This study describes a new variety of chalcedony with a unique inhomogeneous bluish green hue, named aquaprase. It was discovered in Africa and is considered to be a valuable addition to the gem trade. A multi-methodological approach was used to examine its chemistry, mineralogy and microstructure, which were then compared to those of chrysoprase and agate, two of the most popular varieties of chalcedony. Optical microscopy revealed a complex microstructural heterogeneity in the different colour intensity areas/bands of aquaprase and agate, whereas chrysoprase exhibited a more homogeneous coexistence of micro- and cryptocrystalline quartz. High-resolution synchrotron XRD was essential for highlighting the complex assemblage of various types of α-quartz in aquaprase and agate (which differ in terms of crystal size and/or cell parameters). Micro-Raman spectroscopy revealed α-quartz and moganite in all three varieties of chalcedony and the presence of the nickel-bearing layered silicate mineral, willemseite, in chrysoprase, which is responsible for its green colouration. The chemical analysis displayed a homogeneous composition of agate, as well as high levels of nickel content in the chrysoprase variety. Aquaprase showed significant amounts (ppm by weight) of trace elements (Al, Mg, Na, K, Ca, Ti, U and Fe) characteristic of its formation environment, as well as high values of Cr, which are thought to be the cause of its bluish green colouration.

Fire agates from Deer Creek are highly appreciated gemstones due to the presence of optical phenomena and rainbow colours that cause fiery effects to be observed on their characteristic brown base. The specific morphology of poorly ordered chalcedony (crystallinity index = 0.1–1.5) with an admixture of mogánite (av. 6.6%), micro-quartz and opal-C forming a colloform texture seems to be responsible for the presence of fire effect in these agates. The multi-layered silica spheroidal forms (‘bubble’-like structure), already noted in hand specimens, could be the centres of reflection and interference of white light. Numerous, microscopic inclusions of Fe and Ti compounds randomly scattered within some silica zones, together with microstructural features of agate, could determine the colour and size of the domains with the optical effect. Deer Creek fire agates form veins within their host volcanic rocks. The silica mineralisation filling the network of fissures in the host rocks was supplied cyclically with aqueous fluids of varying composition, enriched periodically in CO2, Fe, Ti, Mn, Zn and Ca. As a result, the red-brown colour of fire agates was created by scattered pigments of tiny iron oxides (magnetite, maghemite) and titanium oxides (rutile, anatase) within the silica matrix. The precipitation of strongly disordered silica with a characteristic colloform texture is diagnostic for boiling processes in this area.

Agates with spectacular micro-structural features were found in volcanic rocks at several occurrences in the Saar-Nahe region (Germany). These agates include spirals of several tens up to several hundreds of μm in size within zones lacking the characteristic structural agate banding. A combined mineralogical study by polarising microscopy, scanning electron microscopy, cathodoluminescence microscopy and spectroscopy, and electron backscatter diffraction provided evidence that the spirals consist of well-ordered trigonal α-quartz, whereas the surrounding matrix is composed of strongly disordered or amorphous SiO2 phases. The quartz micro-crystals show a systematic rotation of the crystal orientation perpendicular to the direction of the spiral loops indicating helical growth.

It is assumed that the spiral growth is initiated by dislocations with a screw component. The lacking symmetry of the strongly disordered or amorphous matrix initiated a curved development by a screw dislocation in a system far from equilibrium. The atoms/molecules are packed into spiral layers, which is energetically favoured in comparison with the incorporation into plane crystal faces. Such self-organisation growth and polymerisation initiated by a screw dislocation can produce variable spiral morphologies sometimes resembling living forms.

Agates from the Bighorn district in Montana (USA), the so-called Dryhead area, and their adjacent host rocks have been examined in the present study. Analyses by XRD, polarizing microscopy, LA-ICP-MS, cathodoluminescence (CL), SEM and of oxygen isotopes were performed to obtain information surrounding the genesis of this agate type.

