Pinnoite

Pinnoite is a magnesium borate mineral with formula: MgB₂O(OH)₆[3][2] or MgB₂O₄·3(H₂O).[4] It crystallizes in the tetragonal crystal system and occurs as colorless to yellow or light green radial fibrous clusters and rarely as short prismatic crystals.

Pinnoite
Pinnoite clusters from a salt dome in the Atyrau Oblast, Kazakhstan
General
Category	Borate mineral
Formula (repeating unit)	MgB2O(OH)6
IMA symbol	Pno[1]
Strunz classification	6.BB.05
Crystal system	Tetragonal
Crystal class	Pyramidal (4) H-M symbol: (4)
Space group	P42
Unit cell	a = 7.617 Å, c = 8.19 Å; Z = 4
Identification
Color	Colorless, light yellow, yellow green
Crystal habit	Short prismatic crystals uncommon; radial fibrous clusters
Fracture	Uneven
Mohs scale hardness	3.5
Luster	Vitreous
Diaphaneity	Translucent
Specific gravity	2.27
Optical properties	Uniaxial (+)
Refractive index	nω = 1.565 nε = 1.575
Birefringence	δ = 0.010
References	[2][3][4]

Pinnoite was first described in 1884 for an occurrence in the Stassfurt potash deposit, Saxony-Anhalt, Germany and named for the mine counselor Oberbergrat Pinno of Halle, Germany.[3][2] It occurs in marine evaporite deposits and as efflorescence associated with mineral springs. It occurs with boracite and kaliborite.[3] It also occurs in the borax mines of Death Valley in California, the Da Quidam saline lake of the Qinghai-Xizang Plateau in Tibet and in Socacastro, Salta Province, Argentina.[3][2]