*2.1. Occurrence, General Appearance, Physical Properties and Optical Data*

Krasnoshteinite was found in the core of the borehole #2001, with a depth of 247.6–248 m, drilled in the Romanovskiy area (30 km south of the city of Berezniki) of the Verkhnekamskoe potassium salt deposit, Perm Krai, Western Urals, Russia. The general data on this well-known. Huge deposits are given in monographs [5,6]. Krasnoshteinite occurs in halite-carnallite rock and is associated with dritsite (Li2Al4(OH)12Cl2·3H2O) [7], dolomite, magnesite, quartz, Sr-bearing baryte, kaolinite, potassic feldspar, congolite, members of the goyazite SrAl3(PO4)(PO3OH)(OH)6–woodhouseite CaAl3(PO4)(SO4)(OH)6 series, fluorite, hematite, and anatase. The new mineral was probably formed as a result of diagenetic or post-diagenetic processes in halite-carnallite evaporitic rock of the Layer E of the Verkhnekamskoe deposit.

Krasnoshteinite occurs as separate tabular to lamellar crystals of up to 0.06 × 0.25 × 0.3 mm (Figure 1a,b) and their parallel intergrowths (Figure 1c) embedded in carnallite and halite. In some cases, tiny crystals of krasnoshteinite overgrow its larger crystal in random orientations to form a crystal cluster (Figure 1d). Samples shown in Figure 1 were separated after dissolution of a host halite-carnallite rock in water.

Crystals of krasnoshteinite are flattened on the *ab* plane. The pedions {010} and {0-10} and the pinacoid {100} are major lateral faces of the tabular crystals. The surface of the most developed "face" of a crystal is typically complicated, rough, and demonstrating coarse or/and fine striation along {100} (Figure 1); it is usually composed by several poorly formed faces belonging to the 0*kl* zone.

Krasnoshteinite is a transparent colorless mineral with a white streak and vitreous luster. It is brittle, with a Mohs hardness is ca. of 3. Krasnoshteinite demonstrates perfect cleavage on {010} and an imperfect cleavage on {100}. A fracture is stepped (observed under the microscope). The mineral is non-fluorescent in the ultraviolet light. The density measured by flotation in heavy liquids (bromoform + dimethylformamide) is 2.11 (1) g/cm3, and the density calculated using the empirical formula and the unit-cell parameters determined from single-crystal X-ray diffraction data is 2.115 g/cm3.

In plane polarized light, krasnoshteinite is colorless and non-pleochroic. It is optically biaxial (+), α = 1.563 (2), β = 1.565 (2), γ = 1.574 (2) (589 nm). 2*V* (meas.) = 50 (10)◦ and 2*V* (calc.) = 51◦. Dispersion of optical axes is distinct, *r* > *v*. Optical orientation is: *Y* = *b*, and *X* = *a*.

**Figure 1.** Separate crystals (**a**,**b**) and crystal clusters ((**c**,**d**): small crystals overgrow large crystal) of krasnoshteinite. Blocky crystals of dolomite and distorted quartz crystal are observed in (**b**).
