Minerals In Thin Section


Formula	CaCO₃

Optic class & sign	Biaxial negative

Optical orientation	X = c, Y = a, Z = b

Optical plane	(100)

Relief	Low-negative to high

Refractive indices	n_x = 1.530 -1 531

	n_y = 1.680 -1.681

	n_z = 1.685 -1.686

	n decreases as Sr replaces Ca, but increases with Pb

Birefringence (max.)	0.155 - 0.156

	-

Optic Angle	2V_x = 18° -

	2V_z

Sign of elongation	Length-fast, l (-) for crystals elongate in c

Interference figure	Acute bisectrix figures in sections ⊥ c with numerous isochromes

Colour / pleochroism	Colourless

Zoning


Form	Habit	Granular, columnar crystals elongate in c; pseudohexagonal cross-sections, radiating-fibrous aggregates, encrustations

	Surface	Euhedral to anhedral

Cleavage		{010} imperfect

Twinning		Cyclic {110} twins with 6 segments in sections ⊥ c; multiple twins on {110}

Extinction		Straight to prism faces and {010} cleavage traces in sections parallel to c.


Reaction textures	Partial or complete transformation to calcite is common, with pseudomorphs of calcite after aragonite

Alteration / decomposition	Highly soluble


Occurence	Ign

	Met	Carbonate-bearing high-pressure, low-temperature rocks. Preservation of original aragonite in blueschist-facies rocks is only possible under low-temperatures of retrogression and relatively fast cooling.

	Sed	Cave and hotspring deposits, oolites, hard shells of marine organisms; carbonate-bearing iron-rich sediments

	Hyd	Cavities and amygdales of basaltic and andesitic rocks

	Other


Distinctive properties	Extreme Δn with high-order white interference colours and marked change of relief with rotation in sections at a low angle to c. Biaxial character, twinning, absence of rhombohedral cleavage (as seen in calcite). Staining may also be used to distinguish aragonite from calcite.

Additional comments	Incorporation of Pb increases 2V slightly n-Δn chart: Arg - aragonite, Cer - cerussite, Str - strontianite, Wth - witherite

Aragonite