A nomenclature for tourmaline-supergroup minerals is based on chemical systematics using the generalized tourmaline structural formula: XY3Z6(T6O18)(BO3)3V3W, where the most common ions (or vacancy) ...at each site are X=Na1+, Ca2+, K1+, and vacancy; Y=Fe2+, Mg2+, Mn2+, Al3+, Li1+, Fe3+, and Cr3+; Z=Al3+, Fe3+, Mg2+, and Cr3+; T=Si4+, Al3+, and B3+; B=B3+; V=OH1- and O2-; and W=OH1-, F1-, and O2-. Most compositional variability occurs at the X, Y, Z, W, and V sites. Tourmaline species are defined in accordance with the dominant-valency rule such that in a relevant site the dominant ion of the dominant valence state is used for the basis of nomenclature. Tourmaline can be divided into several groups and subgroups. The primary groups are based on occupancy of the X site, which yields alkali, calcic, or X-vacant groups. Because each of these groups involves cations (or vacancy) with a different charge, coupled substitutions are required to relate the compositions of the groups. Within each group, there are several subgroups related by heterovalent coupled substitutions. If there is more than one tourmaline species within a subgroup, they are related by homovalent substitutions. Additionally, the following considerations are made. (1) In tourmaline-supergroup minerals dominated by either OH1- or F1- at the W site, the OH1--dominant species is considered the reference root composition for that root name: e.g., dravite. (2) For a tourmaline composition that has most of the chemical characteristics of a root composition, but is dominated by other cations or anions at one or more sites, the mineral species is designated by the root name plus prefix modifiers, e.g., fluor-dravite. (3) If there are multiple prefixes, they should be arranged in the order occurring in the structural formula, e.g., "potassium-fluor-dravite."
Abstract
There has been much debate about the fault zone processes that generate slow earthquakes, including tremor and slow slip. Indeed, we still debate whether tremor and slow slip are generated ...by the same process operating at different scales or by two distinct processes. Here we investigate tremor scaling near Parkfield, California; we examine how rupture duration scales with moment. We thoroughly search for and detect the low frequency earthquakes (LFEs) that constitute tremor and robustly estimate their durations. Our results show varying durations (0.1–0.6 s) and spectra for LFEs at the same location. These variations confirm a common assumption, that LFEs’ observed low frequency contents are due to source processes, not path effects. The LFEs’ amplitude and spectra variations are consistent with a linear moment-duration scaling: the same scaling observed among slow slip events. The similar scaling suggests that tremor and slow slip events are governed by the same fault zone process and that when we attempt to identify the process creating slow earthquakes, we should focus on processes which allow higher slip rates on smaller faults.
Nomenclature of the amphibole supergroup Hawthorne, Frank C; Oberti, Roberta; Harlow, George E ...
The American mineralogist,
11/2012, Letnik:
97, Številka:
11-12
Journal Article
Recenzirano
A new classification and nomenclature scheme for the amphibole-supergroup minerals is described, based on the general formula AB2C5T8O22W2, where A=, Na, K, Ca, Pb, Li; B=Na, Ca, Mn2+, Fe2+, Mg, Li; ...C=Mg, Fe2+, Mn2+, Al, Fe3+, Mn3+, Ti4+, Li; T=Si, Al, Ti4+, Be; W=(OH), F, Cl, O2-. Distinct arrangements of formal charges at the sites (or groups of sites) in the amphibole structure warrant distinct root names, and are, by implication, distinct species; for a specific root name, different homovalent cations (e.g., Mg vs. Fe2+) or anions (e.g., OH vs. F) are indicated by prefixes (e.g., ferro-, fluoro-). The classification is based on the A, B, and C groups of cations and the W group of anions, as these groups show the maximum compositional variability in the amphibole structure. The amphibole supergroup is divided into two groups according to the dominant W species: W(OH,F,Cl)-dominant amphiboles and WO-dominant amphiboles (oxo-amphiboles). Amphiboles with (OH, F, Cl) dominant at W are divided into eight subgroups according to the dominant charge-arrangements and type of B-group cations: magnesium-iron-manganese amphiboles, calcium amphiboles, sodium-calcium amphiboles, sodium amphiboles, lithium amphiboles, sodium-(magnesium-iron-manganese) amphiboles, lithium-(magnesium-iron-manganese) amphiboles and lithium-calcium amphiboles. Within each of these subgroups, the A- and C-group cations are used to assign specific names to specific compositional ranges and root compositions. Root names are assigned to distinct arrangements of formal charges at the sites, and prefixes are assigned to describe homovalent variation in the dominant ion of the root composition. For amphiboles with O dominant at W, distinct root-compositions are currently known for four (calcium and sodium) amphiboles, and homovalent variation in the dominant cation is handled as for the W(OH,F,Cl)-dominant amphiboles. With this classification, we attempt to recognize the concerns of each constituent community interested in amphiboles and incorporate these into this classification scheme. Where such concerns conflict, we have attempted to act in accord with the more important concerns of each community.