Investigations of the agate microstructure by polarizing microscopy and CL showed that chalcedony layers and macrocrystalline quartz crystals may have formed by crystallization from the same silica source by a process of self-organization. High defect densities and internal structures (e.g. sector zoning) of quartz indicate that crystallization went rapidly under non-equilibrium conditions. Most trace-element contents in macrocrystalline quartz are less than in chalcedony due to a process of ‘selfpurification’, which also caused the formation of Fe oxide inclusions and spherules.

Although the agates formed in sedimentary host rocks, analytical data indicate participation of hydrothermal fluids during agate formation. Trace elements (REE distribution patterns, U contents up to 70 ppm) and CL features of agate (transient blue CL), as well as associated minerals (fluorite, REE carbonates) point to the influence of hydrothermal processes on the genesis of the Dryhead agates. However, formation temperatures <120°C were calculated from O-isotope compositions between 28.9‰ (quartz) and 32.2‰ (chalcedony).

Crystallite growth in natural agate samples has been investigated at temperatures of 350—550°C and 100 MPa pressure in the presence of water vapour. Initial crystallite coarsening is accompanied by the transformation of moganite to α-quartz that is apparently inhibited by residual moganite when the crystallite sizes reach ~50 nm. At 350—500°C the coarsening kinetics can be described by an empirical law developed to describe Zener pinning which incorporates the maximum crystallite size prior to growth inhibition: . Co = initial crystallite size, Cs = crystallite growth after time t, Cm = the maximum size achieved before inhibition and k is the rate constant that includes the activation energy which was found to be 51(±9) kJ mole—1. A more conventional isothermal growth rate law, = kt with n = 6.5, only applies at 550°C. Limited growth was obtained when small agate cubes were heated in an open furnace up to 122 d at 550°C, demonstrating that water vapour was essential for continued crystallite coarsening. The crystallite size and moganite content of agates formed under normal earth surface conditions from hosts aged 13 Ma to 3.5 Ga have also been determined. The high temperature crystallite growth rate law does not describe natural agate growth quantitatively but a qualitatively similar pattern is observed.

Agates from a 430 Ma host at Stockdale Beck, Cumbria, England have been characterized. The crystallite size of the Stockdale Beck agates was found to be ~60% greater than any other agates from five regions aged 400–1100 Ma. Raman spectroscopy identified moganite in all agates tested except those from Stockdale Beck. Infrared spectroscopy showed that the silanol content of the Stockdale Beck agates was near zero. The properties of agates from Stockdale Beck and the 1.84–3.48 Ga metamorphosed hosts found in Western Australia were similar but different from agates found in other hosts aged 400–1100 Ma. Cathodoluminescence demonstrates further differences between agates from hosts aged 13–1100 Ma and those from Stockdale Beck and Western Australia. Agates from the latter areas have a lower proportion of defects causing a red emission band (~660 nm) but an increased proportion of defects causing blue (~470 nm) and orange (~640 nm) emission bands. Agates found in hosts aged 13–1100 Ma are also differentiated from the Stockdale Beck and Western Australian agates in a ternary plot of the relative intensities of violet to blue to orange emission bands. Single scans producing this combination of colours are only found in the Stockdale Beck and Western Australian agates. The properties shown by the Stockdale Beck and Western Australian agates demonstrate that an agate or chalcedony infill can be used to identify post-deposition palaeoheating within a host rock.

Chalcedony and agates from a variety of world-wide hosts have been examined using cathodoluminescence (CL). Gaussian fitting of the experimental data shows that there are two dominant spectral emissions at ∼400 and ∼660 nm. A third subordinate peak is also found at ∼470, ∼560 or ∼620 nm. An age-related link is shown between the respective decreasing and increasing relative intensities of the 660 and 620 nm emissions. It is proposed that this change is due to a condensation reaction between neighbouring Si–OH groups eliminating water and forming a strained Si-O-Si bond.

