comets (which formed from the dust disk around the young Sun) often have the spectral signature of olivine, and the presence of olivine was verified in samples of a comet from the Stardust spacecraft in 2006. Comet-like (magnesium-rich) olivine has also been detected in the planetesimal belt around the star Beta Pictoris. Crystal structure Minerals in the olivine group crystallize in the orthorhombic system (space group Pbnm) with isolated silicate tetrahedra, meaning that olivine is a nesosilicate. The structure can be described as a hexagonal, close-packed array of oxygen ions with half of the octahedral sites occupied with magnesium or iron ions and one-eighth of the tetrahedral sites occupied by silicon ions. There are three distinct oxygen sites (marked O1, O2 and O3 in figure 1), two distinct metal sites (M1 and M2) and only one distinct silicon site. O1, O2, M2 and Si all lie on mirror planes, while M1 exists on an inversion center. O3 lies in a general position. High-pressure polymorphs At the high temperatures and pressures found at depth within the Earth the olivine structure is no longer stable. Below depths of about olivine undergoes an exothermic phase transition to the sorosilicate, wadsleyite and, at about depth, wadsleyite transforms exothermically into ringwoodite, which has the spinel structure. At a depth of about , ringwoodite decomposes into silicate perovskite () and ferropericlase () in an endothermic reaction. These phase transitions lead to a discontinuous increase in the density of the Earth's mantle that can be observed by seismic methods. They are also thought to influence the dynamics of mantle convection in that the exothermic transitions reinforce flow across the phase boundary, whereas the endothermic reaction hampers it. The pressure at which these phase transitions occur depends on temperature and iron content. At , the pure magnesium end member, forsterite, transforms to wadsleyite at and to ringwoodite at pressures above . Increasing the iron content decreases the pressure of the phase transition and narrows the wadsleyite stability field. At about 0.8 mole fraction fayalite, olivine transforms directly to ringwoodite over the pressure range . Fayalite transforms to spinel at pressures below . Increasing the temperature increases the pressure of these phase transitions. Weathering Olivine is one of the less stable common minerals on the surface according to the Goldich dissolution series. It alters into iddingsite (a combination of clay minerals, iron oxides and ferrihydrite) readily in the presence