Quartz Field Research Articles

Do phylogenetic community metrics reveal the South African quartz fields as terrestrial-habitat islands?

The quartz fields of the Greater Cape Floristic Region (GCFR) are arid and island-like special habitats, hosting ~142 habitat-specialized plant species, of which 81 % are local endemics, characterized by a rapid turnover of species between and among sites. We use several phylogenetic community metrics: (1) to examine species diversity and phylogenetic structure within and among quartz fields; (2) to investigate whether quartz field specialists are evolutionarily drawn from local species pools, whereas the alternative hypothesis posits that there is no significant evolutionary connection between quartz field specialists and the local species pools; and (3) to determine whether there is an association between certain traits and the presence of species in quartz fields. We sampled and developed dated phylogenies for six species-rich angiosperm families (Aizoaceae, Asteraceae, Crassulaceae, Cyperaceae, Fabaceae and Santalaceae) represented in the quartz field floras of southern Africa. Specifically, we focused on the flora of three quartz field regions in South Africa (Knersvlakte, Little Karoo and Overberg) and their surrounding species pools to address our research questions by scoring traits associated with harsh environments. We found that the Overberg and Little Karoo had the highest level of species overlap for families Aizoaceae and Fabaceae, whereas the Knersvlakte and the Overberg had the highest species overlap for families Asteraceae, Crassulaceae and Santalaceae. Although our phylogenetic community structure and trait analyses showed no clear patterns, relatively low pairwise phylogenetic distances between specialists and their local species pools for Aizoaceae suggest that quartz species could be drawn evolutionarily from their surrounding areas. We also found that families Aizoaceae and Crassulaceae in Knersvlakte and Little Karoo were phylogenetically even. Despite their proximity to one another within the GCFR, the studied areas differ in their species pools and the phylogenetic structure of their specialists. Our work provides further justification for increased conservation focus on these unique habitats under future scenarios of global change.

Germination success of habitat specialists from the Succulent Karoo and Renosterveld on different soil types

South African quartz fields in the Succulent Karoo and adjacent biomes are special habitats for a range of highly specialized dwarf succulents, many of which belong to the Aizoaceae. The soil chemistry of the quartz fields ranges from very acidic to slightly alkaline and from moderately to highly saline, and drives species turnover. The soil conditions are therefore critical for the vegetation composition. The aim of our study is to better understand the role of the regeneration niche of habitat specialists from different quartz-field habitats to inform conservation planning and ecological restoration. We tested germination responses to soil chemistry of 14 Aizoaceae species from quartz fields in the Succulent Karoo (Knersvlakte) and the Fynbos (Overberg) biome of South Africa. For each of the two sites we sampled species from two different quartz-field habitats. The seeds from 14 species were germinated under greenhouse conditions for 79 days on their home soils and on the natural soils of the three other habitat types as well as on salinized and acidified horticultural standard substrate and a standard substrate as control.Germination success in the control differed between the species from the two biomes. Of the seven species from the Knersvlakte, six species germinated rapidly, with 38–86% success. Only Dicrocaulon brevifolium germinated with merely 17% success. Of the seven species from Overberg only Gibbaeum hartmannianum germinated at a high percentage (83%). The other species germinated below 20% or not at all.Natural soils and manipulated substrates limited germination of all species to <20%. No seeds germinated on natural saline soils from the Knersvlakte until from day 46 to day 50, when an excess supply of rainwater diluted the ion content of the soil. The subsequent recovery of the seeds resulted in a germination percentage of 4–35% for seven species from different habitat types. Quartz fields provide a plant-adverse environment. Their soils contain a range of toxic ions at high concentrations. High volume and prolonged rainfall events that leach the soils temporarily seemed to be a requisite for successful germination.