Agates from a variety of hosts and regions produced no clear demonstrable CL distinctions. However, a set of Western Australian agates was examined from host rocks that had been subjected to burial metamorphism. Cathodoluminescence produced different spectral emissions in the petrographic fibrous and granular regions of these agates. One agate shows a partial transformation of the petrographic fibrosity into granularity. This conversion is characterized by emission bands at 570 nm and 460 nm. Similar emission-band changes were produced by heating Brazilian agates for 35 days at 300°C. The identification of these changes in agate could serve as an indicator of palaeoheating within the parent rock.

Characterization of Brazilian agates containing a lower horizontally banded section and an upper chamber with bands parallel to the walls shows that these agates formed much later than the 135 Ma Paraná basalt host rock. Age differentiation between the two types of banding shows that the horizontal bands formed between 43 to 63 Ma ago with a final infill of wall-lining bands between 7 and 27 Ma later. The horizontal bands have a higher Al3+ concentration and a greater crystallite size than the wall-lining layers; they have a lower mogánite content and defect-site water content. The formation of these agates appears to be the result of a three-stage process. After the separate formation of horizontally banded and wall-lining agate, a silica infill seals the gap between the agate and the cavity wall. The detection of cristobalite in some specimens indicates that genesis of both the horizontally banded and wall-lining deposits in the Brazilian samples proceeds along an amorphous silica → opal-CT → opal-C → chalcedony pathway.

An ‘Al-thermometer’ was applied to agate samples of volcanic parent rocks to provide information about the temperature of agate formation. The temperatures were calculated from concentrations of [AlO4]0 centres determined by electron paramagnetic resonance (EPR) measurements. The calculations for agate bands in some cases yield temperatures of up to 655°C, which are assumed to be invalid; they are thought to be artifacts resulting from non-equilibrium crystallization processes. In contrast, the formation temperatures (60–198°C) estimated for associated quartz incrustations within the agate geodes are in good accord with those calculated from oxygen isotope data. Direct estimation of the formation temperatures of agates is problematic. The best results will be obtained by analyses of associated phanerocrystalline quartz incrustations.

Natural onyx agate from Mali was investigated in an integrated mineralogical and chemical study to reveal the origin of the unusual black colouration. Detailed studies by polarizing microscopy, scanning electron microscopy and micro-Raman spectroscopy showed that the colour of the dark bands is related to the incorporation of small particles of carbon (low-crystalline graphite) up to 200 nm in size into the cryptocrystalline silica matrix. The dark bands have carbon contents of 1.88 wt.%. The location of the graphite particles is closely related to the primary structural banding in the chalcedony. Cathodoluminescence data shows that the banding is interrupted by small fissures containing secondary hydrothermal quartz. The carbon isotope composition (δ13C value of –31.1±0.2‰) of the carbonaceous material points to an organic precursor. Both the direct hydrothermal formation of graphite from methane under elevated temperature and the graphitization of organic precursors by secondary hydrothermal or metamorphic overprint are possible explanations for the colour of the dark bands. The graphitization of organic precursors results in an intense electron spin resonance line at geff = 2.0026.

Samples of agate and quartz incrustations from different parent volcanic rocks of certain world-wide localities were investigated by EPR, CL and trace element analysis. In all agate samples the following paramagnetic centres were detected:, E′1, [AlO4]0 [FeO4/M+]0 and [GeO4/M+]0. Centres of the type [TiO4/Li+]0 and [TiO4/H+]0, which were detected in quartz of the parent volcanics, are absent in agate. Generally, the abundance of centres (silicon vacancy) and E′1 centres (oxygen vacancy) in agate is remarkably higher than in quartz. The high defect density in agates points to rapid growth of silica from a strongly supersaturated solution probably with a noncrystalline precursor.

CL microscopy reveals internal structures and zoning in agates and quartz incrustations which clearly differ from those discernible by conventional polarizing microscopy. The CL spectra of agates differ from those of quartz from crystalline rocks. At least three broad emission bands were detected in the CL spectra: a blue band of low intensity, a yellow band at about 580 nm, and an intense red band at 650 nm. The CL emission at 650 nm shows some relations to the hydroxyl or alkali content and the abundance of centres and E′1 centres. The emission intensity increases during electron bombardement due to the conversion of different precursors (e.g. ≡Si-O-H, ≡Si-O-Na groups) into hole centres. Another conspicuous feature in the CL spectra of agates is the existence of a yellow emission band centred at around 580 nm. The predominance of the yellow CL emission band and the high concentration of E′1 centres are typical for agates of acidic volcanics and are indicative of a close relationship between the two.

