New Mexico Mineral Symposium — Abstracts

Metamict minerals are a special class of amorphous materials which were initially crystalline. Although the mechanism for the transition is not clearly understood, radiation damage caused by alpha particles and recoil nuclei is critical to the process. Metamict minerals are generally optically isotropic, lack cleavage, display a distinct conchoidal fracture and are less dense than their crystalline equivalents. Metamict minerals recrystallize on heating, but the original pre-metamict phase may not form due to compositional changes caused by post-metamict alteration.

The microlite of the Harding petmatite is noteworthy because it occurs in both the crystalline and metamict state. Microlite belongs to the pyrochlore group, a complex croup of cubic Nb-Ta-Ti oxides with the general formula

A_{2_m}B₂O₆(O,0H,F)_{1_n}•pH₂0

where A = Na, Mg, R, Ca, Mn ,Fe²⁺, Sr, Sn, Sb, Ba, Pb, Bi, REE, Th, U⁶⁺, U⁴⁺

and B = Ta, Nb, Ti, Fe³⁺; m = 0 to 1; n = 0 to 1, p = 0 to ?.

For the Harding microlite, A-site cations are dominantly Ca, Na and U; the dominant B-site cation is Ta. The Harding pegmatite probably contains the largest deposit of microlite in the world. During a five-year period, 1942-1947, the pegmatite yielded over 10,000 kg of Ta-Nb concentrate. Microlite occurs late in the paragensis of the pegmatite and is a common accessory in three of the eight lithologic units, in replacement masses of the wall zones, and in core units of the eastern extensions of the dike system. Anhedral to euhedral crystals range from 0.1 to 25 mm in size, most commonly 0.5 to 3 mm. Crystals occur dispersed and isolated and display modified octahedral and dodecahedral forms. The degree of metamictness is directly related to the amount of uranium and thorium. The maximum uranium content in metamict microlites approaches five percent of the A-site cations. During annealing experiments in air over the range of 300°C to 1000°C, the metamict microlites recrystallize. Recrystallization begins at 300°C with a decrease in unit cell parameters with increasing temperature.