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Globally, borax is one of the most important borate minerals, both industrially and from a technological viewpoint. Despite its importance, there have been only a few reports on the structural changes of borax upon heating. In this study, we have investigated the thermal behavior and structural characteristics of borax using thermal analysis, in situ high-temperature synchrotron X-ray diffraction, in situ variable-temperature single-crystal X-ray diffraction, and quantum chemical calculations. Differential scanning calorimetry (DSC) curve showed a large endothermic peak at 349 K corresponding to the dehydration reaction of borax. X-ray diffraction (XRD) pattern remained almost unchanged up to 353 K. Above 363 K, the XRD peaks gradually became less intense until they disappeared at 403 K. The unit-cell volume continuously increased with increasing temperature and became constant just before its phase transition to tincalconite. The volumetric thermal expansion coefficient between 113 and 323 K was 7.84 × 10–5 K–1. The a and c lattice parameters exhibited a slight increase trend with increasing temperature, whereas the b lattice parameter increased significantly. Therefore, the thermal expansibilities followed the order b/b0 >> c/c0 > a/a0. The increasing b lattice parameter was closely related to the elongation of the O4···H7B hydrogen-bond interaction along the b-axis. Na-O bond lengths were isotropically expanded with increasing temperature, whereas the B-O bond lengths and angles remained unchanged even after the phase transition to tincalconite. Molecular orbital calculations revealed that an electron cloud shared by two borate tetrahedra led to the formation of a large electron cloud distribution over the B4O5(OH)4 cluster. The intramolecular interactions with substantial covalent character made the cluster quite rigid. The existence of borate minerals containing the B4O5(OH)4 clusters in their structure evidenced the presence of moderately acidic or alkaline water, wherein the borate minerals grew via the incorporation of B4O5(OH)4 clusters. Our results indicate that the connection geometry of the fundamental building block consisting of Bφ3 triangles and Bφ4 tetrahedra (φ: O2–, OH–) can potentially be used as a palaeoenvironmental indicator. [ABSTRACT FROM AUTHOR]

Carbonate minerals are major contributors to carbon sequestration in geological deposits; however, their nature and behavior remain unclear. Amorphous magnesium carbonate (AMC) is formed as a precursor to crystalline magnesium carbonates and as a product of thermal decomposition of nesquehonite (NSQ). In this study, the AMCs formed during the crystallization and decomposition of NSQ were investigated using X-ray diffraction (XRD) and atomic pair distribution function (PDF) methods. An AMC with a hydromagnesite-like structure (AMC-I) was formed immediately after mixing MgCl2 and Na2CO3 solutions. After 5 min of stirring, no change was observed in the XRD pattern; however, the PDF pattern changed. This suggests that the medium-range ordered structure of AMC-I transformed into an intermediate structure (AMC-II) between AMC-I and NSQ. After 10 min of stirring, the AMC-II crystallized into NSQ. In the case of Rb2CO3, the AMC-II structure was formed immediately after the mixing of solutions and was stable for three days. AMC-II in the Rb2CO3 solution appeared to be in equilibrium with energetic local minima, indicating the existence of polyamorphism in AMC. When Cs2CO3 solution was used, the first precipitate had an AMC-I structure. By stirring for 5 min, the AMC-I was transformed to AMC-II, and after 10 min of stirring, a few quantities crystallized into NSQ. After three days, NSQ dissolved and transformed back into AMC-I. Thus, it is inferred that the crystallization of NSQ is significantly influenced by alkali cations in aqueous solutions. The AMC formed during the thermal decomposition also possesses the AMC-I structure. [ABSTRACT FROM AUTHOR]

