Karlite, a new borate mineral with the idealized formula Mg7(BO3)3(OH,Cl)5 occurs in a clinohumite-chlorite marble associated with ludwigite at Schlegeistal, Zillertaler Alpen, Austria. The mineral is white to hght green and has a silky luster. It occurs as aggregates of minute needles and prisms elongated parallel to c, as much as 10 mm long, which sometimes have rosette-like forms. Karlite is biaxial negative with a mean 2 V of 24°, α = 1.589, β = 1.632, γ = 1.634, γα = 0.045, X = c, Y = b, Z = a; and is colorless in thin section. The (001) cleavage is perfect. The two most common forms are {110} and {100}. Hardness is 5.5. Streak is colorless.

Karlite is orthorhombic, P212121, with a = 17.929(5), b = 17.600(5), c = 3.102(1)Å, Z = 4, Gcalc = 3.02, Gmeas = 2.80 to 2.85. The six strongest lines of the X-ray pattern (d, in Å, intensity, hkl) are 2.21,100,810,740,441; 2.83,92,620;2.25,87,251; 2.78,82,260,221,540; 12.53,76,110; and 2.89, 68, 160,221,121. IR spectra indicate that water is present as (OH) only. DTA shows only one strong nonreversible endotherm at 830°C. Melting occurs at 1345°C.

Combined microprobe, thermogravimetric and wet chemical analysis yielded SiO2 0.02, TiO2 < 0.01, B2O3 22.92, A12O3 2.96, Cr2O3 < 0.01, FeO 2.27, MnO 0.17, MgO 58.90, CaO 0.14, K2O 0.01, Na2O 0.01, F 0.16, Cl 3.26, H2O+ 10.40, H2O 0.63 (not included in total), total 101.22, O ≡ F 0.82, corr. total 100.40.

The mineral is named in honor of Prof. Dr. Franz Karl, late professor of mineralogy and petrography at the Christian Albrechts University, Kiel, West Germany; in recognition of his geologic studies of the Eastern Alps.

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