Aragonite

Aragonite
Aragonite from Los Molinillos, Cuenca, Spain, sample width about 4 cm
General
CategoryCarbonate minerals
Formula
(repeating unit)
Ca CO3
IMA symbolArg (not to be confused with arginine)[1]
Crystal systemOrthorhombic
Unit celll a = 4.9598(5) Å, b = 7.9641(9) Å, and c = 5.7379(6) Å at 25 °C[2]
Identification
ColorCan come in a variety of colors, but commonly red or white
Crystal habitCommonly dendritic or pseudo-hexagonal; can also be acicular, tabular, prismatic, coral-like
TwinningCyclic on {110}, forms pseudohexagonal aggregates. If polysynthetic, forms fine striations parallel to [110].
CleavageGood on [110], Poor on {110}.
FractureSubconchoidal
TenacityVery brittle
Mohs scale hardness3.5–4
LusterVitreous, waxy, resinous
StreakWhite
DiaphaneityTransparent to opaque
Specific gravity2.94
Optical propertiesBiaxial (−)
Refractive indexnω = 1.550 nε = 1.650
Birefringenceδ = 0.155
2V angleMeasured 18–19°
DispersionWeak
ExtinctionParallel
Ultraviolet fluorescenceFaint white-blue to blue-violet
SolubilitySoluble in acids, and saltwater (but takes longer)
Common impuritiesCommonly strontium, zirconium, lead
Other characteristicsThermodynamically unstable, Morphs slowly back into calcite
References[3][4]

Aragonite is a carbonate mineral and one of the three most common naturally occurring crystal forms of calcium carbonate (Ca CO3), the others being calcite and vaterite. It is formed by biological and physical processes, including precipitation from marine and freshwater environments.

Aragonite crystal structure

The crystal lattice of aragonite differs from that of calcite, resulting in a different crystal shape, an orthorhombic crystal system with acicular crystal.[5] Repeated twinning results in pseudo-hexagonal forms. Aragonite may be columnar or fibrous, occasionally in branching helictitic forms called flos-ferri ("flowers of iron") from their association with the ores at the Carinthian iron mines.[6]

  1. ^ Warr, L. N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ Dickens, B.; Bowen, J. S. (1971). "Refinement of the Crystal Structure of the Aragonite Phase of CaCO(3)". Journal of Research of the National Bureau of Standards Section A. 75A (1): 27–32. doi:10.6028/jres.075A.004. PMC 6715969. PMID 34876711.
  3. ^ "Aragonite Properties, Occurrence » Geology Science". 26 October 2021.
  4. ^ Aragonite, Mindat.org
  5. ^ Bragg, William Lawrence (1924-01-01). "The structure of aragonite". Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character. 105 (729): 16–39. Bibcode:1924RSPSA.105...16B. doi:10.1098/rspa.1924.0002. ISSN 0950-1207.
  6. ^ Sinkankas, John (1964). Mineralogy for amateurs. Princeton, N.J.: Van Nostrand. pp. 371–372. ISBN 0442276249.