Chemical & mechanical properties of Lapis Lazuli

This beautiful stone is technically a rock, not a mineral, because it is made up of various other minerals.

A darkfield photo micrograph of a thin section of a rock containing lapis mineralsLazurite can be thought of as a product of a chemical reaction between the mineral nepheline, similar chemically to the common mineral feldspar, and sodium chloride, NaCl, the mineral halite. As a common, widespread chemical element, chlorine plays a role in numerous geo-chemical reactions and appears in many mineral formulas. Chlorine in lazurite is supplied by sodium chloride. The "driving force" for this reaction is the heat supplied by nearby magma. 1

Lapis is formed by contact metamorphism which occurs deep below the earth's surface. This natural process occurs when magma finds its way deep into cracks in the earth's crust; the intense heat of the fluid causes chemical changes in the rock surrounding the intrusion creating such beautiful stones as Lapis Lazuli.. 2

Lapis lazuli is characteristically recognized by its distinctive blue colour (due to the Lazurite).
It yields H²S gas (rotten egg smell) when exposed to HCl.

  • Lapis Lazuli RocksLapis lazuli is a rock composed mainly of the following minerals:
    • Lazurite: (Na,Ca)8(AlSiO4)6(SO4,S,Cl)2
    • Hauynite: (Na,Ca)4-8(AlSiO4)6(SO4)1-2
    • Sodalite: Na8(AlSiO4)6Cl2
    • Noselite: Na8(AlSiO4)6SO4
    • Calcite: CaCO3
    • Pyrite: FeS2
    • Diopside, augite, mica and hornblende may also be present in small amounts
  • Class: Silicates
    Subclass: Tectosilicates
    Group: Both the Sodalite and Feldspathoid groups.
  • Lazurite CrystalCrystal Habit: Massive as a rock (Lapis Lazuli) forming mineral, but dodecahedral crystals have been found of Lazurite.
  • Index of refraction: approximately 1.5
  • Pleochroism: none (isotropic)
  • Specific Gravity: generally 2.7 to 2.9, higher with increasing pyrite content
  • Hardness: 5.0 to 5.5
  • Durability: sensitive to heat, acid, alkali
  • Colour: deep blue, purplish-blue, greenish blue
  • Lustre: dull
  • Transparency: translucent
  • Cleavage: Poor. In six directions, but rarely seen.
  • Fracture: conchoidal
  • Streak: light blue
  • Index of refraction is 1.5
  • Wearability: good.
faba Chile: design & handicrafts in Lapis Lazuli