Rhyolitic Glass Shards Research Articles

Volcanic glass from the 1.8 ka Taupō eruption (New Zealand) detected in Antarctic ice at ~ 230 CE

Chemical anomalies in polar ice core records are frequently linked to volcanism; however, without the presence of (crypto)tephra particles, links to specific eruptions remain speculative. Correlating tephras yields estimates of eruption timing and potential source volcano, offers refinement of ice core chronologies, and provides insights into volcanic impacts. Here, we report on sparse rhyolitic glass shards detected in the Roosevelt Island Climate Evolution (RICE) ice core (West Antarctica), attributed to the 1.8 ka Taupō eruption (New Zealand)—one of the largest and most energetic Holocene eruptions globally. Six shards of a distinctive geochemical composition, identical within analytical uncertainties to proximal Taupō glass, are accompanied by a single shard indistinguishable from glass of the ~25.5 ka Ōruanui supereruption, also from Taupō volcano. This double fingerprint uniquely identifies the source volcano and helps link the shards to the climactic phase of the Taupō eruption. The englacial Taupō-derived glass shards coincide with a particle spike and conductivity anomaly at 278.84 m core depth, along with trachytic glass from a local Antarctic eruption of Mt. Melbourne. The assessed age of the sampled ice is 230 ± 19 CE (95% confidence), confirming that the published radiocarbon wiggle-match date of 232 ± 10 CE (2 SD) for the Taupō eruption is robust.

Detection of the Askja AD 1875 cryptotephra in Latvia, Eastern Europe

ABSTRACTWe report the first geochemically confirmed findings of the Askja volcano (Iceland) AD 1875 eruption cryptotephra in Eastern Europe. The cryptotephra finding in Latvia is the easternmost finding of the Askja AD 1875 so far, providing an important time marker in the sediments. Although low concentrations of Askja AD 1875 rhyolitic glass shards were recorded, our findings suggest the possibility of also tracing other historical cryptotephras in lacustrine and peat sediments in Eastern Europe. We use the Askja AD 1875 tephra isochrone to synchronize pollen data of human activities, i.e. rye (Secale cereale) cultivation. Our comparison of Secale pollen from two sites reveals that there were minor dissimilarities in the timing of highest rye cultivation, and that a synchronous decrease of rye cultivation occurred at both sites few years after the Askja eruption at AD 1875.

Multiple ash layers in late Quaternary sediments from the Central Indian Basin

We have investigated three sediment cores collected from water depths >5000m along the transect 76°30′E in close proximity to a fracture zone in the Central Indian Basin (CIB). The cores yielded five volcanic horizons of which four have visual and dispersed shards. Rhyolitic glass shards of bubble wall, platy, angular and blocky types were retrieved from various stratigraphic horizons in the cores. The abundance of glass shards, composition of bulk sediments, and 230Thexcess ages of the host sediments were used to distinguish the volcanic horizons. Of the four volcanic horizons, three are now newly reported and correspond to ages of ~85, 107–109 and 142–146ka while the fourth horizon is of 70–75ka. By using trace element ratios and Cr and Nb-based normative calculations, cryptotephra has been identified for the first time from the CIB sediment. The cryptotephra forms the fifth ash horizon and is of ~34ka.A comparison with the published data on volcanic tephra in and around the Indian Ocean indicate the shard rich horizon (SRH) of 70–75ka to resemble the Younger Toba Tuffs (YTT), while the other volcanic horizons that were deposited during different time periods do not correlate with any known marine or terrestrial records. These tephra layers have produced a tephrostratigraphic framework across the tectonically and volcanically complex regions of the CIB. Due to the lack of terrestrial equivalents of these tephra, it is hypothesized that the newly found volcanic horizons may have been derived from submarine volcanic eruptions. Multiple layers of submarine volcaniclastic deposits found at water depths as great as 5300m reaffirm the growing belief that submarine phreatomagmatic eruptions are much more common in the intraplate region of the Indian Ocean than previously reported.