Molecular crystals of an [As.sub.4][S.sub.4-x][Se.sub.x] (0 [less than or equal to] x [less than or equal to] 4) solid solution were grown at 400[degrees]C using vacuum-sublimation and were characterized using single-crystal X-ray diffraction and electron microprobe analyses. The solid solution crystallizes in the monoclinic space group [P2.sub.1]/n with lattice parameters of a = 9.33-9.57 [Angstrom], b = 13.57-13.82 [Angstrom], c = 6.60-6.74 [Angstrom], [beta] = 106.4-106.7[degrees], V = 801-854 [[Angstrom].sup.3] and Z = 4. Substitution of Se for S in the [As.sub.4][X.sub.4] molecule causes nearly isotropic unit-cell expansion retaining identical molecular packing to the low-temperature form [alpha]-[As.sub.4][S.sub.4] (natural realgar). Crystal structure refinements show that Se is distributed with a strong preference for the X2 site. The As-X3 bond is slightly elongated in the solid-solution series. The bond length difference between As-X3 and the other As-X bonds increases with the Se content, thereby inducing atomic shift of the As4 bonds to X3 and X4 toward the side of X3. The atomic position of As4 returns to the place on the bisector between X3 and X4 at the [As.sub.4][Se.sub.4] end-member composition. Although As-As bonds shorten with Se content, the volume of the [As.sub.4][X.sub.4] molecule and the intercentroid distance between the nearest [As.sub.4][X.sub.4] molecules increase continuously, resulting in isotropic expansion of the unit cell in this solid-solution series. The strong site preference of Se for the X2 site suggests that S may be detached from that site more easily than from the remaining ones. On the contrary, very slight substitution of Se for S in the X3 site indicates that S in the X3 site is strongly bonded to two As atoms. Therefore, S is more stably incorporated into the X3 site than into the X2 site; these molecular features of the Se substitution in the [As.sub.4][X.sub.4] molecule are consistent with the proposed photoinduced transformation dynamics in realgar. Continuous illumination of the Se-containing realgar engenders alteration to pararealgar under identical transformation mechanisms to those of the realgar. Keywords: Realgar, [As.sub.4][S.sub.4], seleno-realgar, [As.sub.4] [Se.sub.4], solid solution, crystal structure

Guanacoite with high Mg content from the type locality (El Guanaco mine, Chile) was investigated to elucidate the crystal-chemical characteristics of this arsenate mineral. In this study, WDS X-ray intensity maps revealed that guanacoite single crystals from the investigated specimen show distinct and characteristic compositional zoning, which varies cyclically along [100], which is in the direction of the elongation of the prismatic crystals (Witzke et al. 2006); the boundaries separating different compositional regions lie perpendicular to the prism faces. The crystal structure of a selected fragment [a = 5.459(2), b = 16.808(9), c = 6.917(3) [Angstrom], [beta] = 100.44(1)[degrees], V = 624.1(5) [[Angstrom].sup.3], [P2.sub.1]/c, and Z = 2], was solved using direct methods and refined to an R index of 3.09% for 1385 observed [[I.sub.o] > 4[sigma]([I.sub.o])] reflections measured at 103 K using MoK[alpha] X-radiation. The chemical formula based on the refinement is [([Cu.sub.0.85][Mg.sub.0.15]).sub.2] [Mg.sub.2]([Mg.sub.0.83][CU.sub.0.17])[(OH).sub.4][([H.sub.2]O).sub.4] [(As[O.sub.4]).sub.2]. The structure model obtained is in good agreement with the previous structure determination, although the occupancies of the three M sites (M = Mg or Cu) are distinctly different, i.e., the sites are considerably richer in Mg, especially M1 and M3. These structural results suggest that guanacoite's chemical formula should be enlarged from the Cu-rich composition [Cu.sub.2] [Mg.sub.2](Mg,Cu)[(OH).sub.4][([H.sub.2]O).sub.4][(As[O.sub.4]).sub.2] to [(Cu,Mg).sub.2][Mg.sub.2](Mg,Cu)[(OH).sub.4][([H.sub.2]O).sub.4] [(As[O.sub.4]).sub.2] to include the Mg-rich members (Mg up to 3.395 apfu). It appears that the chemical variability of guanacoite is mainly due to Mg-Cu substitution on the M1 and M2 sites. Keywords: Guanacoite, copper magnesium arsenate hydrate, crystal structure, chemical analysis, Jahn-Teller distortion

