Herbertsmithite
Herbertsmithite from Caracoles, Sierra Gorda District, Tocopilla Province, Chile. Size: 4.5 x 4.4 x 2.7 cm.
General
Category	Halide mineral
Formula (repeating unit)	ZnCu3(OH)6Cl2
IMA symbol	Her[1]
Strunz classification	3.DA.10c
Crystal system	Trigonal
Crystal class	Hexagonal scalenohedral (3m) H-M symbol: (3 2/m)
Space group	R3m
Unit cell	a = 6.834 Å c = 14.075 Å; Z = 3
Identification
Color	Light green, blue-green
Crystal habit	Aggregates of rhombohedral crystals
Cleavage	Good on {1011}
Tenacity	Brittle
Mohs scale hardness	3–3.5
Luster	Vitreous to adamantine
Streak	Light green
Diaphaneity	Transparent
Specific gravity	3.75-3.95
Optical properties	Uniaxial (-)
Refractive index	nε 1.817, nω 1.825
Birefringence	0.0080
References	[2][3]

Herbertsmithite is a mineral with chemical structure ZnCu₃(OH)₆Cl₂. It is named after the mineralogist Herbert Smith (1872–1953) and was first found in 1972 in Chile. It is polymorphous with kapellasite and closely related to paratacamite. Herbertsmithite is generally found in and around Anarak, Iran, hence its other name, anarakite.

Herbertsmithite is associated with copper mineralizations in syenitic porphyries and granites in Chile and in Triassic dolomite formations in Iran. It has also been reported from the Osborn District in the Big Horn Mountains of Maricopa County, Arizona and the Lavrion District Mines of Attica, Greece.^[2]

Herbertsmithite has a vitreous luster and is fairly transparent with a light-green to blue green color. Herbertsmithite has a Mohs hardness of between 3 and 3.5 and is known to have a brittle tenacity. The crystal's density has been calculated at 3.76 g/cm³.

Herbertsmithite, in a pure synthetic form, was discovered in 2012 to be able to exhibit the properties of a quantum spin liquid, a generalized form of strongly correlated quantum spin liquid^[4] due to its Kagome lattice structure.^[5] Herbertsmithite is the first mineral known to exhibit this unique state of magnetism: it is neither a ferromagnet with mostly aligned magnetic particles, nor is it an antiferromagnet with mostly opposed adjacent magnetic particles; rather its magnetic particles have constantly fluctuating scattered orientations.

Optical conductivity observations^[6] suggest the magnetic state in herbertsmithite is a type of emergent gauge field of a gapless U(1) Dirac spin liquid. Other experiments ^[7][8][9] and some numerical calculations suggest instead that it is a ${\displaystyle \mathbb {Z} _{2}}$ spin liquid (or in other words, has a ${\displaystyle \mathbb {Z} _{2}}$ topological order). To clarify the situation, it is useful to carry out a number of experiments.^[10]

References