THE MINERAL LAZURITE

Chemistry: (Na, Ca)8Al6Si6O24(S, SO4) , Sodium Calcium Aluminum Silicate Sulfur Sulfate.
Class: Silicates
Subclass: Tectosilicates
Group: Both the Sodalite and feldspathoid groups.
Uses: mineral specimens and ornamental stones
Specimens

Lazurite is a popular but generally expensive mineral. Well-formed, deep blue crystals are rare and valuable. It is more commonly found massive and combined with other minerals into a rock called lapis lazuli.

Lapis lazuli or lapis for short is mostly lazurite but commonly contains pyrite and calcite and some other minerals. The name means "blue rock" and is always a brilliant blue with violet or greenish tints. The rich blue color is due to the sulfur that is inherent in the structure of lazurite. Small crystals of pyrite are always present in lapis and their brassy yellow color is both attractive and diagnostic in distinguishing lapis from its also blue cousin - sodalite rock, which lacks pyrite. The calcite produces white streaks in the lapis and too much calcite will lower the value of the stone.

Lapis lazuli has been mined for centuries from a locality still in use today in the remote mountain valley called Kokcha, Afghanistan. First mined 6000 years ago, the rock was transported to Egypt and present day Iraq and later to Europe where it was used in jewelry and for ornamental stone. Europeans even ground down the rock into an expensive powdered pigment for paints called "ultramarine". Today ultramarine is manufactured artificially. Although now not the only source of lapis, the source in Afghanistan still produces the finest quality material.

Lazurite is a member of the feldspathoid group of minerals. Minerals whose chemistries are close to that of the alkali feldspars but are poor in silica (SiO2) content, are called feldspathoids. As a result or more correctly as a function of the fact, they are found in silica poor rocks containing other silica poor minerals and no quartz. If quartz were present when the melt was crystallizing, it would react with any feldspathoids and form a feldspar.. Localities that have feldspathoids are few.

The name lazurite is often confused with the bright blue phosphate mineral lazulite. However the two minerals can not be confused with each other identification wise because of lazulite's typical vitreous luster and good crystal habit. The carbonate mineral azurite has a very similar color to lazurite but is associated with the green carbonate mineral malachite and reacts to acids.

PHYSICAL CHARACTERISTICS:

Color is brilliant blue with violet or greenish tints.
Luster is dull to greasy.
Transparency: Crystals are translucent to opaque.
Crystal System: Isometric; bar 4 3/m
Crystal Habits: Dodecahedral crystals have been found, usually massive as a rock (lapis lazuli) forming mineral.
Cleavage is poor, in six directions, but rarely seen.
Fracture is uneven
Hardness is 5 - 5.5
Specific Gravity is 2.3 - 2.4 (somewhat below average)
Streak is bright blue.
Other Characteristics: Index of refraction is 1.5.
Associated Minerals calcite, some pyroxenes and most diagnostic pyrite.
Notable Occurrences include Kokcha River valley, Afghanistan; Ovalle, Cordillera, Chile; near Lake Baikal, Russia; Mt. Vesuvius, Italy; Cascade Canyon, San Bernardino Mountains and Ontario Peak, California and in the Sawatch Mountains, Colorado, USA.
Best Field Indicators are the violet-blue color, pyrite association (unlike sodalite), locality and specific gravity.