Ontario Mineral - Cancrinite

Rocks Minerals Ontario Cancrinite
Chemistry:
 Na7CaAl6Si6O24(CO3)2 , Sodium Calcium Aluminum Silicate Carbonate.
Class:
 Silicates
Subclass:
 Tectosilicates
Group:
 Feldspathoid
Uses:
 Only as mineral specimens
Color:
 yellow, orange, pink, white or blue
Luster:
 vitreous or pearly
Transparency:
 Crystals are translucent to more rarely transparent
Crystal System:
 Hexagonal; 6
Crystal Habits:
 Usually massive or granular. Some prismatic to columnar crystals are found.
Cleavage:
perfect, in three directions, prismatic, but rarely seen in massive specimens.
Fracture:
 conchoidal to uneven.
Hardness:
 5 - 6
Specific Gravity:
 2.4 - 2.5 (average)
Streak
 white
Other Characteristics:
 Will effervescence in warm hydrochloric acid
Associated Minerals:
 include feldspars such as albite , biotite , hornblende , nepheline , sodalite and other feldspathoids
Local Occurance:
 Cancrinite Hill, Davis Quarry, Davis Hill, Goulding-Keene Quarry, Gutz Farm, Morrison Quarry, York River (Tactile) Skarn,
Best Field Indicators:
 reaction to acids, associations, locality and hardness.

Description:

Cancrinite is one of the rarer members 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.
Cancrinite is unusual in that it is one of the few silicate minerals to have a carbonate ion (CO3 -2) present in its structure. It is because of the carbonate ion that it will effervescence in warm hydrochloric acid. The acid reacts with the carbonate ion to liberate carbon dioxide (CO2) gas which forms the bubbles. The effervescence is diagnostic as almost no other silicates will do this, although it is common in the carbonates .
Although feldspathoids are known to both extrusive and intrusive igneous rocks, cancrinite is found almost exclusively in intrusive or plutonic rocks. This may be due to the tremendous pressures required to force the large carbonate ions into the structure of cancrinite. Cancrinite will also contain some percentages of sulfate ions (SO4 -2) and chlorine ions (Cl -1); two other large ions. Feldspathoids are similar to zeolites in that their wide open structures allow large ions to be incorporated into them.