Biomass assessment of microbial surface communities by means of hyperspectral remote sensing data

Dryland vegetation developed morphological and physiological strategies to cope with drought. However, as aridity increases, vascular plant coverage gets sparse and microbially-dominated surface communities (MSC), comprising cyanobacteria, algae, lichens and bryophytes together with heterotropic bacteria, archaea and fungi, gain relevance. Nevertheless, the relevance of MSC net primary productivity has only rarely been considered in ecosystem scale studies, and detailed information on their contribution to the total photosynthetic biomass reservoir is largely missing.In this study, we mapped the spatial distribution of two different MSC (biological soil crusts and quartz fields hosting hypolithic crusts) at two different sites within the South African Succulent Karoo (Soebatsfontein and Knersvlakte). Then we characterized both types of MSC in terms of chlorophyll content, and combining these data with the biocrust and quartz field maps, we estimated total biomass values of MSCs and their spatial patterns within the two different ecosystems. Our results revealed that MSC are important vegetation components of the South African Karoo biome, revealing clear differences between the two sites. At Soebatsfontein, MSC occurred as biological soil crusts (biocrusts), which covered about one third of the landscape reaching an overall biomass value of ~480gha−1 of chlorophyll a+b at the landscape scale. In the Knersvlakte, which is characterized by harsher environmental conditions (i.e. higher solar radiation and potential evapotranspiration), MSC occurred as biocrusts, but also formed hypolithic crusts growing on the lower soil-immersed parts of translucent quartz pebbles. Whereas chlorophyll concentrations of biocrusts and hypolithic crusts where insignificantly lower in the Knersvlakte, the overall MSC biomass reservoir was by far larger with ~780gha−1 of chlorophyll a+b. Thus, the complementary microbially-dominated surface communities promoted biomass formation within the environmentally harsh Knersvlakte ecosystem.

Identification and evaluation of post-depositional mechanical traces on quartz assemblages: An experimental investigation

The evaluation of the state of preservation of archaeological lithic artefacts is the first step before starting a functional analysis. If lithic artefacts show a variety of damage from many contact materials, they are also subject to modifications from “natural” causes, such as water transport, soil abrasion, trampling etc. Studying alteration features gives us information to reconstruct the life story of lithic tools after their abandonment and helps us to better understand formation processes of archaeological sites.Prolonged movements under soil could be responsible for post depositional surface modifications that sometimes look like wear traces due to use. These alterations could have chemical and/or mechanical origin. Recognizing them is of paramount importance especially on quartz assemblages where usewear analysis requires a long and complex methodology that differs to a certain extent from the protocol applied to other artefacts produced with different raw materials such as flint, obsidian etc … For this reason, the use wear and post-depositional surface modification analysis on quartz and quartzite tools have been undertaken by very few researchers until now. The aim of this paper is to increase the experimental quartz field through the attempt to recreate mechanical contact between quartz flakes and sediments such as in archaeological contexts.Through the use of a Polishing Machine MECAPOL P320 equipped with two counter-rotating platens, it was possible to wear out two experimental flakes in a sandy quartzite sediment coming from Sai Island (Sudan) for 240 h.After the experimentation, the two flakes showed no edge damage or other visible modifications, but some micro traces on the crystal surface were present. Although long and demanding, this step sharply reduces possible errors in the recognition of use-wear during the experimental stage. The micro traces definable as abrasions on some parts of the crystal surface, have been analysed by a metallographic microscope and compared with the same crystal portion taken before the experiment. Even if it is difficult to reconstruct post depositional processes in a laboratory, this attempt shows that the contact between sediment and stone tools in a continuous movement can randomly abrade the crystal surface.

Small-scale soil patterns drive sharp boundaries between succulent “dwarf” biomes (or habitats) in the arid Succulent Karoo, South Africa

Quartz fields, which are densely covered by angular quartz gravel, represent edaphically special habitats in the Succulent Karoo, a winter rainfall desert in South Africa. They are home to an endemic flora and vegetation communities that show strong species turnover at their boundaries. We interpret the vegetation changes along these boundaries as analogous to biome boundaries: a functional shift is accompanied by a change in structure. Our study aims to identify which abiotic variables are associated with the abrupt shift in species and life form composition at the boundaries and whether these variables change as abruptly as the species composition.We studied 56 transects of 6×1m2 plots across eight boundary types (6–9 replicate transects per type) between six different habitats (namely, two acidic quartz fields, two saline quartz fields, zonal habitat types and heuweltjies). For each plot, we sampled soil texture, soil pH, electrical conductivity, and the calcium carbonate content at the soil surface (0–2cm), as well as soil depth, plant species composition and species richness. We tested for trends in the group means of the abiotic parameters across the different boundary types with a polynomial-contrast ANOVA. We also aggregated the three plots left (1–3) and right (4–6) of the observed vegetation boundary and tested for differences between the mean ranks of the abiotic parameters using a Wilcoxon rank sum test. Abiotic differences were compared to an indicator of the fidelity of species to the vegetation types, identified through an Indicator Species Analysis.All boundary types exhibited a strong trend in either soil pH and/or electrical conductivity. Only in a few cases did calcium carbonate and soil texture also vary across boundary types. The boundary types between the most acidic quartz fields and adjacent vegetation types showed the strongest level of fidelity of species to one of the two habitat types. Acidic quartz fields (pH<5) were most species-poor and had only one species with a significant indicator value.We discuss the influence of parent material, microtopography, climatic aridity, size of plants, and properties of winter rainfall precipitation on the abrupt difference in soil properties and vegetation patterns. By comparing the soil patterns and plant responses on quartz fields to studies of other edaphically special habitats (e.g., gypsum, serpentine and saltmarsh soils), we argue that some of the morphological features of the dominant taxa (Aizoaceae lineages) may determine their success in these habitats.

