Rynersonite, Ca(Ta,Nb)<2) O<6), a new mineral from San Diego County, California

Rynersonite, a new species isostructural with synthetic CaTa₂O_e, is found in the Himalaya pegmatite-aplite dike system, Mesa Grande district, San Diego County, California. It occurs as creamy-white to reddish-pink felted masses of fibrous to lath-like crystals. Individual laths average less than 0.5 mm in length and 0.05 mm in thickness. Electron diffraction patterns show that the laths are elongated along a and flattened on (010). Hardness is about 4.5; luster earthy; streak white; fracture uneven. Rynersonite is biaxial positive with moderate 2V; indices of refraction are greater than 2.05; birefringence is about 0.14; X = c, Y = b, Z = a; with X = Y = straw-yellow, Z = light straw-yellow.

Reddish-pink rynersonite is orthorhombic, space group Pmnb, with a = 7.505(1), b = 11.063(2), c = 5.370(1)A, Z = 4, Gcalc = 6.394; for the composition Ca(Ta_1.21Nb_0.77)O₆, Gmeas = 6.402. The six strongest lines of the X-ray pattern (d, in A; intensity; hkl) are 3.038 100 031, 4.835 91 011, 2.964 91 211, 3.754 71 200, 2.683 50 002, and 2.359 50 231. Heating in air for 20 hours at 550°, 800°, and 1050°C produces no irreversible changes in cell dimensions or structure.

Electron microprobe analyses indicate a range of composition from Ca(Ta_1.44Nb_0.51)O₆ to Ca(Ta_1.21Nb_0.77)O₆. Rynersonite formed as an alteration product of stibiocolumbite—stibiotantalite crystals and is intimately associated with minor amounts of antimonian microlite and fersmite, Ca(Nb,Ta)₂O₆. Although rynersonite and fersmite are both orthohombic AB₂O₆ compounds, they differ structurally. The new mineral has space group Pmnb; fersmite, Pcan.

The mineral is named for Eugene B. and Buel F. Rynerson, and their late father, Fred J. Rynerson, in recognition of their lifelong interest in mining the gem-bearing pegmatites in Mesa Grande, California.