Gypsum: Difference between revisions
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[[Category:Gipsum]][[Category:Sulphate]][[Category:Salt]][[Category:InProgress]][[Category:Sulfate]] | [[Category:Gipsum]][[Category:Sulphate]][[Category:Salt]][[Category:InProgress]][[Category:Sulfate]] | ||
Revision as of 12:44, 24 January 2012
<bibimport/>
| Gypsum[1][2] | |
| |
| Mineralogical name | Gypsum |
| Chemical name | Calcium sulfate dihydrate |
| Trivial name | plaster of paris, Alabaster, Satin Spar, Selenite |
| Chemical formula | Ca[SO4]•2H2O |
| Other forms | Anhydrite (CaSO4) Hemihydrate (CaSO4•0.5H2O) |
| Crystal system | monoclinic |
| Crystal structure | |
| Deliquescence humidity 20°C | |
| Solubility (g/l) at 20°C | 2.14 g/l |
| Density (g/cm³) | 2.2-2.4 g/cm³ |
| Molar volume | 74.69 cm3/mol |
| Molar weight | 172.17g /mol |
| Transparency | transparent to opaque |
| Cleavage | perfect, clearly visible formation of fibre |
| Crystal habit | flat, prismatic, needle- like crystal, granular, massive aggregate |
| Twinning | very common |
| Phase transition | |
| Chemical behavior | hardly soluble in water |
| Comments | |
| Crystal Optics | |
| Refractive Indices | α = 1.519-1.521 β = 1.522-1.523 γ = 1.529-1.530 |
| Birefringence | Δ = 0.010 |
| Optical Orientation | biaxial positive |
| Pleochroism | colorless |
| Dispersion | 58° |
| Used Literature | |
| {{{Literature}}} | |
Authors: Hans-Jürgen Schwarz , Nils Mainusch, Tim Müller
English Translation by Sandra Leithäuser
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| This article will be released soon. |
Microscopy[edit]
Laboratory examination: Gypsum is slightly water soluble, therefore gypsum-containing sample material only dissolves slightly, when mixed with distilled Water. In solution, gypsum- containing sample material recrystallizes by carefully concentrating the solvent. At first, single needles form, then increasingly needle- like gypsum aggregate in proximity of the seam of the solvent emerges. Alternatively, sample material can be dissolved in hydrochloric acid, which also leads to the formation of crystal needles. Compared to other salts that can recrystallize in needle-like shapes, e.g. sodium carbonate, gypsum needles are clearly shorter.
Refraction indices:
nx = 1.521; ny =1.523; nz =1.530
birefringence: Δ = 0.009
crystal class: monoclinic
Polarized light microscopy examination:
Apart from the typical acicular habit of gypsum crystals, (especially in recrystallized material) different morphological characteristics appear. These can be useful for identifying gypsum. Gypsum particles (in raw material samples) display shapes of rounded fragments and plate- like rhombohedra, clearly showing the inner cleavage planes. Furthermore, the occurrence of twinning shapes is typical for gypsum crystals, whether they are lath- shaped, tabular or lamellar. The assignment of refractive indices is carried out in accordance with the immersion method using media with indices nD=1,518 und nD=1,53. Due to the often small- scale particles the examination using the Schoeder van der Kolk method is more significant and reliable than the Becke- Line test.
Gypsum crystals belong to the class of monoclinic crystals. Thus, they show, depending on the orientation of the single particle under the microscope, a parallel or respectively a symmetrical extinction, but mainly exhibit a characteristicly oblique axis position in the extinction position. On well developed crystal rhombi the oblique extinction can clearly be measured. Of all calcium sulfate crystals, gypsum has the lowest birefringence. Under crossed polarizers, gypsum has very low interference colors, lying within the gray to yellowish white range of the first order, (of course depending on the thickness of the particles).
Possibility for mistakes:
The Analysis methods mentioned above clearly identify gypsum, provided the following evaluation criteria are explicitly clarified.
- low water solubility
- characteristic needle- like morphology of the recrystallized particles
- all observable indices have a nD –value from 1,518 and 1,530
- gypsum crystal show low interference colors
- gypsum crystals have an oblique extinction
| salt phase | differentiating features |
| Syngenite K2Ca(SO4) • 2H2O | all observable indices; 1,518 |
| Tachyhydrite CaMg2Cl6 • 12H2O | mostly observable index < 1,518 / only parallel and symmetrical extinction |
| Hydromagnesite Mg5[OH(CO3)2]2 • 4H2O | mostly one index > 1,53 |
Weblinks[edit]
- ↑ http://webmineral.com/data/Gypsum.shtml seen on 30.07.2010
- ↑ http://www.mindat.org/min-1784.html seen on 30.07.2010
Literatur[edit]
[Filter missing]