Macphersonite
General
Category	Carbonate mineral
Formula (repeating unit)	Pb4(SO4)(CO3)2 (OH)2
IMA symbol	Mps[1]
Strunz classification	5.BF.40
Dana classification	17.01.04.01
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Pcab
Unit cell	a = 10.37 Å, b = 23.10 Å, c = 9.25 Å, β = 106.43°; Z = 8
Identification
Formula mass	1,078.90 g/mol
Color	White, pale amber, colorless
Crystal habit	Pseudo hexagonal, tabular
Twinning	Polysynthetic, lamellar, contact
Cleavage	Perfect on {010}
Fracture	Uneven
Mohs scale hardness	2.5 - 3
Luster	Adamantine, otherwise resinous
Streak	White
Specific gravity	6.50
Optical properties	Biaxial (-)
Refractive index	nα = 1.870 nβ = 2.000 nγ = 2.010
Birefringence	δ = 0.140
2V angle	35-36°
Dispersion	r > v
Ultraviolet fluorescence	A very strong and vivid yellow
Other characteristics	Polymorph of leadhillite and susannite
References	[2][3][4][5][6]

Macphersonite, Pb₄(SO₄)(CO₃)₂ (OH)₂, is a carbonate mineral that is trimorphous with leadhillite and susannite. Macphersonite is generally white, colorless, or a pale amber in color and has a white streak. It crystallizes in the orthorhombic system with a space group of Pcab. It is fairly soft mineral that has a high specific gravity.

Macphersonite is named after Harry Gordon Macpherson,^[4] a keeper of minerals at the Royal Scottish Museum. It was discovered and accepted in 1984.

Structure

The structure of macphersonite is represented as a sequence of three layers stacked along the [001]. The first layer is a sulfate tetrahedra, the second is of lead and hydroxide, and the third is a layer composed of lead and carbonate. Stacking of the three layers can be detailed as ...BABCCBABCC...^[5] similar to leadhillite. Two C layers of lead carbonate in the BAB stacking provide a weak connection that leads to the perfect {001} cleavage.

Physical properties

The Leadhills macphersonite is a very pale amber to colorless in color, while the Argentolle mine macphersonite is colorless to white.^[4] It has a luster of adamantine on fresh surfaces and elsewhere it is resinous. Macphersonite is soft with a 2.5-3 on the Mohs hardness, has an uneven fracture with a high density of 6.5g/cm³.^[3]

Macphersonite has a very strong yellow fluorescence under both long and short wave, ultraviolet is displayed by the Leadhills specimens, the Argentolle material does not fluoresce.

Occurrence

Macphersonite is found in the Leadhills region of southwest Scotland and in the Saint-Prix, Saône-et-Loire region of France. It is the rarest of the three polymorphs. It occurs in lead deposits associated with cerussite, susannite, caledonite, scotlandite, leadhillite, galena and pyromorphite.

References