Charles W. Burnham

Pyroxferroite: Stability and X-ray Crystallography of Synthetic Ca0.15Fe0.85SiO3 Pyroxenoid.

Synthetic Ca0.15Fe0.85SiO3 pyroxenoid has the same (pyroxmangite) structure and very nearly the same composition as pyroxferroite, a new mineral found in Apollo 11 lunar samples. The synthetic material is not stable below pressures of approximately 10 kilobars. It appears likely that the lunar pyroxferroite has persisted in a metastable state for some billions of years.

The crystal structures of high and low clinohypersthene

Abstract The crystal structures of high ( C 2/ c ) and low ( P 2 1 / c ) monoclinic polymorphs of a natural ferrohypersthene (Mg 0.3 Fe 0.7 SiO 3 ) have been analysed from single-crystal intensity data obtained at 900° and 25°C respectively. The C -centered structure appears to be well ordered with nearly straight silicate chains and a six-coordinated M2 site. At 900°C, the space group is C 2/ c and the cell dimensions are: a = 9.869(3)A; b = 9.059(3)A; c = 5.334(2)A; β = 109.91(2)°. At room temperature, the space group is P 2 1 / c , and the cell dimensions are: a = 9.696(3); b = 8.994(3); c = 5.228(3); β = 108.60(2)°.

Positron Imaging Instrumentation

Positron imaging systems provide a unique and powerful means to examine the spatial and temporal course of radiopharmaceuticals in the body to observe normal and abnormal physiology in vivo. The information so obtained in conventional or transverse section mode complements the information on structure obtained from conventional x-ray and computerized tomography (CT) imaging. Powerful instruments are now available to provide high resolution conventional and transverse section positron images.

Transverse Section Imaging with the MGH Positron Camera

Transverse section emission imaging provides a method to quantitatively measure physiological parameters on a three dimensional basis in the body. PCII, the MGH Positron Camera, is widely used for such studies. New instruments promise higher sensitivity and resolution. The ultimate instrument would use as many of the annihilation events occurring with the body as possible.