Coalingite, a new mineral from the New Idria serpentinite, fresno and san benito counties, California

A new mineral, coalingite, has been identified in the surface weathering zone of the New Idria serpentinite, Fresno and San Benito Counties, California. It occurs to a depth of 2030 ft as soft, reddish-brown platelets, 0.1 to 0.2 mm in size, with resinous luster. Individual grains are contaminated with either chrysotile or a pyroaurite mineral, intimately intergrown with the coalingite, which has prevented the separation of absolutely pure fractions of this phase. Analyses of four concentrates and of a nearly-pure, hand-picked 500-mg sample have yielded the following empirical formula: Mg₁₀Fe₂(CO₃)(OH)₂₄· 2H₂O.

Coalingite is uniaxial (—), with ϵ=1.563 and ω=1.594. It is pleochroic from goldenbrown to colorless and is length slow. The measure density is 2.32 g/cc. X-ray diffraction studies clearly distinguish coalingite from closely related minerals such as brucite, pyroaurite, hydrotalcite and brugnatellite and suggest hexagonal symmetry. Differential thermal analyses of coalingite show three endothermic peaks at about 175°, 310° and 400° C. Thermal gravimetric analyses, x-ray diffraction and infrared absorption studies of heated specimens suggest that at about 120° C. a reversible dehydration takes place, accompanied by a rearrangement of the carbonate ions. At about 260° C. further rearrangement occurs without loss of CO₂, and at about 320° C. both CO₂ and hydroxyl water are evolved from the lattice, and a periclase phase is formed.

X-ray and petrographic studies of dense, dark green, unweathered portions of residual serpentinite, and of brown, friable, highly weathered counterparts indicate that coalingite forms in situ from pre-existing brucite, an important constituent of the serpentinite. Electron probe analyses have shown the presence of up to 18 weight per cent iron in brucite from this body, sufficient to form coalingite directly without the introduction of iron from an external source. These data, combined with the observed transformation of brucite-rich specimens to coalingite in the laboratory after beng exposed to the atmosphere for several months, strongly indicate that coalingite is formed by the oxidation and carbonation of iron-rich brucite in the surface weathering zone of the serpentinite. Artinite and hydromagnesite are also found in the weathering zone, but are decidedly later and probably precipitated directly from magnesium-rich ground waters.

The name coalingite (kol-inǵ-glt) is proposed for this new mineral referring to the nearby town of Coalinga, California. It is likely that many of the so-called “ferrobrucites,” inadequately described in the literature, are actually coalingite or coalingite-rich mixtures.