Light-induced degradation in realgar (arsenic sulfide) has been studied by means of four-circle single-crystal X-ray diffraction and X-ray photoelectron spectroscopy. Because of the alteration of realgar exposed to light, the a lattice parameter and c sin[beta] value increase linearly from 9.327 to 9.385 [Angstrom] and from 6.320 to 6.364 [Angtrom], respectively. In contrast, the b lattice parameter remains substantially constant. Anisotropic variations of the lattice parameters engender a continuous increase of the unit-cell volume from 799.5 to 810.4 [Angstrom].sup.3]. Nevertheless, no correlation exists between the continuous increase of the unit-cell volume and the bond distance variations in [As.sub.4][S.sub.4] molecules because the [As.sub.4][S.sub.4] molecule in the unit cell expands very little during light exposure. The most pronounced change was in the distance between centroids in [As.sub.4][S.sub.4] cages. The spread of [As.sub.4][S.sub.4] intermolecular distances increases continuously from 5.642 to 5.665 [Angstrom], which directly affects the unit-cell volume expansion of realgar. In addition, the Ols peak increases rapidly after light exposure. The result substantiates the following reaction proposed by Bindi et al. (2003): 5[As.sub.4][S.sub.4] + 3[O.sub.2] [right arrow] + 4[As.sub.4][S.sub.5] + 2[As.sub.2][O.sub.3]. That is, realgar is transformed into pararealgar if oxygen exists and produces the [As.sub.4][S.sub.4] molecule. The additional S atom contributes to anisotropic expansion for the a and c axes because the direction of the additional S atom points toward [414] in the unit cell. Furthermore, an S atom in the [As.sub.4][S.sub.5] molecule is released from one of the equivalent As-S-As linkages in [As.sub.4][S.sub.5] which becomes the [As.sub.4][S.sub.4] molecular of pararealgar. After the [As.sub.4][S.sub.5] molecule is divided into an S atom (radical) and the [As.sub.4][S.sub.4] (pararealgar type) molecule, the free S atom is re-attached to another [As.sub.4][S.sub.4] (realgar type) molecule, and reproduces an [As.sub.4][S.sub.5] molecule. The reproduced [As.sub.4][S.sub.5] molecule is again divided into an S atom (radical) and an [As.sub.4][S.sub.4] (pararealgar type) molecule. This cycle whereby realgar is indirectly transformed into pararealgar via the [As.sub.4][S.sub.5] molecule is promoted by light and repeated during light exposure.

The crystal structures of chalcostibite CuSb[S.sub.2] (orthorhombic, space group Pnma, a = 6.018(l), b = 3.7958(6), and c = 14.495(7) A, V 331.1(1) [A.sup.3] Z = 4, R1 = 0.040, wR2 = 0.155 for 533 reflections) and emplectite CuBi[S.sub.2] (orthorhombic, space group Prima, a = 6.134(1), b = 3.9111 (8), and c = 14.548(8) A, V = 348.8(2) [A.sup.3], Z = 4, R1 = 0.037, wR2 = 0.112 for 492 reflections) were redetermined using a four-circle diffractometer and graphite-monochromatized MoK[alpha] radiation. These two crystal structures are composed of M[S.sub.5] square pyramids (M = Sb and Bi) and nearly regular Cu[S.sub.4] tetrahedra. The five M-S bond distances in the Sb[S.sub.5] square pyramid in chalcostibite are always shorter than corresponding distances in the Bi[S.sub.5] square pyramid in emplectite because the Sb atom is smaller than the Bi atom. The a cell parameter increases appreciably from chalcostibite to emplectite not only because of increasing M-S bond distances in the M[S.sub.5] square pyramid, but also because of increasing Cu-S2-Cu bond angles along a. The increase in the b cell parameter is caused mainly by increasing M-S bond distances along b. In contrast, the slight increase of the c cell parameter is largely brought about by decreasing Cu-S2-Cu bond angles ascribed to weakened stereochemical activity of Bi 6[s.sup.2] lone-pair electrons. The anisotropic change of unit-cell parameters from chalcostibite to emplectite is strongly associated with the positions of the lone-pair electrons in the unit cell.

