Class: Carbonates (Nitrates)
Group: Aragonite
In evaporite deposits; in sinter at hot springs; characteristic of high-pressure, low-temperature metamorphism; a secondary component in altered
ultramafic rocks (
Anthony et al., 2001—2005). Faceted specimens are very rare. The largest faceted stone is a 110 carat from Bílina, Czech Republic (
Arem, 1987: 46).
Original description: ‘Arragonite’ Hauy, O., 1791. Sur l’Arragonite de Werner — Bulletin des Sciences, par la Société Philomathique de Paris 2: 67—68 [
view in ‘Library’].
Type locality: Gallo river, Molina de Aragón, Guadalajara, Castile-La Mancha, Spain.
Type material: unknown.
Etymology: named for the type locality, the village of Molina de Aragón, Spain.
Distribution: many localities, but fine crystals are uncommon: Czech Republic: Bohemia, Ústecký kraj, Bílina; Italy: Sicily, Agrigento; Morocco: Oriental Region, Oujda; Namibia: Tsumeb; Spain: Castile-La Mancha, Guadalajara, Molina de Aragón (
Anthony et al., 2001—2005).
Chemistry
CaCO3
Essential elements: carbon (C), oxygen (O), calcium (Ca).
Crystal data
Crystallography: orthorhombic — dipyramidal.
Crystal habit: isolated crystals short to long prismatic along [010], also acicular with chisellike terminations, dipyramidal, thick tabular with forms {001}, {110} and {010}, to 30 cm; as columnar crystal aggregates and crusts, globular, reniform, pisolitic, coralloidal, internally fibrous or banded.
Twinning: common by repeated contact twinning on {110}, yielding pseudohexagonal columnar trillings (
Anthony et al., 2001—2005).
Physical properties
Cleavage: on {010}, distinct; on {110} and {011}, imperfect (
Anthony et al., 2001—2005).
Fracture: subconchoidal (
Arem, 1987: 46).
Tenacity: brittle (
Arem, 1987: 46).
Hardness: 3.5—4 (
Arem, 1987: 46).
Density: 2.93—2.96 g/cm
3 (
Lazzarelli, 2012).
Luminescence: fluoresce red or yellow under UV, phosphorescent, cathodoluminescent (
Anthony et al., 2001—2005).
Optical properties
Colour: colourless, white, also grey, pale shades of yellow, blue, violet, green, red; colourless in transmitted light (
Anthony et al., 2001—2005).
Diaphaneity: transparent to translucent (
Anthony et al., 2001—2005).
Lustre: vitreous, resinous on fracture surfaces (
Anthony et al., 2001—2005).
Refractive index: 1.525—1.686 — anisotropic [biaxial (-)] (
Lazzarelli, 2012).
Birefringence: 0.155.
Dispersion: weak (
Anthony et al., 2001—2005).
Pleochroism: none (
Arem, 1987: 46).
Material from ‘Repository’
1 specimen: 0190 — 2.16 ct, Czech Republic, Bohemia, Ústecký kraj, Bílina.
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