Tazzoliite, ideally Ba2CaSr0.5Na0.5Ti2Nb3SiO17[PO2(OH)2]0.5, is a new mineral (IMA 2011-018) from Monte delle Basse, Euganei Hills, Galzignano Terme, Padova, Italy. It occurs as lamellar pale orange crystals, which are typically a few m m thick and up to 0.4 mm long, closely associated with a diopsidic pyroxene and titanite. Tazzoliite is transparent. It has a white streak, a pearly lustre, is not fluorescent and has a hardness of 6 (Mohs' scale). The tenacity is brittle and the crystals have a perfect cleavage along {010}. The calculated density is 4.517 g cm–3. Tazzoliite is biaxial (–) with 2Vmeas of ~50º, it is not pleochroic and the average refractive index is 2.04. No twinning was observed. Electronmicroprobe analyses gave the following chemical formula: (Ba1.93Ca1.20Sr0.52Na0.25Fe0.102+)Σ4 (Nb2.88Ti2.05Ta0.07Zr0.01V0.015+)Σ5.02SiO17[(P0.13Si0.12S0.07)Σ0.32O0.66(OH)0.66][F0.09(OH)0.23]Σ0.32.

Tazzoliite is orthorhombic, space group Fmmm, with unit-cell parameters a = 7.4116(3), b = 20.0632(8), c = 21.4402(8) Å, V = 3188.2(2) Å3 and Z = 8. The crystal structure, obtained from single-crystal X-ray diffraction data, was refined to R1(F2) = 0.063. It consists of a framework of Nb(Ti) octahedra and BaO7 polyhedra sharing apexes or edges, and Si tetrahedra sharing apexes with Nb(Ti) octahedra and BaO7 polyhedra. The structure, which is related to the pyrochlore structure, contains three Nb(Ti) octahedra: two are Nb dominant and one is Ti dominant. Chains of A2O8 polyhedra [A2 being occupied by Sr(Ca, Fe)] extend along [100] and are surrounded by Nb octahedra. Channels formed by six Nb(Ti) octahedra and two tetrahedra, or four A1O8(OH) polyhedra (A1 being occupied by Ba), alternate along [100]. The channels are partially occupied by [PO2(OH)2] in two possible mutually exclusive positions, alternating with fully occupied A3O7 polyhedral pairs [A3 being occupied by Ca(Na)]. The seven strongest X-ray powder diffraction lines [d in Å (I/I0) (hkl)] are: 3.66 (60) (044), 3.16 (30) (153), 3.05 (100) (204), 2.98 (25) (240), 2.84 (50) (064), 1.85 (25) (400) and 1.82 (25) (268). Raman spectra of tazzoliite were collected in the range 150–3700 cm–1 and confirm the presence of OH groups. Tazzoliite is named in honour of Vittorio Tazzoli in recognition of his contributions to the fields of mineralogy and crystallography.

The crystal structure of zoned philipsbornite – hidalgoite, hexagonal (rhombohedral), Rm , Z = 3: a = 7.1142(4), c = 17.0973(9) A ˚ , V = 749.4(1) Å3, from the Tsumeb mine, Namibia, has been refined to R1 = 1.68% for 301 unique reflections collected on a Bruker D8 three-circle diffractometer equipped with a rotating-anode generator, multilayer optics and an APEX-II CCD detector. Chemical analysis by electron microprobe showed zoned crystals with a rim enriched in S and Fe relative to the core. The core composition is SO3 3.31, As2O5 30.57, Al2O3 23.05, FeO 1.44, PbO 33.94, H2Ocalc 9.58, total 101.79 wt.%, corresponding to Pb0.982+(Al2.92Fe0.132+)(AsO4)[(As0.72S0.27)O3.14(OH)0.85](OH)6; and the rim composition is SO3 8.88, As2O5 22.63, Al2O3 22.90, FeO 2.57, PbO 34.91, H2Ocalc 9.27, total 101.16 wt.%, corresponding to Pb0.992+(Al2.85Fe0.232+)(AsO4)[(As0.25S0.70)O3.30(OH)0.50](OH)6. Philipsbornite – hidalgoite has the alunite-type structure, sheets of corner-sharing octahedra, decorated on top and bottom by [(As,S)O4] and (AsO3OH) tetrahedra, that are linked into a three-dimensional structure by [12]-coordinated Pb2+ cations and hydrogen bonds. A new hydrogen-bonding scheme for the D2+G33+(T5+O4)(TO3OH)(OH)6 minerals is proposed.