The magnitude of postseismic slip is useful for constraining physical models of fault slip. Here we examine the postseismic slip following intermediate‐magnitude (M 4 to 5) earthquakes by ...systematically analyzing data from borehole strainmeters in central and northern California. We assess the noise in the data and identify 11 earthquakes that generated interpretable strain records. We estimate the earthquakes' postseismic to coseismic moment ratios by comparing the coseismic strain changes with strain changes induced by afterslip in the following 1.5 days. The median estimated postseismic moment is 0.45 times the coseismic moment, with a 90% confidence interval between 0.25 and 0.60. This postseismic moment is slightly larger than typically observed following large (M > 6) earthquakes but smaller than observed following small (M2 to 4) earthquakes. The intermediate‐magnitude postseismic slip suggests a size dependence in the dynamics of earthquakes or in the properties of fault areas that surround earthquakes.
Key Points
We examine postseismic slip following M4‐5 earthquakes using borehole strain data
The median estimated postseismic moment is between 0.25 and 0.6 times the coseismic moment
The postseismic moment ratio is intermediate between estimates for small and large earthquakes
The patterns of linkage of chemical bonds in space contain significant energetic information that may be used as the basis of a theoretical approach to the structure and chemical composition of ...minerals. This approach combines aspects of graph theory, bond-valence theory, and the moments approach to the electronic-energy density-of-states to interpret topological aspects of crystal structures, and allows consideration of many issues of crystal structure, mineral composition, and mineral behavior that are not addressed by established theoretical methods. The chemical composition of a mineral is controlled by the weak interaction between the structural unit and the interstitial complex. The principle of correspondence of Lewis acidity-basicity asserts that stable structures will form when the Lewis-base strength of the structural unit closely matches the Lewis-acid strength of the interstitial complex. This principle allows analysis of the factors that control the chemical compositions and aspects of the structural arrangements of minerals, and provides a mechanism to understand the relations between structure, the speciation of its constituents in aqueous solution, and its mechanism of crystallization. (H2O) groups in the structural unit limit the polymerization of the structural unit in one or more directions, controlling the polymerization of the structural unit. This is a major cause of structural diversity in oxygen-based minerals, and accounts for the systematic distribution in mineral species from the core to the surface of the Earth. The moments approach to the electronic-energy density-of-states provides a bond-topological interpretation of the energetics of a structure. When comparing structures, the most important structural differences involve the first few disparate moments of the electronic-energy density-of-states. We may classify chemical reactions according to the lowest-order moment of the electronic-energy density-of-states that is conserved, which allows us to identify the principal structural changes that drive chemical change: (1) coordination number for discontinuous reactions, and (2) short-range order for continuous reactions. This relation between the bond topology of a structure and its enthalpy of formation from constituent oxides is indicated by a correlation between change in anion-coordination number and reduced enthalpy of formation for the reactions 6Mgm 4SinO(m+2n) = mMgO+nSiO2.