Structural refinements of single crystal X-ray diffraction data for synthetic [(Sb,Bi).sub.2][S.sub.3] solid solutions revealed structural variations in the stibnite ([Sb.sub.2][S.sub.3])-bismuthinite ([Bi.sub.2][S.sub.3]) series. Coordination environments of the M cations are (3 + 4)-fold for the M1 site and (5 + 2)-fold for the M2 site. For the M1 and M2 polyhedra, the short M-S bond lengths increase constantly with increasing Bi concentration, whereas the long M-S bond lengths decrease continuously. The S-M-S interatomic angles interposing lone-pair electrons increase continuously from stibnite to bismuthinite. Stereochemical activity of the lone-pair electrons induces configurational changes of ligands around the M cations from elongated ellipsoidal coordinations to spheroidal ones. Substitution of [Bi.sup.3+] for [Sb.sup.3+] in the solid solution expands the basic building block, which causes linear increase of the b lattice parameter with slight positive deviation from Vegard's law. This feature is ascribed to order-disorder with concentration of Sb at the M1 site and Bi at the smaller M2 site. Furthermore, increased Bi content engenders both expansion of the basic building block and contraction of intervals between these blocks, contributing to smaller changes in the a and c lattice parameters than in the b lattice parameter. The M2 polyhedra expand relative to the M1 polyhedra with increasing Bi content because the large Bi cation is concentrated at the smaller M2 site. One striking characteristic of [(Sb,Bi).sub.2][S.sub.3] crystal structures is that geometries of central M cation and ligand atoms can be adapted flexibly to transformation of stereochemical activity from 5[s.sup.2] lone-pair electrons to Bi 6[s.sup.2] lone-pair electrons by altering the centroid-central atom distance and by changing angles of the centroid-central atom to the a axis.

The crystal structure of getchellite, [As.sub.0.98] [Sb.sub.1.02] [S.sub.3.00], from the type-locality (the Getchell mine, Humboldt County, Nevada), monoclinic, [P2.sub.1]/a, a = 11.949(3), b = 9.028(1), c = 10.130(2) [Angstrom], [beta] = 116.15(1)[degrees], V = 980.9(4) [[Angstrom].sup.3], Z = 8, was solved by direct methods and refined with full-matrix least-squares techniques to R = 0.058 and Rw = 0.062 for 505 observed reflections [[I.sub.o] > 5 [sigma]([I.sub.o])] collected using MoK[alpha] radiation. The structure is made up of (As,Sb)[S.sub.3] trigonal pyramids forming an eight-membered [(As,Sb).sub.8] [S.sub.8] ring; the sheets formed by the eight-membered rings are connected to each other by S1 and S6 atoms are parallel to (001). The sheet structure is responsible for the cleavage and twin planes of getchellite. The cations predominantly occupy the M2 site in natural getchellite, because the M2[S.sub.6] polyhedron has the most distorted (irregular) geometry compared with other metal polyhedra in the structure. The amount of As incorporated at the M1 sites correlates strongly with M2 site composition. An important feature of [As.sup.3+] sulfide minerals is that the crystal structures are composed of a 3-dimensional network consisting not only of As[S.sub.3] pyramids but also containing trigonal pyramids of other constituent cations because the structures are governed strongly by the stereochemical requirement of [As.sup.3+] cations for [MS.sub.3] trigonal pyramids.