Origins of felsic magmas in Japanese subduction zone: Geochemical characterizations of tephra from caldera‐forming eruptions <5 Ma

AbstractDacitic to rhyolitic glass shards from 80 widespread tephras erupted during the past 5 Mys from calderas in Kyushu, and SW, central, and NE Japan were analyzed. Laser ablation inductively coupled plasma mass spectrometry was used to determine 10 major and 33 trace elements and 207Pb/206Pb‐208Pb/206Pb isotope ratios. The tephras were classified into three major geochemical types and their source rocks were identified as plutonic, sedimentary, and intermediate amphibolite rocks in the upper crust. A few tephras from SW Japan were identified as adakite and alkali rhyolite and were regarded to have originated from slab melt and mantle melt, respectively. The Pb isotope ratios of the tephras are comparable to those of the intermediate lavas in the source areas but are different from the basalts in these areas. The crustal assimilants for the intermediate lavas were largely from crustal melts and are represented by the rhyolitic tephras. A large heat source is required for forming large volumes of felsic crustal melts and is usually supplied by the mantle via basalt. Hydrous arc basalt formed by cold slab subduction is voluminous, and its heat transfer with high water content may have melted crustal rocks leading to effective felsic magma production. Coincidence of basalt and felsic magma activities shown by this study suggests caldera‐forming eruptions are ultimately the effect of a mantle‐driven cause.

Towards a protocol for the trace element analysis of glass from rhyolitic shards in tephra deposits by laser ablation ICP‐MS

ABSTRACTLaser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) is used widely to analyse single volcanic glass shards from Quaternary tephra deposits for stratigraphic correlation. As yet, no generally accepted protocol for these analyses exists and published methods report significant differences in crater size and calibration strategies. Using Ca as the ‘internal standard’ (CaI.S.), and not SiI.S., for the analysis of rhyolitic glass can cause significant problems, because of ablation of (i) Ca‐rich phenocrysts, notably ‘ubiquitous’ plagioclase, but also calcic‐amphibole, apatite and allanite or (ii) mounting epoxy resin which can contribute to the Ca internal standard signal. Using CaI.S. can cause underreporting of many elements which concentrate in Ca‐rich phenocrysts, making their ablation difficult to recognize, but when using SiI.S., no such underreporting occurs. Additionally, larger ablation craters (50 µm diameter) potentially incorporate even small volumes of phenocryst material, whereas smaller craters (<20 µm diameter) more frequently miss phenocrysts. Thus, for the LA‐ICP‐MS analysis of rhyolitic glass shards, smaller ablation craters should be employed and calibrated using SiI.S., allowing recognition and removal of analyses incorporating Ca‐rich phenocrysts and avoiding issues related to the ablation of the mounting resin.

Provenance of glass shards in archaeological lime plasters by LA-ICP-MS: implications for the ancient routes from the Gulf of Mexico to Teotihuacan in Central Mexico

We present the chemical characteristics of samples collected from the central courtyard of Teopancazco, a neighbourhood centre in Teotihuacan (Mexico), the most important classic Mesoamerican city. The rhyolitic glass shards used as aggregates have a composition comparable to the glass material sourced from the magmatic system of Altotonga (located along the route to the Gulf Coast). People travelling from the Gulf Coast may have transported the glass shards to Teotihuacan during the Xolalpan phase (350–550 AD). However, the reasons for using glass shards as a construction material remain unknown. These new data confirm the close relationship between inhabitants of Teopancazco and the Gulf Coast already revealed by previous archaeological and archaeometric studies.

The systematics of chlorine, fluorine, and water in Izu arc front volcanic rocks: Implications for volatile recycling in subduction zones

