Palygorskite

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Palygorskite
A sample of palygorskite from Hnúšťa, Slovakia.
General
Category	Phyllosilicate[1][2]
Formula (repeating unit)	(Mg,Al)2Si4O10(OH)·4(H2O) Al2Mg2◻2Si8O20(OH)2(H2O)4 · 4H2O[3]
IMA symbol	Plg[4]
Strunz classification	9.EE.20[3]
Crystal system	Monoclinic,[3] orthorhombic[5]
Crystal class	Prismatic (2/m)[3] (same H-M symbol)
Space group	B2/m and setting C2/m, [3] P 21 21 21[6]
Unit cell	a = 12.78 Å, b = 17.86 Å, c = 5.24 Å; β = 95.78°; Z = 4[3]
Identification
Color	White, grayish, yellowish, gray-green[3]
Crystal habit	Commonly fibrous, tangled mats known as mountain leather. Individual, small crystals are lath-shaped[3]
Cleavage	Distinct/good, good on {110}[3]
Tenacity	Tough[3]
Mohs scale hardness	2 – 2.5[3]
Luster	Waxy, earthy[3]
Diaphaneity	Translucent[3]
Specific gravity	1 – 2.6[3]
Density	2.1 - 2.6 g/cm3 (Measured); 2.35 g/cm3 (Calculated)[3]
Optical properties	Biaxial (−)[3]
Refractive index	nα = 1.522 – 1.528 nβ = 1.530 – 1.546 nγ = 1.533 – 1.548[3]
Birefringence	δ = 0.011 – 0.020[3]
Pleochroism	X= pale yellow Y=Z= pale yellow-green[3]
Common impurities	Fe,K [3]
References	[1][2][3][7]

Palygorskite (Russian: Палыгорскит) or attapulgite is a magnesium aluminium phyllosilicate with the chemical formula (Mg,Al)₂Si₄O₁₀(OH)·4(H₂O) that occurs in a type of clay soil common to the Southeastern United States. It is one of the types of fuller's earth. Some smaller deposits of this mineral can be found in Mexico, where its use is tied to the manufacture of Maya blue in pre-Columbian times.^[2][3][8]