Physical Properties |
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| Chemical Composition | Lazurite is a mineral in the sodalite group: it is common for two or more of the minerals of this group to be represented in the same rock. A glance at their formulae will show they are very similar chemically, and, as might be expected, very similar structurally. Lazurite, however, constitutes the bulk of lapis-lazuli. The chemical formula of lazurite may be expressed as (Na,Ca)8[(S,Cl,SO4,OH)2|(Al6Si6O24)]. The sodium is often replaced in part by potassium and the solphur by S04, chlorine or selenium. Hauynite may be given as (Na,Ca)4-8Al6Si6(O,S)24(SO4,Cl)1-2and sodalite as (Na4Al3(SiO4)3Cl) . |
| Crystallographic Character | Cubic. Gem material is in the form of a granular aggregate |
| Hardness | 5 to 6. The variation is related to the amount of impurities present. |
| Toughness | Fair |
| Cleavage | None |
| Fracture | Granular, uneven |
| Specific Gravity | 2.50 to 3.0; normal, 2.75. The higher S. G.s are caused by excessive amounts of included pyrite. |
| Streak | White to light blue |
| Characteristic Inclusions | Brassy-yellow pyrite and white calcite. |
Optical Properties |
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| Degree of Transparency | Opaque to semi-translucent, |
| Luster | Polished surfaces are vitreous to waxy; fractured surface are dull. |
| Refractive Index | 1.50 |
| Birefringence | Strong, but not apparent by any gemological test. |
| Optic Character | None |
| Pleochroism | None |
| Dispersion | None |
| Phenomena | None |
| X-Ray Fluorescence | Very weak to none. The calcite inclusions, however, may fluoresce brightly I creating a mottled effect. |
| Transparency to X-Rays | Transparent. |
| Ultraviolet Fluorescence | The calcite inclusions fluoresce moderately in a pink color under long wavelength. Lapis itself is generally inert but may fluoresce weak of moderate green or yellowish green under short wavelength. |
| Color-Filter Reaction | Dull reddish brown. |
| Absorption Spectra | None. |
| Effects Caused by heat | Pale-colored stones sometimes darken and improve in appearance when brought to a dull-red heat, but an undesirable green may be produced. If heated too strongly, lapis will lose all color |
| Effects Caused by Acids | Decomposed slowly by hydrochloric acid, giving the odor of rotten eggs (hydrogen sulphide). The calcite matrix effervesces. |
| Effects Caused by irradiation | No effect |