Alcaparrosaite, ideally K3Ti4+Fe3+(SO4)4O(H2O)2, is a new mineral from the Alcaparrosa mine, Cerritos Bayos, El Loa Province, Antofagasta, Chile (IMA2011-024). The mineral occurs on and intergrown with coquimbite, and is also associated with ferrinatrite, krausite, pertlikite, pyrite, tamarugite and voltaite. It is a relatively early phase which forms during the oxidation of pyritic masses under increasingly arid conditions. Alcaparrosaite crystallizes from hyperacidic solutions in a chemical environment that is consistent with its association with coquimbite. It occurs as pale yellow blades and tapering prisms up to 4 mm in length, flattened on {010} and elongated along [100]. The observed crystal forms are {010}, {110}, {1.13.0} and {021}. The mineral is transparent and has a white streak, vitreous lustre, Mohs hardness of about 4, brittle tenacity, conchoidal fracture and no cleavage. The measured and calculated densities are 2.80(3) and 2.807 g cm–3, respectively. It is optically biaxial (+) with α = 1.643(1), β = 1.655(1), γ = 1.680(1) (white light), 2Vmeas = 70(2)° and 2Vcalc = 70.3°. The mineral exhibits strong parallel dispersion, r < v. The optical orientation is X = b; Y^c = 27° in the obtuse angle β. No pleochroism was observed. Electron-microprobe analyses (average of 4) provided: Na2O 0.32, K2O 20.44, Fe2O3 11.58, TiO2 11.77, P2O50.55, SO347.52, H2O 5.79 (calc); total 97.97 wt.%. The empirical formula (based on 19 O) is (K2.89Na0.07)Σ2.96Ti0.984+Fe0.973+(S0.99P0.01O4)4O0.72(OH)0.28(H2O)2. The mineral is hydrophobic, insoluble in cold and hot water, very slowly soluble in acids and decomposes slowly in bases. Alcaparrosaite is monoclinic, C2/c, with the cell parameters a = 7.55943(14), b = 16.7923(3), c = 12.1783(9) Å, β = 94.076(7)°, V = 1542.01(12) Å3 and Z = 4. The eight strongest lines in the X-ray powder diffraction pattern [dobs in A ˚ (Irel) (hkl)] are 6.907 (41) (021,110); 3.628 (34) (023,13); 3.320 (32) (02); 3.096 (100) (202,33,150); 3.000 (40) (51); 2.704 (38) (23,152); 1.9283 (30) (55); 1.8406 (31) (53,06). In the structure of alcaparrosaite (R1 = 2.57% for 1725 Fo > 4σF), Ti4+ and Fe3+, in roughly equal amounts, occupy the same octahedrally coordinated site. Octahedra are linked into dimers by corner sharing. The SO4 tetrahedra link the dimers into chains parallel to [001] and link the chains into undulating sheets parallel to {010}. The sheets link via 10- and 11-coordinated K atoms in the interlayer region. The structure shares some features with that of goldichite.

