The formula for talmessite is Ca2Mg2.2H2O. It is a triclinic mineral, crystal class, space group P. There is one formula unit per unit cell and the unitcell parameters are variously given as a = 5.87 Å, b = 6.94 Å, c = 5.53 Å, α = 97.3°, β = 108.7°, γ = 108.1°, or a = 5.89 Å, b = 7.69 Å, c = 5.56 Å, α = 112.633°, β = 70.817°, γ = 119.417°,. These values give a calculated specific gravity varying from 3.42 to 3.63.
The structure is dominated by chains of tetrahedral AsO4 and octahedral groups that parallel the c crystal axis. The octahedral are compressed, resulting in chain disorder
Appearance
Talmessite occurs as prismatic crystals to 3 mm, as radiating fibrous aggregates or as fine crystalline aggregates; it may also be stalactitic or in crusts. Pure talmessite is white or colourless, and colourless in transmitted light, but nickel-rich varieties are pale green and cobalt-rich varieties may be brownish, pink or the purple colour typical of many cobalt minerals. The streak is white and crystals are transparent to translucent with a vitreous lustre.
Physical properties
Talmessite is a moderately hard mineral, with Mohs hardness 5, harder than fluorite but not as hard as quartz. The specific gravity calculated from the formula and the cell dimensions is 3.49, but the measured value is less for ordinary talmessite, at 3.42 and more for the cobaltoan variety, at 3.57. The mineral displays polysynthetic twinning. It is not radioactive and it loses water of crystallisation at 450o.
Optical properties
crystals have two directions in which light travels with zero birefringence; these directions are called the optic axes, and the crystal is said to be biaxial. Talmessite is triclinic, so it is biaxial.
The speed of a ray of light travelling through the crystal differs with direction. The direction of the fastest ray is called the X direction and the direction of the slowest ray is called the Z direction. X and Z are perpendicular to each other, and a third direction Y is defined as perpendicular to both X and Z; light travelling along Y has an intermediate speed. Refractive index is inversely proportional to speed, so the refractive indices for the X, Y and Z directions increase from X to Z. The refractive indices are nα = 1.672, nβ = 1.685 and nγ = 1.698 with slightly higher values for the cobaltoan variety of nα = 1.695 and nγ = 1.73. The maximum birefringence δ is the difference between the highest and lowest refractive index; for talmessite δ = 0.026.
The angle between the two optic axes is called the optic angle, 2V, and it is always acute, and bisected either by X or by Z. If Z is the bisector then the crystal is said to be positive, and if X is the bisector it is said to be negative. Talmessite may be biaxial or biaxial. The measured value of 2V is about 90o. Also 2V can be calculated from the values of the refractive indices, giving a value of 88°
2V depends on the refractive indices, but refractive index varies with wavelength, and hence with colour. So 2V also depends on the colour, and is different for red and for violet light. This effect is called dispersion of the optic axes, or just dispersion. If 2V is greater for red light than for violet light the dispersion is designated r > v, and vice versa. For talmessite dispersion is strong, with r > v. The cobaltoan variety is pleochroic colourless to pale rose.