Livestock paths on Namaqualand quartz fields: Will the endemic flora disappear?

Quartz fields are rare features that contribute significantly to vegetation diversity and endemism of South Africa's Succulent Karoo Biome. The Riethuis-Wallekraal quartz fields in the north-western Namaqualand area of South Africa contain 17 quartz field specialist species of which seven are endemic to this specific area. Hoof-action by livestock has formed paths of approximately 0.30m on these quartz fields. It would be important to conservationists to understand whether direct (e.g. trampling) and indirect effects (e.g. burial of flora by sediment movement) associated with the livestock paths holds any threat to the dwarf succulent (<0.05m) and micro-chamaephytes (0.06–0.15m) endemic to the quartz fields. We tested the hypotheses that the unique quartz field vegetation and biological soil crusts would be affected by loose soil particles transported downslope from the paths. The soil stability index, total vegetation cover, cover of specialized quartz field species and species diversity were lower on livestock paths but did not differ between upslope and downslope locations. Livestock paths also had lower cover and fewer quartz field specialist species. It is concluded that under conditions of intense and continuous grazing, livestock are likely to have an even stronger negative impact on the specialist quartz field flora.

Effects of livestock grazing and habitat characteristics on small mammal communities in the Knersvlakte, South Africa

We studied the influence of different livestock grazing intensities and habitat types on small mammals in the Knersvlakte, South Africa. The Knersvlakte belongs to the Succulent Karoo Biome, a biodiversity hotspot. We placed a total of 22 plots in areas of two grazing intensities (moderate and intensive) and two major habitat types (quartz fields and loamy soils). On these plots, we carried out small-mammal live trapping (90 traps during four nights) and estimated the cover of different vegetation layers. In total, 140 small mammal individuals from ten species were caught during 7920 trap nights. All species occurred on loamy soils, but only four in quartz fields. The small-mammal community on intensively-grazed plots was a subset of that on moderately-grazed plots. The most abundant species on loamy soils were Macroscelides proboscideus and Rhabdomys pumilio and in quartz fields M. proboscideus and Gerbillurus paeba. Generalised linear models revealed that habitat type had the strongest influence on abundance and diversity of small mammals. Vegetation cover mainly had indirect influences, via the habitat type, on abundance, diversity and the body condition of small mammals. Overall, the intensities of livestock grazing showed no effect on abundance and diversity.

A conservation and floristic assessment of poorly known species rich quartz–silcrete outcrops within Rûens Shale Renosterveld (Overberg, Western Cape), with taxonomic descriptions of five new species