The structure topology and crystal chemistry have been considered for ten astrophyllite-group minerals that contain the HOH layer, a central trioctahedral (O) sheet and two adjacent (H) sheets of [5]- and [6]-coordinated D polyhedra and the astrophyllite (T4O12) ribbons. The HOH layer is characterized by a planar cell with a ∼5.4, b ∼11.9 A ˚ and a^b ∼103°. The ideal composition of the O sheet is Fe72+ (astrophyllite) or Mn72+(kupletskite). All structures consist of an HOH layer and an I (intermediate) block that consists of atoms between two HOH layers. In the astrophyllite group, there are two types of structures based on the type of linkage of HOH layers: (1) HOH layers link directly where they share common vertices of D octahedra, and (2) HOH layers do not link directly via polyhedra of the H sheets. The type-1 structure occurs in astrophyllite, niobophyllite, nalivkinite, tarbagataite, kupletskite, niobokupletskite and kupletskite-(Cs); the type-2 structure occurs in magnesioastrophyllite, sveinbergeite and devitoite. The general formulae for the eight astrophyllite-group minerals (astrophyllite, niobophyllite, nalivkinite, tarbagataite, kupletskite, niobokupletskite, kupletskite-(Cs), magnesioastrophyllite) and for the extended astrophyllite group including devitoite and sveinbergeite are A2BC7D2T8O26(OH)4X0–1 and A2pBrC7D2(T4O12)2IXD2OXA4OXDnP, respectively, where C and D are cations of the O and H sheets, C =[6](Fe2+, Mn, Fe3+, Na, Mg, Zn) at the M(1–4) sites; D = [6,5](Ti, Nb, Zr, Fe3+); T = Si, minor Al; A2pBrI is the composition of the I block where p = 1,2; r = 1,2; A = K, Cs, Li, Ba, H2O, ☐; B = Na, Ca, Ba, H2O, ☐; I represents the composition of the central part of the I block, excluding peripheral layers of the form A2B; X = O, OH, F and H2O; n = 0, 1, 2. Two topological issues have been considered: (1) the pattern of sizes of the M octahedra in the O sheet, M(1) > M(2) > M(3) > M(4) and (2) different topologies of the HOH layer in magnesioastrophyllite and all other structures of the astrophyllite group.

Two new lead oxychloride minerals, hereroite [Pb32(O, ☐)21](AsO4)2((Si,As,V,Mo)O4)2Cl10 and vladkrivovichevite [Pb32O18][Pb4Mn2O]Cl14(BO3)8·2H2O occur in association with asisite, damaraite, kombatite, sahlinite, copper, quartz, barysilite, Mn silicates and a number of Mn oxyhydroxide minerals on a specimen from the Kombat mine in Namibia. The minerals formed as late-stage products of hydrothermal reworking of primary sulfide minerals.

Hereroite is monoclinic, C2/c with a = 23.14(1), b = 22.65(1), c = 12.39(1) Å, β = 102.00(5)°, V = 6351.6(41) Å3 from powder-diffraction data and a = 23.139(4), b = 22.684(4), c = 12.389(2) Å, β = 102.090(3)°, V = 6358.8(18) Å3 from single-crystal data. It is bright orange, with white streak and adamantine lustre. It is brittle with no observed parting or cleavage and has a conchoidal fracture. The calculated density is 8.15 g cm–3. The mean refractive index in air at 589 nm is 2.38. The six strongest reflections in the X-ray powder diffraction pattern [d in Å, (I), (hkl)] are as follows: 2.982(100)(51); 2.795(47)(80); 1.986(24)(8); 1.641(24)(11..); 3.512(23)(61); 3.901(21)(511). Hereroite is named for the Herero people, one of the indigenous tribal groupings in the region where the Kombat mine is located.

Vladkrivovichevite is orthorhombic, Pmmn with a = 12.87(5), b = 27.7(4), c = 11.46(3) Å, V = 4080.1(5) Å3, from powder-diffraction data and a = 12.759(1), b = 27.169(4), c = 11.515(1) Å, V = 3992.0(9) Å3, Z = 2, from single-crystal data. It is pale greenish yellow, with white streak and adamantine lustre. It is brittle with no observed parting or cleavage and has a conchoidal fracture. The calculated density is 7.40 g cm–3. The mean refractive indices in air at 589 nm are 2.30 and 2.34. The six strongest reflections in the X-ray powder diffraction pattern [d in Å, (I), (hkl)] are as follows: 2.860(100)(370); 2.733(84)(073); 3.707(49)(073); 3.068(37)(401); 2.075(32)(473); 1.601(32)(3.14.3). Vladkrivovichevite is named in honour of Prof. Dr Vladimir Gerasimovich Krivovichev (b. 24.04.1946), Head of the Department of Mineralogy, Geological Faculty, St Petersburg State University.

The crystal structures of hereroite and vladkrivovichevite consist of alternating litharge-like O – Pb double layers and chlorine sheets and both are structurally related to other layered lead oxychlorides. In hereroite, tetrahedral AsO4 and (Si, As, V, Mo)O4 groups locate in defects within the O – Pb block, which combines square 'symesite-type' and double-square 'kombatite-type' cavities in its crystal structure. The structure of vladkrivovichevite is based on O – Pb derivative blocks with the interlayer occupied by Cl– anions and oxocentred OPb4Mn2 octahedra whose eight triangular faces are capped by triangular borate anions, BO33–.