Here, I describe a theoretical approach to the structure and chemical composition of minerals based on their bond topology. This approach allows consideration of many aspects of minerals and mineral ...behaviour that cannot be addressed by current theoretical methods. It consists of combining the bond topology of the structure with aspects of graph theory and bond-valence theory (both long range and short range), and using the moments approach to the electronic energy density-of-states to interpret topological aspects of crystal structures. The structure hierarchy hypothesis states that higher bond-valence polyhedra polymerize to form the (usually anionic)
structural unit
, the excess charge of which is balanced by the
interstitial complex
(usually consisting of large low-valence cations and (H
2
O) groups). This hypothesis may be justified within the framework of bond topology and bond-valence theory, and may be used to hierarchically classify oxysalt minerals. It is the weak interaction between the structural unit and the interstitial complex that controls the stability of the structural arrangement. The principle of correspondence of Lewis acidity–basicity states that stable structures will form when the Lewis-acid strength of the interstitial complex closely matches the Lewis-base strength of the structural unit, and allows us to examine the factors that control the chemical composition and aspects of the structural arrangements of minerals. It also provides a connection between a structure, the speciation of its constituents in aqueous solution and its mechanism of crystallization. The moments approach to the electronic energy density-of-states provides a link between the bond topology of a structure and its thermodynamic properties, as indicated by correlations between average anion coordination number and reduced enthalpy of formation from the oxides for
6
Mg
m
4
Si
n
O
(
m
+2
n
)
and MgSO
4
(H
2
O)
n
.
Background
Several methods have been proposed to measure cerebrovascular autoregulation (CA) in traumatic brain injury (TBI), but the lack of a gold standard and the absence of prospective clinical ...data on risks, impact on care and outcomes of implementation of CA-guided management lead to uncertainty.
Aim
To formulate statements using a Delphi consensus approach employing a group of expert clinicians, that reflect current knowledge of CA, aspects that can be implemented in TBI management and CA research priorities.
Methods
A group of 25 international academic experts with clinical expertise in the management of adult severe TBI patients participated in this consensus process. Seventy-seven statements and multiple-choice questions were submitted to the group in two online surveys, followed by a face-to-face meeting and a third online survey. Participants received feedback on average scores and the rationale for resubmission or rephrasing of statements. Consensus on a statement was defined as agreement of more than 75% of participants.
Results
Consensus amongst participants was achieved on the importance of CA status in adult severe TBI pathophysiology, the dynamic non-binary nature of CA impairment, its association with outcome and the inadvisability of employing universal and absolute cerebral perfusion pressure targets. Consensus could not be reached on the accuracy, reliability and validation of any current CA assessment method. There was also no consensus on how to implement CA information in clinical management protocols, reflecting insufficient clinical evidence.
Conclusion
The Delphi process resulted in 25 consensus statements addressing the pathophysiology of impaired CA, and its impact on cerebral perfusion pressure targets and outcome. A research agenda was proposed emphasizing the need for better validated CA assessment methods as well as the focused investigation of the application of CA-guided management in clinical care using prospective safety, feasibility and efficacy studies.
Tidal modulation of slow slip in Cascadia Hawthorne, Jessica C.; Rubin, Allan M.
Journal of Geophysical Research: Solid Earth,
September 2010, Letnik:
115, Številka:
B9
Journal Article
Recenzirano
Odprti dostop
Several studies have shown that the seismic tremor in episodic tremor and slip is tidally modulated, suggesting a sensitivity to the rather small tidal stresses. We address whether the slip rate in ...slow slip events is also tidally modulated by examining data from six borehole strainmeters in northwest Washington and southern Vancouver Island. We simultaneously fit data from multiple stations and from slow slip events occurring over a 3 year interval from January 2007 to June 2009, as we are unable to extract a meaningful signal from a single record. We find modulation of the strain rate with a 12.4 h period, that of the tide with the largest amplitude, that is significant at the 99% level. The amplitude of this modulation suggests that the slip rate during slow slip events oscillates, on average, 25% above and below its mean value during a tidal cycle. Tidal modulation estimates at three other periods are significant with more than 70% probability. The phase of maximum strain rate in the 12.4 h M2 period coincides with the phase of the maximum tremor rate taken from a catalog in an overlapping region. Comparison with a simple tidal loading model shows that the phase of maximum strain rate in the M2 period may occur at the maximum shear stress or up to 90° before it, depending on the location of slip in the subduction zone.
