the celsian-Geologic occurrence
Celsian is of limited occurrence. Most of the barium feldspars are associated with exhalative hydrothermal processes and low-and medium-grade metamorphism (Moro and Cembranos and Fernandez, 2001). It is also associated with sedimentary and meta sedimentary rocks, manganese, ferromanganese and barite deposits. Celsian can be found in places like Wales, Zamora (Spain), Alaska, California, Sweden and Japan, also with hendricksite on the Franklin mines in New Jersey.Structure
The symmetry in celsian is somewhat different from the symmetry normally found in feldspars. It is monoclinic with a body centered lattice similar to those of anorthite. Insufficient evidence has been found to suggest that celsian lacks a center of symmetry, so its space group is I 2/c (Newnham and Megaw, 1960). The space group differs from others of its group like orthoclase, albite and body center anorthite are C2/m, C1bar and I1bar. X-ray analysis shows that the values for the lattice parameters ''a'', ''b'', ''c'' axes and angles are approximately ''a''=863 pm, ''b''=131.0 pm, ''c''=1400 pm and ''β''=116°, ''θ''=90° (Gay, 1956). There are 8 formula units per cell, and the general position is eightfold, so all atoms can lie in general positions (Newnham and Megaw, 1960). This structure is very similar to that of orthoclase and sanidine but differs in a couple of ways: :1. The distribution of Si and Al. :2. The coordinates of all the atoms. The distribution of silicon and aluminium along the tetrahedral sites mixed with the nature of the barium atom makes an impact on the surrounding silicate framework (Newham and Megaw, 1960). The Si-Al bonds are partially ordered, and in some cases the aluminium substitute's silicon. The order in celsian is very simple, each aluminium tetrahedron is surrounded by four silicon tetrahedra, and vice versa (Newham and Megaw, 1960). Also there is another type of transformation besides aluminium-silicon, where silicon-poor goes into a silicon-rich network that involves having to simultaneously be a replacement of Al, and Si at other sites. The barium ion has an irregular configuration close to the one in potassium in the feldspars. Each barium has an oxygen close, and thanks to this configuration it has a strong effect on the silicon-oxygen-silicon bond angles.Polymorphism of celsian
There are four distinctive polymorphs of celsian, two of them are the natural minerals and the other two are synthetic products. The first are paracelsian and celsian, the second ones are hexacelsian and the other one is related to the mineral cymrite (Lin and foster, 1967). The order of increasing stability is paracelsian→hexacelsian→celsian in a temperature range between 500 °C to the 1000 °C. As temperature rises from the 1,600 °C to 1,760 °C it goes from celsian to a reversible form of hexacelsian. Paracelsian is less stable than the other two and celsian is the most stable.Twinning
Barium feldspars occur in optically uniform crystals where the twinning is poorly developed, except on coarse crystals. Eighteen crystal forms have been identified; eleven of them coincide with those known for orthoclase. Observed twining includes Manebach twins on (001) and Baveno twins on (021). Some samples of celsian were found to have a rare lamellar twinning (Spencer, 1941).Properties
Physical properties
Celsian shows a c(001) perfect cleavage and a b(010) good cleavage, which marks the difference with its polymorph paracelsian which has a 10indistinct cleavage. There are different crystals habits likeOptical properties
Some other optical properties are the 2V angle which is approximately 88° with a maximum birefringence of 0.014, biaxial with a negative sign (Newnham and Megaw, 1960). It has a moderate relief.Uses
The uses of celsian are mostly related to glass and ceramics. This uses are usually achieved by the preparation of pure synthetic monoclinic celsian. Celsian has very attractive features such as chemical stability and high mechanical resistance, which can be favourably exploited in order to obtain enhanced-performance composites with respect to bulk glass. (Cannillo, Carlier, Manfredini, Montorsi, and Siligardi 2006). Many studies show that by increasing the amount of celsian phases in the glasses results in increased bulk of crystallization (Khater and Idris, 2004).See also
*References
*Cannillo V, Carlier E, Manfredini T, Montorsi M, Siligardi C. "Design and Optimisation of Glass-Celsian Composites". ''Composites Part A – Applied Science and Manufacturing''. 2006, vol. 37, pp 23–30. *Gay P. "A note on celsian". ''Acta Crystallographica''. 1956, vol 9, pp 474. *Khater G, Idris M. "The use of a Saudi kaolinitic clay for the production glass-ceramic materials". ''Indus. Ceramics''. 2004, vol. 24, pp. 43–50. *Lin H.C and Foster W.R. "The polymorphism of Celsian". ''The Canadian Mineralogist''. 1967, vol. 9, p. 295. *Lin H.C and Foster W.R. "Studies in the system Ba0-Al2O3Si02. The polymorphism of Celsian". ''American Mineralogist''. 1968, vol. 53, pp. 134–144 *Moro M.C, Cembranos M.L and Fernandez A. "Celsian, (Ba,K)-feldspar and cymrite from sedex barite deposits of Zamora, Spain". ''The Canadian Mineralogist''. 2001, vol. 39, pp. 1039–1051. *Newham R.W and Megaw H.D. "The crystal structure of celsian (barium feldspar)". ''Acta Crystallographica'', 1960, vol. 13, pp. 303–312 *Spencer L.J. "Barium-feldspar (celsian and paracelsian) from Wales". ''The Mineralogical Magazine''. 1942, vol. 26, pp. 231–243. {{Refend Barium minerals Aluminium minerals Feldspar Monoclinic minerals