Studies of the stability of the oxides schafarzikite, FeSb2O4, and tripuhyite, FeSbO4, have been undertaken to clarify the roles these secondary minerals may have in determining the dispersion of antimony in oxidizing environments. Solubilities were determined at 298.15 K in aqueous HNO3, and these data were used to calculate values of ΔGfϴ at the same temperature. The derived Δ Gfϴ (s, 298.15 K) values for FeSb2O4 and FeSbO4 are – 959.4±4.3 and – 836.8±2.2 kJ mol–1, respectively. These results have been compared with electrochemically derived data, extrapolated from 771–981 K. The present study shows conclusively that although the mobility of Sb above the water table is limited by simple Sb(III) and Sb(V) oxides and stibiconite-group minerals, depending upon the prevailing redox potential and pH, tripuhyite is an important ultimate sink for Sb in the supergene environment. It is highly insoluble even in strongly acidic conditions and its anomalous stability at ambient temperatures causes the common mineral goethite, FeOOH, to react to form tripuhyite at activities of Sb(OH)5(aq) as low as 10–11. The comparatively limited numbers of reported occurrences of tripuhyite in the supergene zone are almost certainly due to the fact that its physical properties, especially colour and habit, are remarkably similar to those of goethite. In contrast, the small number of reported occurrences of schafarzikite can be related to its decomposition to tripuhyite as redox potentials rise at the top of the supergene zone and the fact that it decomposes to sénarmontite, Sb2O3, in acidic conditions, releasing Fe2+ ions into solution. In general, the findings confirm the immobility of Sb in near-surface conditions. Geochemical settings favouring the formation of the above minerals have been assessed using the results of the present study and data from the literature.

We report a single-crystal Fourier-transform infrared (FTIR) study of a sample of pollucite from Maine, USA. Prior to our work, the sample had been characterized by single-crystal X-ray diffraction, neutron diffraction and electron-probe microanalysis. It is cubic Ia3d, with a crystal-chemical formula Na1.93(Cs10.48Rb0.31K0.04)Σ=10.83(Al14.45Si33.97)Σ=48.42O96·3.92H2O, and an H2O content, determined by thermogravimetric analysis, of 1.6 wt.%. The single-crystal FTIR spectrum has a doublet of intense bands at 3670 and 3589 cm–1, which are assigned to the ν3 and ν1 stretching modes of the H2O molecule, respectively. A very intense and sharp peak at 1620 cm–1 is assigned to the ν2 bending vibration. In the near-infrared region there is a relatively intense peak at 5270 cm–1, which is assigned to a combination (ν2 + ν3) mode of H2O, and a weak but well defined doublet at 7118 and 6831 cm–1, which is assigned to the first overtones of the fundamental stretching modes. A relatively weak but extremely sharp peak at 2348 cm–1 shows that the pollucite contains CO2 molecules in structural cavities. Mapping the sample using FTIR indicates that both H2O and CO2 are homogeneously distributed. Secondary ion mass spectrometry yielded an average CO2 content of 0.09±0.02 wt.%. On the basis of this value, we determined the integrated molar absorption coefficient for the spectroscopic analysis of CO2 in pollucite to be εiCO2 = 11,000±3000 l mol–1 cm–2; the linear molar absorption coefficient for the same integration range is εlCO2 = 1600±500 l mol–1 cm–1.

The high-pressure monazite – scheelite structure transition has been observed at P >4.57 GPa in CaSeO4 by synchrotron X-ray powder diffraction. It is a first-order transition with a 4.5% volume change and is severely hindered kinetically. Scheelite-type CaSeO4 remains to a maximum experimental pressure of 42.2 GPa and no (002) reflection, specifically indicative of a subgroup transition to a fergusonite-type structure, is observed. Scheelite-type CaSeO4 remains at ambient conditions, where the tetragonal unit cell has parameters of a = 5.04801(11) c = 11.6644(5) Å and V = 297.21(3) Å3 with Dcalc = 4.090 g cm–3. The diffraction pattern of the recovered material was refined in space group I41/a to Rp = 0.98%, wRp = 1.91%, GoF = 0.59, RFobs = 5.04%, wRFobs = 4.27%. The oxygen is located on the general 16f site at (0.2578(8) 0.3699(14) 0.5755(4)) and shares four identical bonds with Se (4a: ½ ½ ½) at 1.644(5) Å. The Ca (4b: 0, 0, ½) is eight-coordinated via O at 4 × 2.440(6) Å and 4 × 2.504(5) Å. This is further evidence of the dissimilarity of sulfate and selenate at high pressure and temperature conditions and the closer resemblance of the selenates to the orthophosphates, arsenates and vanadates, where this type of transition sequence has been described.