Quartz fields are islands of distinct vegetation in southern Africa. Such habitats differ from surrounding shale substrate in geomorphological and climatic attributes, and are dominated by a predominantly succulent flora with between 14% and 91% endemism. Previous studies have identified and surveyed quartz fields within the Succulent Karoo biome, but little is known about the Overberg quartz–silcrete fields located in the Renosterveld vegetation (Fynbos biome). This study maps the occurrence of quartz–silcrete fields in the Overberg (Eastern and Central Rûens Shale Renosterveld) and investigates if such fields support distinct vegetation compared with surrounding shales. Forty-seven plant species were recorded, 19 of which are endemic to the quartz–silcrete patches, including several newly discovered species in vascular plant families Cyperaceae (Ficinia), Iridaceae (Hesperantha) and Fabaceae (Aspalathus, Otholobium, Polhillia, Xiphotheca). Five species among these are described in this paper: Aspalathus quartzicola C.H.Stirt. & Muasya, Aspalathus microlithica C.H.Stirt. & Muasya, Ficinia overbergensis Muasya & C.H.Stirt., Otholobium curtisiae C.H.Stirt. & Muasya, and Polhillia curtisiae C.H.Stirt. & Muasya. Detailed floristic composition of 25 quartz fields is surveyed, together with their conservation status. The Overberg Quartzveld is a critically endangered vegetation type that is the sole locality for 18 Red Listed plant species; which include seven new taxa. It is a distinct vegetation unit embedded within Rûens Shale Renosterveld in the eastern Overberg region of South Africa. These quartz patches have been overlooked in the past and we emphasize the need for further research and conservation attention of these habitats.

Ecological characterization of soil-inhabiting and hypolithic soil crusts within the Knersvlakte, South Africa

Within the Knersvlakte, cyanobacteria occur hypolithically underneath translucent quartz stones in areas with quartz pavement and, outside pavement areas, they are soil-inhabiting within the uppermost millimeters of the soil. Both habitats were characterized in terms of biomass and growth patterns of cyanobacteria. Long-term microclimatic conditions were determined. Biomass of organisms within both habitats was determined by means of chlorophyll analyses. A transect approach was used to determine the frequency of hypolithic growth depending on the size, weight, and embedding depth of the quartz pebbles. Organisms were identified by means of microscopic analyses of the samples. Microclimatic conditions within both habitats, i.e., temperature, light intensity, air humidity, and soil moisture, were recorded bi-hourly from September 23, 2004 through September 7, 2006. The biomass of hypolithic and soil-inhabiting crusts was almost identical, 88 vs. 86 mg Chla/m2 and 136 vs. 134 mg Chla+b/m2. Within the quartz fields, 46.8% of the surface area was covered by quartz stones with 69% of translucent quartz stones colonized by hypolithic cyanobacteria and algae. Colonized quartz stones were significantly thicker, heavier, and more deeply embedded in the soil than uncolonized ones. Whereas the annual mean temperature on top of quartz stones was nearly identical to that underneath thin and thick quartz stones, daily temperature amplitudes were largest on the stone surface (up to 48.1K), compared to the hypolithic habitats (up to 39.4K). Light intensity in the hypolithic habitat was between 15 and 30% of the ambient light intensity during daytime. Water condensation in the absence of rain occurred during 50% of the nights on the quartz stone surface, but only during 34% of the nights on the soil surface during winter months within 1 year. Soil moisture beneath quartz layers was greater and less variable than beneath soil-inhabiting crusts. In spite of the large differences in the microclimatic conditions, both habitats seem to be similarly well suited for cyanobacterial growth, resulting in equal biomass values but some differences in taxonomic composition.

A first formal classification of the Hardeveld vegetation in Namaqualand, South Africa

AbstractQuestionsThe Succulent Karoo is a winter‐rainfall desert in southern Africa with a highly diverse flora. Which environmental factors differentiate vegetation composition at the plot level: How can compositional pattern be best represented by a formal phytosociological classification.LocationSoebatsfontein communal area and surroundings in the Namaqualand Hardeveld Bioregion of the Succulent Karoo Biome, Northern Cape Province, South Africa.MethodsWe recorded vascular plant species composition, together with a range of structural and soil parameters as well as grazing intensity, in 355 plots of 100 m² size. This sample covered all vegetation types of the study area except those of river beds and anthropogenic sites. Vegetation classification was carried out using modified TWINSPAN followed by manual re‐arrangement of a subset of plots with the aim of increasing floristic distinctiveness of the vegetation units. For the final vegetation units, we determined diagnostic species and environmental parameters, according to phi‐values and ANOVA, respectively, and translated these into a phytosociological classification.ResultsThe major factor influencing species composition and richness was soil salinity. At a broad scale, the less saline sites could be differentiated into sandy plains and rocky hills, and the saline sites into quartz fields, the centres of heuweltjies (termitaria) and the matrix between. We translated these patterns into two phytosociological classes with four orders, six alliances and 16 associations. Most of the syntaxa are very well defined in floristic terms, and all except the Othonnion cylindricae are new to science. The Hermannio trifurcae‐Zygophylletea morgsanae represents the zonal vegetation of marginally saline soils, the Didelto carnosae‐Cephalophylletea inaequalis the vegetation of saline soils. The vegetation of the extremely saline quartz patches probably belongs to a third class not described here due to the scarcity of representative plots.ConclusionsOur phytosociological approach allowed the various degrees of ecological and floristic similarity to be represented by syntaxa of different ranks. A comparison with previous classifications from the Succulent Karoo revealed a high spatial turnover, with only two of our alliances and none of our associations coinciding with those found in previously studied areas of the Succulent Karoo. Our formal classification has the potential to be a powerful tool for local land use, ecological comparison and conservation, and to contribute to a future national phytosociological classification of South Africa.

