Methodological strategies to assess the degree of bone preservation for ancient DNA studies.

Background: Archaeological bones contain only small amounts of DNA due to post-mortem DNA degradation and the changes endogenous DNA is subjected to during diagenesis. An important step before undertaking such time-consuming and costly analyses as ancient DNA investigation is to predict the presence of DNA in ancient samples. To date, the leading screening method has been amino acid racemization; however, other analytical techniques can also be used to assess the degree of bone preservation.

Effects of merohedric twinning on the diffraction pattern.

In merohedric twinning, the lattices of the individuals are perfectly overlapped and the presence of twinning is not easily detected from the diffraction pattern, especially in the case of inversion twinning (class I). In general, the investigator has to consider three possible structural models: a crystal with space-group type H and point group P, either untwinned (H model) or twinned through an operation t in vector space (t-H model), and an untwinned crystal with space group G whose point group P' is obtained as an extension of P through the twin operation t (G model). In 71 cases, consideration of the reflection conditions may directly rule out the G model; in seven other cases the reflection conditions suggest a space group which does not correspond to the extension of H by the twin operation and the structure solution or at least the refinement will fail.

New RE microporous heteropolyhedral silicates containing 4(1)5(1)6(1)8(2) tetrahedral sheets.

Four heteropolyhedral microporous silicates, A(3)RESi(6)O(15).2.25H(2)O, crystallizing in the Cmm2 space group and based on 4(1)5(1)6(1)8(2) tetrahedral sheets [A(3) = Na(2.

Two new silicate structures based on a rhodesite-type heteropolyhedral microporous framework.

Two new members of the mero-plesiotype rhodesite series [Sr(2)Na(2)(Si(8)O(19)).4H(2)O, abbreviated as TR09; SrNa(4)(Si(8)O(19)).4H(2)O, TR10] have been hydrothermally synthesized in Teflon-lined autoclaves at 503 K and structurally characterized using X-ray diffraction single-crystal data.

Order-disorder character and twinning in the structure of a new synthetic titanosilicate: (Ba,Sr)4Ti6Si4O24 x H2O.

Prismatic crystals of the title compound, up to 100 microm long and {001} twinned by metric merohedry, were obtained as a side-product of a hydrothermal run devoted to synthesizing the heterophyllosilicate lamprophyllite. Single-crystal X-ray diffraction data were collected on a Bruker-AXS Smart Apex diffractometer from a crystal with approximate composition (Ba(0.80)Sr(0.

Penkvilksite-2O: Na2TiSi4O11.2H2O.

The crystal structure of synthetic penkvilksite-2O, disodium titanium tetrasilicate dihydrate, Na(2)TiSi(4)O(11).2H(2)O, a microporous titanosilicate, confirms the major features of a previous model that had been obtained by order-disorder (OD) theory from the known structure of penkvilksite-1M. An important difference from the previous model involves the hydrogen bonding of the water molecule which, on the basis of a Raman spectrum and the finding of only one of the two H atoms, is proposed to be disordered about a fixed O-H direction.

Overlooked problems in manifold twins: twin misfit in zero-obliquity TLQS twinning and twin index calculation.

It is shown that the twin index n calculated, according to Friedel, as a function of the indices (hkl) and [uvw] of the lattice plane and lattice direction defining the cell of the twin lattice applies only to twofold twins, i.e. twins where the twin element is of order 2.

The derivation of twin laws in non-merohedric twins. Application to the analysis of hybrid twins.

An algorithm is presented to derive the twin laws in non-merohedric twins through the systematic search for quasi-perpendicular lattice planes/directions. The twin lattice, i.e.

Applied geminography--symmetry analysis of twinned crystals and definition of twinning by reticular polyholohedry.

The common classification of twinning into the four categories of twinning by merohedry (complete and exact overlap of the lattices of the twinned crystals), pseudomerohedry (complete but approximate overlap), reticular merohedry (partial but exact overlap) and reticular pseudomerohedry (partial and approximate overlap) is revised in terms of the complete (translational and point) lattice symmetry of the twin and of the individual. The new category of reticular polyholohedry is introduced for twins where the twin lattice has the same point symmetry but a different orientation of the individual lattice. It is shown that the degeneration to twin index 1 relates, in a parallel way, reticular merohedry to metric merohedry and reticular polyholohedry to syngonic merohedry.