Stable perovskite and metastable post-perovskite NaCoF3 were deformed in pure-shear geometry in a deformation-DIA press with radiographic monitoring of the sample strain. In isothermal experiments where there was no transformation, post-perovskite was found to be 5 times weaker than perovskite. In temperature-ramping experiments where post-perovskite transformed to perovskite during the deformation experiment the initial post-perovskite sample was 5–10 times weaker than perovskite under comparable conditions and their strengths converged during the transformation, being equal on completion of the transformation. These results confirm recent findings which show that postperovskite is weaker than perovskite, regardless of the prior history of the sample.

The low-temperature structural behaviour of natural cancrinite with a formula Na6.59Ca0.93[Si6.12Al5.88O24](CO3)1.04F0.41·2H2O has been investigated by means of in situ single-crystal X-ray diffraction and Raman spectroscopy. High quality structure refinements were obtained at 293, 250, 220, 180, 140, 100 and at 293 K again (at the end of the low-T experiments). The variation in the unit-cell volume as a function of temperature (T) exhibits a continuous trend, without any evident thermoelastic anomaly. The thermal expansion coefficient αV = (1/V)∂V/∂T is 3.8(7) × 10–5 K–1 (between 100 and 293 K). The structure refinement based on intensity data collected at ambient conditions after the low-T experiment confirmed that the low-T induced deformation processes are completely reversible. The extraframework population does not show significant variations down to 100 K. The strong positional disorder of the carbonate groups along the c axis persists within the T range investigated. The structural behaviour of cancrinite at low-T is mainly governed by the continuous framework rearrangement through the ditrigonalization of the six-membered rings which lie in a plane perpendicular to [0001], the contraction of the four-membered ring joint units, the decrease of the ring corrugation in the (0001) plane, and the flattening of the cancrinite cages. A list of the principal Raman active modes in ambient conditions is provided and discussed.

The effect of pressure on the naturally occurring hydroxide-perovskite stottite, FeGe(OH)6, has been studied in situ by micro-Raman spectroscopy to 21 GPa at 300 K. The ambient spectrum contains six OH-stretching bands in the range 3064 3352 cm–1. The presence of six non-equivalent OH groups is inconsistent with space group P42/n. In view of this inconsistency a new ambient structure determination of stottite from Tsumeb was carried out, but this did not allow the clear rejection of P42/n symmetry. However, a successful refinement was also carried out in space group P2/n, a subgroup of P42/n, which allows for six non-equivalent O atoms. The two refinements are of comparable quality and do not allow a choice to be made based purely on the X-ray data. However, taken with the ambient and 150 K Raman spectra, a good case can be made for stottite having P2/n symmetry at ambient conditions. On this basis, the pressure induced spectroscopic changes are interpreted in terms of a reversible phase transition P2/n ↔ P42/n.

Ambient temperature X-ray diffraction data were collected at different pressures from two crystals of β-As4S4, which were made by heating realgar under vacuum at 295ºC for 24 h. These data were used to calculate the unit-cell parameters at pressures up to 6.86 GPa. Above 2.86 GPa, it was only possible to make an approximate measurement of the unit-cell parameters. As expected for a crystal structure that contains molecular units held together by weak van der Waals interactions, β-As4S4 has an exceptionally high compressibility. The compressibility data were fitted to a third-order Birch–Murnaghan equation of state with a resulting volume V0 = 808.2(2) Å3, bulk modulus K0 = 10.9(2) GPa and K' = 8.9(3). These values are extremely close to those reported for the low-temperature polymorph of As4S4, realgar, which contains the same As4S4 cage-molecule. Structural analysis showed that the unit-cell contraction is due mainly to the reduction in intermolecular distances, which causes a substantial reduction in the unit-cell volume (∼21% at 6.86 GPa). The cage-like As4S4 molecules are only slightly affected. No phase transitions occur in the pressure range investigated.

