THE MINERAL PIRSSONITE

Chemistry: Na₂Ca(CO₃)₂ - 2H₂O , Hydrated Sodium Calcium Carbonate.
Class: Carbonates
Uses: Only as mineral specimens.
Specimens

Pirssonite is named after American geologist Louis Valentine Pirsson. Pirssonite is one of several carbonate minerals that form in non-marine evaporite deposits. Other evaporite carbonates include trona, gaylussite, northupite, nahcolite and thermonatrite. Evaporite minerals are geologically important because they clearly are related to the environmental conditions that existed at the time of their deposition, namely arid. They also can be easily recrystallized in laboratories in order to confirm their specific characteristics of formation.

Pirssonite and gaylussite, Na₂Ca(CO₃)₂ - 5H₂O, differ only in the number of water molecules, yet their symmetries are quite different. This is an indication of a change in their respective crystal structures. The two are best distinguished by their crystal habits in which pirssonite has a distinctive tabular diamond-shaped crystal form. Pirssonite can lose its water molecules and specimens should be stored in a sealed container.

PHYSICAL CHARACTERISTICS:

Color is colorless, white or yellowish.
Luster is vitreous.
Transparency: Crystals are transparent to translucent.
Crystal System is orthorhombic.
Crystal Habits include prismatic and tabular diamond-shaped or distorted hexagon-shaped crystals, but also massive and encrusting.
Fracture is conchoidal.
Hardness is 3 - 3.5
Specific Gravity is 2.4 (slightly below average)
Streak is white.
Associated Minerals include gaylussite, northupite, trona, analcime and halite.
Notable Occurrences include the type locality at Searles Lake, San Bernardino County, California as well as Deep Spring Lake, Inyo County and Borax Lake, Lake County, California, USA; Mont Saint-Hilaire, Quebec, Canada and elsewhere.
Best Field Indicators: Crystal habit, environment of formation, density and locality.