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Idrialite, Skaggs Springs Mine, Sonoma County, California (size: 6.3 x 4.1 x 1.8 cm
Category Organic mineral
(repeating unit)
Strunz classification 10.BA.20
Dana classification
Crystal system Orthorhombic
Unknown space group
Unit cell a = 8.07, b = 6.42
c = 27.75 [Å]; Z = 4
Color Greenish yellow, light brown, colorless
Cleavage {001}, perfect; {100}, poor
Fracture Conchoidal
Mohs scale hardness 1.5
Luster Vitreous to adamantine
Specific gravity 1.236
Optical properties Biaxial (+)
Refractive index nα= 1.557 nβ = 1.734 nγ = 2.07
Pleochroism X = pale yellow; Y = Z = yellow
2V angle 84°
Ultraviolet fluorescence Short UV=blue, orange, yellow, green white
References [1][2][3]

Idrialite, or idrialine, is a soft, orthorhombic hydrocarbon mineral with chemical formula: C22H14.[1][2][3] It is usually greenish yellow to light brown in color with bluish fluorescence.

Raman spectroscopy studies indicate that it may be a mixture of complex hydrocarbons including benzonaphthothiophenes (chemical formula: C16H10S) and dinaphthothiophenes (chemical formula: C20H12S).[4]

Discovery and occurrence[edit]

It was first described in 1832 for an occurrence in the Idrija region west of Ljubljana, northwestern Slovenia.[2] It also occurs at Skaggs Springs, Sonoma County, western Lake County and the Knoxville Mine in Napa County, California.[1] It has also been reported from localities in France, Slovakia and Ukraine.[2]

It can be found mixed with clay, pyrite, quartz and gypsum associated with cinnabar in the Idrija occurrence and with metacinnabar, realgar and opal in the Skaggs Spring location.[1]

Its combustibility gave rise to the term "inflammable cinnabar", which is one of its synonyms.