Vegetation dynamics of endemic‐rich quartz fields in the Succulent Karoo, South Africa, in response to recent climatic trends

AbstractQuestionHow do species composition and abundance change in the medium term and can the detected fluctuations and trends be attributed to weather conditions or climate change?LocationQuartz fields of the Knersvlakte, Succulent Karoo, South Africa, a plant diversity hotspot.MethodsWe monitored vascular plant composition and abundance of all taxa on 12 permanent plots (25 m²) over a period of 12 yr. With multiple regressions, we tested the effect of rainfall in different periods on species richness and abundance.ResultsThe plots showed significant inter‐annual changes in population size, species richness and species composition. In general, plant populations showed a significant positive trend over the 12 yr analysed, while therophytes decreased significantly. Number of chamaephyte and geophyte individuals was positively influenced by rainfall of the preceding year, whereas therophytes were significantly positively influenced by rainfall of the same year.ConclusionsWe conclude that the increase in inter‐annual variability of rainfall has so far had no negative effects on the quartz field vegetation. However, observations of recent climatic trends and climate change projections suggest that there will be much stronger changes in rainfall and temperature patterns in the next decades. Continuous long‐term monitoring of biodiversity is thus crucial to understand potential impacts of climate change in the Knersvlakte.

Sapphirine + quartz assemblage from the Southern Granulite Terrane, India: diagnostic evidence for ultrahigh‐temperature metamorphism within the Gondwana collisional orogen

AbstractA synthesis of the petrological characters of granulite facies rocks that contain equilibrium sapphirine + quartz assemblage from three localities (Rajapalaiyam, Ganguvarpatti and Panangad) in the Southern Granulite Terrane (SGT), India, provides unequivocal evidence for extreme crustal metamorphism associated with the collisional assembly of the Gondwana supercontinent during the Late Neoproterozoic to Early Cambrian. The reaction microstructures associated with sapphirine + quartz vary among the samples, probably suggesting different tectonic conditions during the metamorphic evolution. Sapphirine and quartz in pelitic granulites from Rajapalaiyam in the Madurai Block occur as an intergrowth in poikiloblastic garnet, whereas the two minerals show host‐inclusion relationship (quartz inclusion in coarse‐grained sapphirine) in a Mg‐Al‐rich rock from Ganguvarpatti in the same block. The sapphirine–quartz association and related microstructures from the two localities are consistent with isobaric cooling from sapphirine + quartz to orthopyroxene + sillimanite + quartz fields, possibly along a counterclockwise P‐T path. In contrast, sapphirine + quartz occurs as coronas around Mg‐rich (XMg ∼ 0.58) staurolite within poikiloblastic garnet in an Mg‐Al‐rich rock from Panangad in the Palghat–Cauvery Suture Zone (PCSZ). The texture suggests decompression from the stability field of Mg‐rich staurolite (P &gt; 15 kbar) at T &gt; 1000 °C possibly along a clockwise P‐T trajectory. Such contrasting P‐T paths are consistent with a recent tectonic model of the SGT, according to which the metamorphism in the Madurai Block is related to heat (and fluid) input from the mantle, whereas that in the PCSZ is related to the subduction‐collision tectonics between the Madurai Block and the Dharwar Craton, where high‐pressure and ultrahigh‐temperature (UHT) metamorphic rocks were extruded along the subduction channel. Copyright © 2010 John Wiley &amp; Sons, Ltd.

