DYPINGITE MG5(CO3)4(OH)2▪5H2O FROM RAPID CREEK, YUKON, CANADA. CHEMICAL COMPOSITION, OPTICAL PROPERTIES AND COMPARISON WITH SIMILAR HYDRATED MAGNESIUM CARBONATES

Dypingite Mg₅(CO₃)₄(OH)₂▪5H₂O, closely associated with hydromagnesite Mg₅(CO₃)₄(OH)₂▪4H₂O, was collected from an open fracture system above permafrost near Rapid Creek, Yukon, Canada in August 2012. The Rapid Creek area lies within the Richardson Mountains of the northeastern Yukon Territory, Canada, near the border with the Northwest Territories. There have been seven new minerals discovered there such as arrojadite-(K,Na), gormanite and kulanite; some of these minerals have never been found anywhere else in the world. Dypingite occurs on the fracture surfaces with a botryoidal habit consisting of fine radiating crystals forming a translucent crust of spheroidal aggregates. Some the spheroids can be up to 1 mm in diameter. The dypingite is often covered with loose and chalky hydromagnesite that is brittle and falls away from the fracture surface. The relationship suggests that the hydromagnesite forms as a dehydration product of dypingite at this locality.

Several mineral species belong to the hydrous magnesium carbonate mineral group. Anhydrous magnesite MgCO₃ is followed by barringtonite MgCO₃▪2H₂O , nesquehonite MgCO₃▪3H₂O and lansfordite MgCO₃▪5H₂O as hydration increases. Pokrovskite MgCO₃(OH)₂ and Mcguinnessite (Mg,Cu)CO₃(OH)₂ have hydroxyl as an additional anion. Dypingite Mg₅(CO₃)₄(OH)₂▪5H₂O, hydromagnesite Mg₅(CO₃)₄(OH)₂▪4H₂O and artinite Mg₅(CO₃)₄(OH)₂▪3H₂O have both hydroxyl and water as essential components. The relationship between mineral composition and physical properties of these minerals is discussed. As expected, the mean refractive index is found to be lower in mineral species containing more water and the unit cell volume normalised to magnesium content increases with increased water. Increase of OH within in these mineral structures does not cause a similar decrease in mean refractive index or an increase in cell volume normalized to magnesium atoms per formula unit. Gladstone-Dale calculations show agreements of excellent or superior for all minerals except barringtonite where the agreement is good according to the classification scheme of Mandarino (2007).Attempts to synthesis dypingite are in progress. Comparison of the atomic structures of these chemically similar phases is used to suggest possible atomic structures for dypingite.