We studied the systematics of Cl, F and H 2O in Izu arc front volcanic rocks using basaltic through rhyolitic glass shards and melt inclusions (Izu glasses) from Oligocene to Quaternary distal fallout tephra. These glasses are low-K basalts to rhyolites that are equivalent to the Quaternary lavas of the Izu arc front (Izu VF). Most of the Izu glasses have Cl ∼400–4000 ppm and F ∼70–400 ppm (normal-group glasses). Rare andesitic melt inclusions (halogen-rich andesites; HRA) have very high abundances of Cl (∼6600–8600 ppm) and F (∼780–910 ppm), but their contents of incompatible large ion lithophile elements (LILE) are similar to the normal-group glasses. The preeruptive H 2O of basalt to andesite melt inclusions in plagioclase is estimated to range from ∼2 to ∼10 wt% H 2O. The Izu magmas should be undersaturated in H 2O and the halogens at their preferred levels of crystallization in the middle to lower crust (∼3 to ∼11 kbar, ∼820° to ∼1200°C). A substantial portion of the original H 2O is lost due to degassing during the final ascent to surface. By contrast, halogen loss is minor, except for loss of Cl from siliceous dacitic and rhyolitic compositions. The behavior of Cl, F and H 2O in undegassed melts resembles the fluid mobile LILE (e.g.; K, Rb, Cs, Ba, U, Pb, Li). Most of the Cl (>99%), H 2O (>95%) and F (>53%) in the Izu VF melts appear to originate from the subducting slab. At arc front depths, the slab fluid contains Cl = 0.94 ± 0.25 wt%, F = 990 ± 270 ppm and H 2O = 25 ± 7 wt%. If the subducting sediment and the altered basaltic crust were the only slab sources, then the subducted Cl appears to be almost entirely recycled at the Izu arc (∼77–129%). Conversely, H 2O (∼13–22% recycled at arc) and F (∼4–6% recycled) must be either lost during shallow subduction or retained in the slab to greater depths. If a seawater-impregnated serpentinite layer below the basaltic crust were an additional source of Cl and H 2O, the calculated percentage of Cl and H 2O recycled at arc would be lower. Extrapolating the Izu data to the total length of global arcs (∼37,000 km), the global arc outflux of fluid-recycled Cl and H 2O at subduction zones amounts to Cl ∼2.9–3.8 × 10 12 g/yr and H 2O ∼0.7–1.0 × 10 14 g/yr, respectively—comparable to previous estimates. Further, we obtain a first estimate of global arc outflux of fluid-recycled F of ∼0.3–0.4 × 10 12g/yr. Despite the inherent uncertainties, our results support models suggesting that the slab becomes strongly depleted in Cl and H 2O in subduction zones. In contrast, much of the subducted F appears to be returned to the deep mantle, implying efficient fractionation of Cl and H 2O from F during the subduction process. However, if slab devolatilization produces slab fluids with high Cl/F (∼9.5), slab melting will still produce components with low Cl/F ratios (∼0.9), similar to those characteristic of the upper continental crust (Cl/F ∼0.3–0.9).

Were rhyolitic glasses in the Vedde Ash and in the North Atlantic's Ash Zone 1 produced by the same volcanic eruption?

New evidence from North Atlantic deep-sea sediment cores suggests that rhyolitic glass shards in Ash Zone 1 and in the Vedde Ash may not have been produced by the same eruption as has been widely assumed. Our argument is based on (1) a marine ice-rafted deposit approximately 1000 years older than the Vedde Ash contains rhyolitic shards with the same major element chemistry as the Vedde Ash itself and (2) coincidence of Ash Zone 1 with an abrupt increase in discharge of glacial icebergs into the North Atlantic. Hence, we cannot rule out the possibility that Vedde-like rhyolitic glasses were erupted onto Icelandic glaciers before eruption of the Vedde Ash, stored in the glacial ice, then dispersed by icebergs into the North Atlantic and deposited partly or entirely as Ash Zone 1. While not disproving that Ash Zone 1 and the Vedde Ash came from the same eruption, our findings indicate that further study is needed to establish the exact relation between the two ash deposits.

New occurrence of Youngest Toba Tuff in abyssal sediments of the Central Indian Basin

Volcanic glass and pumice found in siliceous abyssal sediments of the Central Indian Basin, south of the Equator, have previously been assigned various origins including intra-basin volcanism, Indonesian arc, and Krakatau. Rhyolitic glass shards dispersed in sediments from 8 cores that we have examined are compositionally identical to those of fallout deposits of the Youngest Toba Tuff erupted at 74 ka from northern Sumatra. The correlation extends the distribution of coarse (>63 μm) glass shards some 1500 km south of the previously known fallout zone, and into the Southern Hemisphere (reaching latitudes of ca. 14°S). This provides evidence for bi-hemispheric dispersal of the ash cloud and supports global dispersal of gas and aerosols from the eruption. Such dispersal could have facilitated the global impact of the eruption.