How does grazing intensity affect different vegetation types in arid Succulent Karoo, South Africa? Implications for conservation management

Abstract The Knersvlakte in the Succulent Karoo Biome (South Africa) is known for its high plant diversity and endemism. In the course of establishing a conservation area there, we assessed baseline data for future management. We investigated the effects of grazing on the vegetation in terms of species diversity and composition as well as reproduction of selected species. Data were sampled on four adjacent farms, which were ungrazed, moderately or intensively grazed by sheep and goats. The data were collected in 27 quartz and 24 non-quartz plots, representing two major habitat types of the region. Within each of the 1000-m 2 plots, 100 subplots of 400 cm 2 size were sampled. ANOVAs revealed that species richness and abundance of endemic species on quartz fields decreased with grazing. Abundance of annuals did not increase significantly due to grazing. Fidelity analyses indicated that species composition differed between grazing intensities and that the ungrazed and moderately grazed plots both contained unique locally endemic habitat specialists. Reproduction of two endemic dwarf shrubs Drosanthemum schoenlandianum and Argyroderma fissum (both Aizoaceae) increased under moderate grazing, which in the case of D. schoenlandianum was interpreted as an effect of grazing. We attribute the low number of seedlings and annuals on the moderately grazed farm to lower seasonal rainfall on these plots. From a conservation perspective, no or moderate grazing appear to be necessary to preserve plant diversity and vegetation patterns, and their underlying processes.

Mild experimental climate warming induces metabolic impairment and massive mortalities in southern African quartz field succulents

Current thermal regimes for many southern African succulent species of the subfamily Ruschioideae, which rapidly diversified during the cooler Pleistocene period, may be close to their tolerable extremes, and likely exceeded with anticipated future climate warming. This hypothesis was tested by exposing succulent species of different size and architecture to differently elevated temperatures approximating future African climate scenarios (2.5–3.8 °C increases in mean annual daily temperature maxima) using transparent hexagonal open-top chambers of different heights. Air temperatures, soil water potentials and amounts of fog and dew precipitation were monitored hourly in the differently heated open-top chambers and ambient environment, and changes in species leaf densities and canopy covers precisely determined in these chambers and ambient environment from high resolution digital images taken at 3-monthly intervals spanning a 12-month monitoring period. Photochemical efficiencies and activities of the photosynthetic enzyme Rubisco were also measured in one widespread dwarf succulent species following 2-h exposures of its populations in a forced draft oven to eight different heat intensities (range: 40–54 °C), the highest closely matching the temperature extreme of 54.8 °C recorded in the most intensively heated open-top chambers. After 12-months warming, all succulent species displayed massively (up to 90.2%) reduced leaf densities and canopy covers in the differently heated open-top chambers, with small sparsely branched species comprising single leaf pairs per axis exhibiting much greater reductions than large, shrubby or creeping species with multiple leaves. Noteworthy, was that fog and dew precipitation levels and soil water potentials at the centres of the least intensively heated chambers did not differ significantly from those in the ambient environment, even during the critical dry summer and early autumn seasons. However, leaf density and canopy reductions in these chambers were of similar magnitude to those in the most intensively heated chambers where fog and dew precipitation levels and soil water potentials were significantly reduced. These findings identified elevated temperatures as the principal cause of the observed massive reductions in leaf density and canopy cover and supported the hypothesis that mild anthropogenic warming could exceed the thermal thresholds of many southern African quartz field succulents leading to metabolic impairment. This impairment explained by an observed loss in the catalytic efficiency of Rubisco at daytime temperature extremes exceeding 54 °C, preceded by a decrease in PSII electron transport commencing at temperatures much lower than the threshold for Rubisco de-activation.

Vegetation of quartz fields in the Little Karoo, Tanqua Karoo and eastern Overberg (Western Cape Province, South Africa)

Vegetation of quartz fields in the Little Karoo, Tanqua Karoo and eastern Overberg (Western Cape Province, South Africa)

Quartz exsolution in clinopyroxene is not proof of ultrahigh pressures: Evidence from eclogites from the Eastern Blue Ridge, Southern Appalachians, U.S.A.