The low frequency earthquakes (LFEs) that constitute tectonic tremor are often inferred to be slow: to have durations of 0.2-0.5 s, a factor of 10-100 longer than those of typical MW 1-2 earthquakes. ...Here we examine LFEs near Parkfield, CA in order to assess several proposed explanations for LFEs' long durations. We determine LFE rupture areas and location distributions using a new approach, similar to directivity analysis, where we examine how signals coming from various locations within LFEs' finite rupture extents create differences in the apparent source time functions recorded at various stations. We use synthetic ruptures to determine how much the LFE signals recorded at each station would be modified by spatial variations of the source-station traveltime within the rupture area given various possible rupture diameters, and then compare those synthetics with the data. Our synthetics show that the methodology can identify interstation variations created by heterogeneous slip distributions or complex rupture edges, and thus lets us estimate LFE rupture extents for unilateral or bilateral ruptures. To obtain robust estimates of the sources' similarity across stations, we stack signals from thousands of LFEs, using an empirical Green's function approach to isolate the LFEs' apparent source time functions from the path effects. Our analysis of LFEs in Parkfield implies that LFEs' apparent source time functions are similar across stations at frequencies up to 8-16 Hz, depending on the family. The interstation coherence observed at these relatively high frequencies, or short wavelengths (down to 0.2-0.5 km), suggest that LFEs in each of the seven families examined occur on asperities. They are clustered in patches with sub-1-km diameters. The individual LFEs' rupture diameters are estimated to be smaller than 1.1 km for all families, and smaller than 0.5 km and 1 km for the two shallowest families, which were previously found to have 0.2-s durations. Coupling the diameters with the durations suggests that it is possible to model these MW 1-2 LFEs with earthquake-like rupture speeds: around 70 per cent of the shear wave speed. However, that rupture speed matches the data only at the edge of our uncertainty estimates for the family with highest coherence. The data for that family are better matched if LFEs have rupture velocities smaller than 40 per cent of the shear wave speed, or if LFEs have different rupture dynamics. They could have long rise times, contain composite sub-ruptures, or have slip distributions that persist from event to event.
The occurrence and binding energies of the U
6+, U
5+ and U
4+ bands in the U 4f
7/2 peak of 19 uranyl minerals of different composition and structure were measured by XPS. The results suggest that ...these minerals can be divided into the following four groups: (1) Uranyl-hydroxy-hydrate compounds with no or monovalent interstitial cations; (2) Uranyl-hydroxy-hydrate minerals with divalent interstitial cations; (3) Uranyl-oxysalt minerals with (
TO
n
) groups (
T
=
Si, P, and C) in which all equatorial O-atoms of the uranyl-polyhedra are shared with (
TO
n
) groups; (4) Uranyl-oxysalt minerals with (
TO
n
) groups (
T
=
S and Se), in which some equatorial O-atoms are shared only between uranyl polyhedra. The average binding energies of the U
6+and U
4+ bands shift to lower values with (1) incorporation of divalent cations and (2) increase in the Lewis basicity of the anion group bonded to U. The first observation is a consequence of an increase in the bond-valence transfer from the interstitial species (cations, H
2O) groups to the O-atoms of the uranyl-groups, which results in an electron transfer from O to U
6+. The second trend correlates with an increase in the covalency of the U
O bonds with increase in Lewis basicity of the anion group, which results in a shift of the electron density from O to U. The presence of U
4+ on the surface of uranyl minerals can be detected by the shape of the U 4f
7/2 peak, and the occurrence of the U 5f peak and satellite peaks belonging to the U 4f
5/2 peak. The presence of U
4+ in some of the uranyl minerals and synthetics examined may be related to the conditions during their formation. A charge-balance mechanism is proposed for the incorporation of lower-valence U in the structure of uranyl minerals. Exposure of a Na-substituted metaschoepite crystal in air and to Ultra-High Vacuum results in dehydration of its surface structure associated with a shift of the U
6+ bands to higher binding energies. The latter observation indicates a shift in electron density from U to O, which must be related to structural changes inside the upper surface layers of Na-substituted metaschoepite.
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