Evidence for recent hydrothermal activity in the Central Indian Basin

This study documents the first actual proof of recent intraplate volcanic-hydrothermal activity in the Central Indian Basin (CIB). Twenty-six surface sediments and a spade core (37 cm long) from the CIB were examined for the presence of volcanogenic-hydrothermal materials (vhm). High concentrations of vhm were discovered in a grab and the core top—both located at the base of an intraplate seamount. The vhm consist of ochrous metalliferous sediments, volcanic spherules and glass shards. The radiolaria associated with the vhm suggest a ∼ 10 ka age for the hydrothermal episode. The metalliferous sediments are semi-indurated, yellow to orange colored FeSi oxyhydroxides with FeO and Si0 2 contents between 54–73% and 16–30%, respectively, and have been derived as a result of hydrothermal precipitation. Incipient formation of nontronite is noted to co-occur with these sediments. The CIB metalliferous sediments have close similarities to those reported from the intraplate regions of the Pacific Ocean. The volcanic spherules occur in various shapes and sizes and are dominantly composed of magnetite and lesser amounts of ilmenite, hematite and maghemite. Electron microscopy shows the arrangement of magnetite crystals in various textural forms. Inclusions within the spherules are of olivine, pyroxene and feldspar. The spherules have formed by a process of liquid immiscibility of a silicic-basic magma, dependent on oxygen fugacity. Rhyolitic glass shards are ubiquitous at 1–2 cm depth in the core and constitute 55% of the coarse fraction. Microprobe analyses of the CIB shards show clear differences in Ti- and SiAl ratios that, together with the vast differences in age of eruption, preclude their derivation from Toba (Indonesia).

Methods of identifying late Quaternary rhyolitic tephras on the ring plains of Ruapehu and Tongariro volcanoes, New Zealand

On the ring plains of Ruapehu and Tongariro volcanoes, distal rhyolitic marker tephras provide a valuable stratigraphic framework. However, identification of many of these tephras has been imprecise. Here we provide a quantitative approach for identifying tephras within the ring‐plain sequences. We extend from simple canonical discriminant function models of glass chemistry to show how these, in conjunction with other geological information, can be used in a practical field‐based study. In two stratigraphically distinguishable groups (10–22 ka and 22–65 ka), we established discriminant models for possible tephra correlatives from standard glass analyses. Testing analyses from unknown tephras against the models classified 34 of the 41 samples with probabilities >0.75 to tephras that were consistent with mineralogical and stratigraphic evidence. Unknowns with lower probabilities of classification had several possible correlatives. Some of these were improved when the tephras classified with >0.75 probability, and which were consistent with stratigraphic and other evidence, were added to the discriminant models. The classifications were improved because of an increased number of samples for each tephra and also because the added analyses were produced by the same EMP operator under the same instrument conditions. Classifications of other unknowns were improved by considering them as mixed tephras. In addition to more rigorously correlating several tephras previously identified in this area, we have identified four tephras in the area for the first time—the Okaia, Omataroa, and Hauparu Tephras and the Rotoehu Ash. These occur as microscopic accumulations of rhyolitic glass shards within weathered andesitic tephra deposits.

Volcanic ash from Icelandic ∼57,300 Yr BP eruption found in GISP2 (Greenland) Ice Core

We have found volcanic ash in Wisconsinan ice from the GISP2 ice core retrieved from the Summit location in Central Greenland. The ash consists, predominantly of large rhyolitic glass shards similar to the ones found in Wisconsinan Dye 3 ice from Southern Greenland. We give the results of our analysis of the ash and present evidence that correlates this ash with the Z2 ash layer found in Atlantic sediment cores, with the Dye 3 ash, with ash recently found in the GRIP ice core from Central Greenland and with ash associated with the Icelandic Thorsmörk ignimbrite. The mineralogy of the ash is very similar to that of ash from the Icelandic Tindfjallajökull volcano that is also present in the Thorsmörk ignimbrite and we suggest that this is the source volcano. Tindfjallajökull has also been suggested as the source of the Z2 ash. Based on the excellent dating of the GISP2 ice core, we determined that the eruptions that produced the GISP2 ash occured 57,300±1,700 yr BP which agrees extremely well with the most recent dating of 57,500±1,300 yr BP of the Z2 ash based on the SPECMAP chronology. This provides us with a reliable revision of the Dye 3 ice core dating at a depth of ∼1950 m. This ice was originally dated at ∼70,000 yr BP.

