Calcium is a silvery white metal which is obtained by the electrolysis of molten calcium chloride.  Calcium metal melts at 842 °C and boils at 1494 °C;   these values are higher than those for magnesium and strontium, the neighbouring group 2 metals. It crystallises in the face-centered cubic arrangement like strontium; above 450 °C, it changes to an anisotropic hexagonal close-packed arrangement like magnesium.  Its density of 1.55 g/cm3 is the lowest in its group. It is one of the most abundant metal elements on Earth (41000 ppm), but it never occurs as the pure element, the main deposits being limestone (CaCO₃), Dolomite (CaCO₃.MgCO₃) and Gypsum (CaSO₄.2H₂O). Calcium is harder than lead but can be cut with a knife with effort. While calcium is a poorer conductor of electricity than copper or aluminium by volume, it is a better conductor by mass than both due to its very low density.  While calcium is infeasible as a conductor for most terrestrial applications as it reacts quickly with atmospheric oxygen, its use as such in space has been considered. Calcium compounds have many applications;  for example, calcium chloride is used extensively to dry gases as it is capable of absorbing moisture from the atmosphere. Calcium oxide (lime), produced by heating limestone, has many applications including the chemical, agricultural, construction and water industries. Calcium is an essential constituent of the human body which contains, on average, 1kg.  of the element.