A simple method for indexing powder diffraction patterns of cubic materials:(1) using the θ-values of reference

From the values of sin2θ taken from powder photographs of the cubic crystal system, a variable constant, K, which is a multiple of (h2 + k2 + l2) can be determined. The smallest value of sin2θ1 gives rise to a multiple of one, two, or three which corresponds to the smallest value of Σhl2 for the primitive, bcc and fcc crystals, respectively. The old assumption that when indexing, the occurrence of numbers of the form (h2 + k2 + l2) = 8n + 7, where n = 0.1.2.3… should cause each of (h2 + k2 + l2) to be multiplied first by two before indexing, is disapproved. Tanzania Journal of Science 28(1) 2002: 1-

Mobility and immobility of mid-ocean ridges and their implications to mantle dynamics

In the past two decades, the mobility of mid-ocean ridges relative to the mantle (absolute migration) have been correlated with major observable features, such as, spreading asymmetry and asymmetry in the abundance of seamounts. The mobility of mid-ocean ridges is also thought to be an important factor that influences the diversity of ridge-crest basalts. However, the mobility of mid-ocean ridges have not yet been defined and mapped. The absolute migration of global mid-ocean ridges since 85 Ma has been computed and mapped. Global mid-ocean ridges have migrated extensively at varying velocities during that period. Presently, the fast-migrating ridges are the Pacific-Antarctic ridges, migrating at velocities between 3.3 and 5.5 cm/yr. The slow-migrating ridges are the Mid-Atlantic and the southwest Indian ridges migrating at velocities between 0.3 and 2.0 cm/yr. Comparison of these results with mantle tomography results shows that the slow-migrating ridges have deeper depth of origin than the fast-migrating suggesting a correlation between the absolute migration velocity and the depth of origin of ridges. Furthermore, the southwest Indian ridge appears to be tapping the same portion of mantle as did the Central Indian ridge. These results have important thermo-chemical implications, such as variations in the extent of melting and mineralogical composition of the mantle beneath different ridges, which may influence mantle dynamics. Tanzanian Journal of Science Vol. 28(1) 2002: 37-4

Structure of tert-ButyloxycarbonyI-L-alanyI-L-proline Monohydrate (t-Boc-Ala-Pro)

$C_{13}H_{22}N_2O_5.H_2O$, $M_r$ = 304.37, is orthorhombic, space group ${P2}_12_12_l$, with a = 20.751 (2), b = 13.457 (1), c = 5.875 (1) A, V = 1640.49 $A^3$, $D_o$ = 1.20, $D_c$ = 1.231 Mg $m^{-3}$; Z = 4; \lambda(Cu Ka) = 1.5418 A; Z = 4; F(000) = 656. Final R = 3.9% for 1664 observed reflexions. There is one molecule of water as well as an N-H...O hydrogen bond, rendering high stability to the crystal packing. The water-bridge bond is of the same type as that of a triple helix: $O_w...O$ = 2.78, $O_w...O'$ = 2.69 and $O_w...OH$ = 2.50 A. $C^{\alpha}$ is in the trans configuration. The absolute configuration of the non-centro-symmetric structure and, therefore, of the molecular conformation was determined by anomalous dispersion. The $N'C^{\alpha}C^{\gamma}C^{\delta}$ group in the pyrrolidine ring is fairly planar. $C^{\beta}$ is readily displaced from this best plane of the five-membered ring and deviates by 0.489 A. N' and $C^{\gamma}$ are on the same side of this plane in relation to the carboxyl C'. Thus t-Boc-Ala-Pro is $C_s-C^{\gamma}-endo(C^{\beta}-exo)$. This derivative belongs to conformation B, since the dihedral angle $x_1$ = 28.55 deg takes a positive value, and it has collagen-like characteristics, with $N'-C^{\alpha}-C'-O$ , that is, the dihedral angle ${\psi}_1$ Pro, equal to 161 deg. The $C^{\alpha}$ atoms of the prolyl and alanyl residues are trans with respect to the peptide bond