THE NEW - METAMORPHOGENIC GENETIC TYPE OF CALZIRTITE

摘要

The calzirtite has been found out within metamorphized titanum-bearing magnesian skarns developed at the contact between low-alkaline ore-bearing gabbroids of Kusinsky massif and xenoliths of dolomite marbles (Zelentsovskaya pit, Kusinskoye Fe-Ti ore deposit, the Southern Urals). Metamorphogenic calzirtite (MC) participates there within mineral aggregates appeared under conditions of the prehnite-pumpellyite facies of metamorphism (clinochlore, perovskite, hydrotalcite, manasseite, magnesioferrite, etc). MC forms pseudocubic tri-unit intergrowths up to 2 mm in size. VHN_(50-100)=985-1180, average 1070 kg/mm~2 (n=19). Reflection in air, R, percent: 16.5 (400 nm), 13.6 (500), 12.3 (600), 12.6 (700). Chemical composition MC (wt. percent, n=8): ZrO_2 67.35, HfO_2 1.59, ThO_2 0.03, TiO_2 17.49, Nb_2O_5, Ta_2O_5 not detected; FeO 0.13, MnO 0.03, MgO 0.11, CaO 12.01, SrO 0.58, Ce_2O_3 0.06, La_2O_3, Y_2O_3 not detected; Yb_2O_3, V_2O_3- traces, Na_2O 0.05, K_2O 0.01, the sum 99.44. Formula MC calculated for 9 atoms of metals is as following: (Ca_(1.93)Sr_(0.05)Ce_(0.01)Na_(0.01))_(2.00)(Ti_(1.97)Mg_(0.02)Fe_(0.02))_(2.01)(Zr_(4.92) X Hf_(0.07))_(4.99)O_(16), which is quite near Ca_2Zr_5Ti_2O_(16) or else 5ZrO_2 X 2CaTiO_3. MC distinguishes itself from the calzirtite- the typomorphic mineral of carbonatites, alkaline-ultrabasic rocks and alkaline metasomates- by the minor contents of Nb, Ta, REE, Th and the lower Zr/Hf value (40-45 against 60-95 in the calzirtite of alkaline associations). Its unit cell parameters: a_0=15.189 (2) A, c_0=10.111 (2) A, V=2333 A~3, Z=8. X-ray density 5.070 g/cm~. Space group I4_1/acd. The structure of MC is completely ordered, thus differing also from the calzirtite of alkaline rocks, which has the structure only partly ordered. It is possible that MC has appeared while the metamorphism of magnesian skarns due to reaction between perovskite and baddeleyite.