Composition and Provenance of Volcanic Glass in Late Eocene Manning Formation, East-Central Texas: ABSTRACT

ABSTRACT Airfall volcanic ash beds in the Late Eocene Manning Formation, Brazos River Valley, Texas, contain large quantities of hydrated, but otherwise unaltered and non-corroded, rhyolitic glass shards, along with mica, sanidine, and quartz crystals. Enclosing deposits contain well preserved siliceous microfossils (diatoms, sponge spicules, phytoliths). Unaltered glass occurs only where volcanic ash was deposited in nonmarine, non-swamp environments: lacustrine, fluvial, and forested soil surface. Composition of glass shards from the upper part and top of the Manning Formation was determined by broad-beam electron microprobe analysis. Glasses from four ash beds analyzed to date are very similar in major and minor element composition. The mean and standard deviation in weight% oxides for 398 shards are: 72.7% SiO2 (Standard deviation 0.7%), 11.6% Al2O3 (0.2),5.7% K20 (0.5),2.4% Na2O (0.3),0.7% FeO (0.2), 0.4% CaO (0.1), and 6.5% H2O (by difference). Na and possibly K have likely been substantially leached from glass shards during hydration. MgO, MnO, TiO2, P2O5, and SO3 are all less than 0.2% (below detection limit). The youngest ash layer (top Manning) contains 0.9% FeO compared to 0.7% FeO for two ash layers in the middle Manning and 0.5% FeO for the oldest ash layer in the lower Manning. A possible source for these glasses would be the Sierra Madre Occidental in west Mexico. Age (34.9 and 34.4 Ma for two of the ash beds) and composition is dissimilar to most rhyolites in the Davis Mountains of west Texas.

Mobility of trace elements and leaching rates of rhyolitic glass shards from some New Zealand tephra deposits

It is shown that glass shards from volcanic eruptions of known age are useful in natural analogue studies connected with nuclear waste disposal. They may be sequentially leached in bulk with HF and the hydration layer thickness determined. If they have also been irradiated with neurons in a reactor, some elemental profile information is available at the same time. Applying this to New Zealand volcanic glass shards gave as many as 15 different fractions, and information about the distribution of up to 16 trace elements. Those shards from a 22 ka eruption yielded hydration rates of ∼2.5 × 10 −11 g/cm 2/d while rates from a 340 ka eruption were 1.5 × 10 −12 −1.2 × 10 −11 g/cm 2/d. The percentage of the total mass hydrated ranged between 5 and 35%. As found in some other studies, many elements accumulate on the surface of the shards, particularly Cr, Co and Fe, but it is shown here that they mostly originate in surrounding groundwater rather than the bulk glass. Rubidium and Cs tend to be removed instead. There was no obvious correlation between degree of hydration and the environment of deposition. The use of glass shards from volcanic eruptions is recommended for such studies because they are ubiquitous, and occurrence is less dependent on local geology than for some sample types.

The plinian eruptions of 1912 at Novarupta, Katmai National Park, Alaska

The three-day eruption at Novarupta in 1912 consisted of three discrete episodes. Episode I began with plinian dispersal of rhyolitic fallout (Layer A) and contemporaneous emplacement of rhyolitic ignimbrites and associated proximal veneers. The plinian column was sustained throughout most of the interval of ash flow generation, in spite of progressive increases in the proportions of dacitic and andesitic ejecta at the expense of rhyolite. Accordingly, plinian Layer B, which fell in unbroken continuity with purely rhyolitic Layer A, is zoned from >99% to ∼15% rhyolite and accumulated synchronously with emplacement of the correspondingly zoned ash flow sequence in Mageik Creek and the Valley of Ten Thousand Smokes (VTTS). Only the andesiterichest flow units that cap the flow sequence lack a widespread fallout equivalent, indicating that ignimbrite emplacement barely outlasted the plinian phase. On near-vent ridges, the passing ash flows left proximal ignimbrite veneers that share the compositional zonation of their valley-filling equivalents but exhibit evidence for turbulent deposition and recurrent scour. Episode II began after a break of a few hours and was dominated by plinian dispersal of dacitic Layers C and D, punctuated by minor proximal intraplinian flows and surges. After another break, dacitic Layers F and G resulted from a third plinian episode (III); intercalated with these proximally are thin intraplinian ignimbrites and several andesite-rich fall/flow layers. Both CD and FG were ejected from an inner vent <400 m wide (nested within that of Episode I), into which the rhyolitic lava dome (Novarupta) was still later extruded. Two finer-grained ash layers settled from composite regional dust clouds: Layer E, which accumulated during the D-F hiatus, includes a contribution from small contemporaneous ash flows; and Layer H settled after the main eruption was over. Both are distinct layers in and near the VTTS, but distally they merge with CD and FG, respectively; they are largely dacitic but include rhyolitic shards that erupted during Episode I and were kept aloft by atmospheric turbulence. Published models yield column heights of 23–26 km for A, 22–25 km for CD, and 17–23 km for FG; and peak mass eruption rates of 0.7–1x108, 0.6–2x108, and 0.2–0.4x108 kg s-1, respectively. Fallout volumes, adjusted to reflect calculated redistribution of rhyolitic glass shards, are 8.8 km3, 4.8 km3, and 3.4 km3 for Episodes I, II, and III. Microprobe analyses of glass show that as much as 0.4 km3 of rhyolitic glass shards from eruptive Episode I fell with CDE and 1.1 km3 with FGH. Most of the rhyolitic ash in the dacitic fallout layers fell far downwind (SE of the vent); near the rhyolite-dominated ignimbrite, however, nearly all of Layers E and H are dacitic, showing that the downwind rhyolitic ash is of ‘co-plinian’ rather than co-ignimbrite origin.