We have carried out TEM observations of agates of geode origin and Beltane opals. Optically observable individual fibres in agates are composed of many fine fibres which consist of quartz crystallites of 8 to 100 nm in length stacked together parallel to <110> or <100> with c-axes perpendicular to the fibre elongation. The optically observable systematic striations in agate are found to consist of cyclic alternation of layers due to variation in grain size and porosity. Large quartz crystals, protruding into the spaces of geodes, represent the last stage of formation of these bands, and are merely a continuation of the banding sequence. Nanometre scale textures of cristobalite fibres were revealed in Beltane opals. The cristobalite crystallites have the size of 3 to 20 nm in length and are also stacked together. Our TEM results suggest that embryonic particles were formed in their corresponding growth environments and agglutinated to form fibres.

Dehydration of silanol and molecular water in 60 agates from 12 hosts with ages between 23 to 2717 Ma has been investigated using desiccators and high-temperature furnace heating. There are wide differences in the water data obtained under uncontrolled and fixed atmospheric water vapour pressure conditions. After agate acclimatization at 20°C and 46% relative humidity, the total water (silanol and molecular) was determined in powders and mini-cuboids by heating samples at 1200°C. Agates from hosts < 180 Ma all showed a greater mass loss using powders and demonstrate that after prolonged high-temperature heating, silanol water is partially-retained by the mini-cuboids. Desiccator dehydration of powders and slabs shows that powder preparation can produce water losses; this is particularly relevant in agates from hosts < 180 Ma. The identified problems have consequences for water quantification in agate and chalcedony using infrared or thermogravimetric techniques. Mobile and total water in agate is considered in relation to host-rock age, mogánite content and crystallite size. Links are observed between the various identified water contents allowing comment on quartz development and agate genesis. The water data also supports previous claims that agates from New Zealand and Brazil were formed long after their host.

A systematic investigation of agates from different occurrences in Europe, Northern and Southern America reveals that macrocrystalline quartz and chalcedony within them have an unusually high uranium content. Whereas agates may contain >70 ppm of U, quartz from magmatic and metamorphic rocks as well as pegmatite quartz commonly exhibit U concentrations at sub-ppm levels. Spatially resolved trace-element analyses by laser ablation inductively coupled plasma mass spectrometry show that the distribution of U within the agate samples is heterogeneous and coincides with the structural banding. The results indicate that U is incorporated into agate as uranyl ions. These ions, which are bound to the silica surface, are interpreted to originate from the parallel accumulation of Si and U by alteration processes of surrounding host rocks during agate formation.

The uranyl ion is the cause of greenish photoluminescence (PL) in agate, which can only be excited by short wavelengths (<300 nm). The green PL is due to the electron transition from an excited to a ground state of the uranyl ion and is shown by a typical emission line at ∼500 nm accompanied by several equidistant lines. These are due to the harmonic vibration of oxygen atoms along the uranyl axis. Luminescence can be detected in samples with a U content down to the 1 ppm level.

Imazethapyr at 0.07 and 0.10 kg ai ha-1 applied preplant incorporated (PPI), preemergence (PRE), and postemergence (POST) was evaluated at two locations in 1988 and 1989 for safety to dry edible beans. Bean stunting, leaf crinkling, and interveinal chlorosis were evident from imazethapyr and varied from 1 to 57%. Imazethapyr significantly reduced bean height and delayed maturity. PPI and POST applications of imazethapyr at 0.07 and 0.10 kg ha-1 did not reduce bean seed yields compared to yields of the untreated control. Bean cultivar by herbicide interactions were significant for bean injury but varied with year and location.