Thomas P. Rooney

Hornblende deformation features

We have plastically deformed hornblende in laboratory compression experiments. (101) twinning is a prominent deformation mechanism in single-crystal tests. The twin system is K 1 = (101), N 1 = (101), K 2 = (100), N 2 = [001]. Translation gliding has also been induced on the system T = (100), t = [001]. Although intragranular plastic deformation effects are not commonly observed in naturally deformed hornblende, we describe features in amphibolite from the southwestern Alps that are similar to those produced experimentally.

Deformation Twins in Hornblende

Hornblende deformation twins with twin planes parallel to (�101) are produced experimentally in single crystals by compression parallel to the c axis. Twinning occurs at confining pressures from 5 to 15 kilobars and temperatures from 400� to 600�C (strain rate, 10-5 per second).

Mineralogic Nature and Origin of Phosphorite, Beaufort County, North Carolina

A large Miocene phosphorite deposit on the Atlantic Coastal Plain in Beaufort County, North Carolina, contains estimated reserves of 10 billion tons. The phosphorite zone, where examined, is 60 feet thick, and the P 2 O 5 content of the untreated ore is about 18 per cent. The sediment consists of phosphate pellets, sand-sized quartz grains, and clay, with thin beds of indurated coquina and phosphatic dolomite. The phosphate pellets contain quartz grains, glauconite, carbonaceous matter, and organic remains. Although most of the pellets are structureless, some show concentric layering around a nucleus. Chemical, X-ray-diffraction, infrared-spectral, and other data demonstrate that the phosphorite mineral is francolite, and that CO 2 is an integral part of the apatite structure. The clay fraction of the phosphorite includes montmorillonite, illite and clinoptilolite. The clay minerals and zeolite probably originated through alteration of volcanic detritus from an undetermined source. Studies indicate origin of the phosphorite to have been in a shallow marine basin characterized by a reducing environment, phosphate replacement of calcareous sediments, local reworking and alteration of certain of the pellets, and coincidence of phosphorite deposition with pyroclastic activity.

Weakening of Dunite by Serpentine Dehydration

A shear press has been used to determine the mechanical behavior of serpentinized dunite and forsterite at normal pressures to 50 kilobars, temperatures to 900�C; and strain rates from 10-1 to 10-4 per second. The shear strength of dunite, containing less than 5 percent by volume of serpentine, is reduced by at least 30 percent as the temperature is raised from 300� to 520�C. Abundant kink bands develop at normal pressures above 35 kilobars at 27�C and at lower pressures as the temperature is increased.

High-Pressure Reactions and Shear Strength of Serpentinized Dunite

The recently reported Pronounced decrease in shear strength of serpentine-bearing rocks at 30 to 40 kilobars in the temperature range 300� to 520 � C may be attributed to the transformation of serpentine to a Pressure-dependent, 10-angstrom,2: 1 layer silicate plus brucite and periclase. This reaction increases density by about 8.5 percent.

Chinoptilolite: A New Occurrence in North Carolina Phosphorite.

The zeolite, clinoptilolite, has been identified in a Miocene phosphorite deposit in North Carolina. It occurs in great abundance in the clay fraction of the phosphorite both as well formed, separate crystals and as clusters of crystals attached to granular particles.