Petrogenesis of a basalt-rhyolite tephra from the west-central Afar, Ethiopia

The Cindery Tuff is an unusual tephra fall deposit that contains evidence for the mixing of basaltic and rhyolitic liquids prior to eruption. It contains clear rhyolitic glass shards together with brown basaltic glass spheres and a broadly bimodal phenocryst assemblage. Brown glasses are ferrobasaltic in composition and are similar to the voluminous Pliocene tholeiites of the surrounding west-central Afar volcanic field; both are enriched in the light rare earth and incompatible elements and possess higher 87Sr/86Sr and lower 143Nd/144Nd than MORB. Rhyolitic glasses are subalkaline and, compared to the basaltic glasses, are strongly depleted in the compatible elements and enriched in the incompatible elements. Both glass types have similar incompatible element and isotopic ratios, and with the rhyolite glass showing a 2-fold parallel enrichment in rare earth element abundances over the basaltic glass. These observations suggest that the two glasses are genetically related.

Refractive index and hydration of rhyolitic glass from Holocene tephras, North Island, New Zealand

Abstract An accurate hot-stage microscope method of determining refractive index of rhyolitic glass shards, by using the thermal variation of refractive index of methyl phenyl ether in which the shards are immersed, enables the distinction of glass samples of Mairoa Ash from those of Tirau Ash and late Pleistocene tephras, The accuracy and precision of the method clearly show the contamination of Mairoa Ash with Oruanui Ash. Comparison of refractive index of glass of Oruanui Ash from various sites shows little difference. However, water contents (H20>−) of the glasses, calculated from the difference between the measured refractive index (being that of hydrated glass) and that determined empirically from chemical composition (being that of the glass before hydration-effectively obsidian), vary with the square root of the time of hydration and show an Arrhenian dependence with the temperature of hydration. The activation energy for diffusion of water into the glasses is calculated from these relationships as 166 kJ mol−1.

Determination Of densities of rhyolitic glass shards by a gradient method (note)

Abstract Densities of rhyolitic glass shards recovered from late Pleistocene tephras in the eastern Waikato region were determined using density gradient columns. There are significant differences in mean density of the glasses, but the widths of the bands in the columns limit the method's tephrostratigraphic use. Okareka Ash is shown to contain two glasses, indicative perhaps of there being two eruptive events. The use of density gradient methods to improve the physical separation of minerals by heavy liquids is discussed.

Alteration of Volcanic Ash Near Denver, Colorado

ABSTRACT A vitric, crystal, lithic volcanic ash near Denver, Colorado, consists predominantly of fresh, highly vesicular, rhyolitic glass shards and smaller amounts of partly altered platy glass shards. The alteration products of the platy shards are predominantly illite randomly interlayered with a small amount of montmorillonite and also containing a small amount of free montmorillonite. Among the lithic and crystal particles in the ash is biotite, probably of volcanic origin, which is partly altered to vermiculite, part of which is randomly interlayered with biotite. Some of the altered biotite crystals are long and vermicular and appear to be authigenic.