Oriented quartz needles in clinopyroxene have become one of the diagnostic indicators of ultrahighpressure (UHP) metamorphism. The presence of apparently exsolved quartz is taken as evidence of decompression of a non-stochiometric Ca.Eskola component (Ca 0.5 ⃞ 0.5 AlSi 2 O 6 , CaEs) that is presumed to be stable only at UHP conditions. Eclogite from the Eastern Blue Ridge, North Carolina, contains clinopyroxene (Jd 20 CaTs 5 Ac 5 CaEs 0 Di 65 Hd 5 ) with oriented needles of quartz and calcic amphibole that appear to have exsolved together. The quartz + amphibole intergrowths are surrounded by 1.5 µm haloes of neoformed pyroxene (Jd 10 CaTs 10 Ac 5 CaEs 0 Di 70 Hd 5 ). The modes of quartz, amphibole, and clinopyroxene haloes were determined using BSE images, and reintegrated with the host clinopyroxene. Viewing the quartz and amphibole needles down the c-axis of the pyroxene host provides a better estimate of their proportions than in prismatic sections. Reintegrated pyroxene compositions were nearly identical to the analyzed host pyroxene with no CaEs component. Clinopyroxene with CaEs solid solution has been repeatedly synthesized at UHP conditions. However, examination of the phase equilibria usually cited as evidence for CaEs stability at conditions of ≥25 kbar shows that clinopyroxene with 10 mol% CaEs is stable well within the quartz field, and provides a pressure minimum similar to the albite = jadeite + quartz barometer. Exsolution of quartz and associated amphibole is commonplace in clinopyroxene from the Blue Ridge eclogite that lacks coesite or other evidence for UHP metamorphism. The presence of a diluted (5.10%) CaEs component in clinopyroxene does not require UHP conditions.

Habitat ecology of southern African quartz fields: studies on the thermal properties near the ground

Desert pavements of white quartz stones (quartz fields) represent azonal habitats in several arid regions of southern Africa. The vegetation of these quartz fields is characterised by dwarf and highly succulent growth forms which contrast strongly with the shrubby vegetation of the surroundings. Incoming and reflected global solar radiation, air temperature near the ground, soil-surface temperatures and leaf-surface temperatures of dwarf plants were determined under natural habitat conditions inside and outside of the quartz fields. Surface temperatures of quartz and shale stones were compared. The study was conducted in the Kners­vlakte and the Little Karoo (Succulent-Karoo Biome), South Africa. The daily maximum temperatures of the air near the ground of quartz fields was several degrees lower than the air near the ground of neighbouring soils without quartz cover. Maximum soil-surface temperatures of the quartz fields, however, were only lower in summer. Accordingly, the leave surfaces of dwarf plants (Aizoaceae) growing inside quartz fields were up to 3 K cooler than identical plants outside the quartz fields. The lower maximum temperatures is associated with an about 5% higher reflection of quartz fields compared to neighbouring soils without quartz cover. At night, the minimum temperatures of the air near the ground and the soil surface temperatures on quartz fields were above those of soils without quartz cover. The nocturnal surface temperatures of quartz stones did not differ considerably from that of shale and often ranged above the temperatures and dew-point of the ambient air. Consequently, the relatively cooler quartz fields seem to provide less adverse growing conditions for plants near the ground compared to surrounding soils without quartz cover.

Pelargonium quarciticola (Geraniaceae), a new species from the Knersvlakte

Pelargonium quarciticola Meve & E.M. Marais is described as a new species. It is a deciduous geophyte belonging to the section Hoarea (Sweet) DC, and is endemic to the quartz fields of the Knersvlakte in the northern part of the Western Cape, The species is characterized by small glandular, slightly sticky, almost red, pinnately compound or coralliform leaves with subsucculent pinnae, a leaf type not typical for Pelargonium section Hoarea P. quarciticola flowers in August when the leaves are still alive. Illustrations of the species, the glandular hairs, a pollen grain, and a distribution map are provided.

Al zoning in orthopyroxene in a sapphirine quartzite: evidence for &gt;1120 °C UHT metamorphism in the Napier Complex, Antarctica, and implications for the entropy of sapphirine

A unique sapphirine + orthopyroxene + quartz granulite from Mt. Riiser-Larsen in the Tula Mountains of Enderby Land, East Antarctica, preserves two generations of coarse and texturally equilibrated orthopyroxene and sapphirine coexisting with quartz. Initial subhedral orthopyroxene porphyroblasts retain core compositions enriched in Al2O3 (12.2 ± 0.5 wt%) compared with their rims and finer orthopyroxene (9.6 ± 0.5 wt% Al2O3) that forms granoblastic textures with sapphirine. Sapphirine and quartz also form symplectites on and along cleavage planes within orthopyroxene. These compositional and textural features are consistent with the reaction [2MgAl2SiO6=Mg2Al4 SiO10 + SiO2] leading to the formation of sapphirine + quartz at the expense of aluminous orthopyroxene. Calculations in the MAS and FMAS systems and theoretical considerations involving the phases enstatite, sapphirine, sillimanite, quartz and cordierite indicate that the reaction above progresses from left to right with decreasing temperature in the orthopyroxene + sapphirine + quartz field, at pressures of ca. 8–10 kbar. The temperature difference required to account for the ca. 2.5–3 wt% decrease in Al2O3 in orthopyroxene is at least 60–80 °C, and implies peak temperatures for the initial assemblage of at least 1120 °C if the second granoblastic assemblage equilibrated at 1040 °C, the P–T conditions required by the sapphirine + quartz association and other P–T-sensitive assemblage indicators in the Napier Complex. It is not possible to distinguish whether the two assemblages are simply related by cooling and re-equilibration or reflect a polyphase evolution involving the superposition of a second UHT event on an earlier, even higher temperature, UHT metamorphism. Preliminary thermodynamic modelling of the reaction above incorporating the observed range in orthopyroxene Al2O3 zoning indicates that present estimates for the entropy of high-temperature sapphirine are potentially too high by 15–18% compared with sapphirine entropy estimates that are consistent with MAS system experiments. The Mt. Riiser-Larsen sapphirine–quartz rocks preserve the first definitive record of regional metamorphic temperatures in excess of 1120 °C in the Napier Complex, or indeed any UHT granulite terrain worldwide. Similarly high peak temperatures may be retrieved from detailed studies of sapphirine–quartz granulites from other regions, further expanding the thermal realm of crustal metamorphism, but progress will critically depend on the experimental acquisition of new entropy data for sapphirine.

Community structure on unusual habitat islands: quartz-fields in the Succulent Karoo, South Africa

Quartz fields are edaphically arid, azonal habitats occurring under different macroclimatic conditions in several arid regions of southern Africa. They are the exclusive home of 142 plant species of which ca. 70% are local or regional endemics. This paper is an analysis of the quartz-field floras and growth form-soil relationships in two quartz field regions: the Knersvlakte in the Namaqualand-Namib Domain of the Succulent Karoo, and the western Little Karoo in the Southern Karoo Domain. The Knersvlakte supported 52 quartz-field specialists of which 39 were endemic to the region. Corresponding data for the Little Karoo were 11, and 10 species, respectively. In both regions, the average canopy cover on the quartz-field releves was ca. 8%, and more than half of this comprised contracted, succulent nanochamaephytes. Cover and vegetation stature were markedly higher on adjacent zonal habitats. Quartz fields in both regions supported a similar array of compact, subglobose and subterranean nanochamaephytes, as evidenced by convergent patterns in two distantly related genera (Argyroderma N. E. Brown and Gibbaeum (Haworth) N. E. Brown, both Mesembryanthemaceae), endemic to the Knersvlakte and largely restricted to the Little Karoo, respectively. Analyses of vegetational and edaphic data of quartz fields and adjacent, zonal habitats were carried out using multivariate direct gradient analysis (Canonical Correspondence Analysis) in order to identify those factors that control the peculiar composition of growth forms on quartz fields. The results revealed highly similar patterns of growth form composition in relation to similar edaphic gradients in both regions. In general, the soils of quartz fields were shallower compared to those of adjacent zonal habitats. In both regions, two different groups of quartz-field edaphic habitats, representing extremes of a continuum, were identified. Group 1 was characterized by high salt content, neutral to slightly acid soil pH, and low stone content. Group 2 was characterized by low salt content, low soil pH, and high stone content. Group 1 quartz fields are the most edaphically arid habitats and support the highest relative cover and diversity of subglobose and subterranean chamaephytes. The combination of reduced competition from larger growth forms, shallow soils and high soil salinity, represents a regionally unusual selective regime. Some succulent lineages in the Mesembryanthemaceae have undergone diversification which has resulted in the fine-scale discrimination of subtle edaphic gradients within the saline quartz-patch habitats. Reliable seasonal rainfall and reduced thermal stress have also played a role in the evolution of quartz patch specialists.