Micro-Raman spectra, collected across the entire pressure range, show that the peaks associated with As–As stretching have the greatest pressure dependence; the S–As–S bending frequency and the As–S stretching have a much weaker dependence or no variation at all as the pressure increases; this is in excellent agreement with the structural data.

Arsenoflorencite-(La), ideally LaAl3(AsO4)2(OH)6, was studied at high pressure by single-crystal X-ray diffractometry. The unit cell was determined at nine pressures up to 7.471(8) GPa; no evidence of a phase transformation was found in this range. The pressure volume data (refined simultaneously) were fitted to a third-order Birch Murnaghan equation of state which gave V0 = 710.71(8) Å3, KT0 = 106(2) GPa and K' = 9.2(9). These values were confirmed independently from an FE–fE plot. The crystal structure was refined at 1.596, 3.622, 5.749 and 7.471 GPa, the first time this has been done for a member the alunite supergroup. The compressibility of arsenoflorencite-(La) is strongly anisotropic, with βc > βa. The main compression mechanism was found to be governed by the internal angle O3 La O2 of the La polyhedron, where the O2 and O3 atoms move toward the c axis during compression.

A single-crystal X-ray diffraction study of a sample of natural gedrite from North Carolina, USA, with the crystal-chemical formula ANa0.47B(Na0.03Mg0.97Fe0.942+Mn0.02Ca0.04)C(Mg3.52Fe0.282+Al1.15Ti0.054+)T(Si6.31Al1.69)O22W(OH)2, up to a maximum pressure of 7 GPa, revealed the following bulk and axial moduli and their pressure derivatives: K0T = 91.2(6) GPa [K0T' = 6.3(2)]; K0T(a) = 60.5(6) GPa [K0T(a)' = 6.1(2)]; K0T(b) = 122.8(2.6) GPa [K0T(b)' = 5.7(8)]; K0T (c) = 119.7(1.5) GPa [K0T(c)' = 5.1(5)]. Gedrite has a much higher bulk modulus than anthophyllite (66 GPa) and proto-amphibole (64 GPa). All of the three axial moduli of gedrite are higher than those of these two other orthoamphiboles. The greater stiffness of gedrite along [100] is due to its high ANa content, which is almost zero in anthophyllite and proto-amphibole. The much greater stiffness parallel to the (100) plane of gedrite compared with the two other amphiboles is probably due to its high CAl content. A comparison is made with published data available for orthorhombic B(Mg, Mn, Fe) and monoclinic BCa amphiboles to identify correlations between crystal-chemistry and compressibility in amphiboles.

The thermal stability of the synthetic iodine analogue of vanadinite, Pb5(VO4)3I, in air has been investigated by in situ high temperature X-ray powder diffraction between 300 and 1070 K. Rietveld refinement of phase fractions shows that breakdown to lead orthovanadate, Pb3(VO4)2, begins at temperatures above 540 K, with complete loss of iodine above 680 K. More than 50 K below the onset of breakdown, the unit-cell parameters of Pb5(VO4)3I show anomalous contraction in the crystallographic x–y plane (reduction of a) which we associate with movement of iodine within the [0001] channels of the structure. The implications of these results for immobilization of 129I in potential apatite-related crystalline radioactive waste forms are discussed.

Potassium-rich mafic dykes and lavas from the Highwood Mountains Igneous Province, USA were studied by electron-microprobe and bulk-rock analysis. For the mafic phonolites, compositional trends for olivine and augite phenocrysts and groundmass biotite, alkali feldspar and titanomagnetites are presented and substitution mechanisms discussed. Phenocrysts of biotite and augite in the minettes are also characterized, together with groundmass alkali feldspar and titanomagnetite. The alkali feldspars and biotites are commonly enriched in Ba. Olivine, clinopyroxene and biotite phenocrysts are generally quite magnesium-rich, which is consistent with the primitive natures of the least evolved rocks.

Bulk-rock major-element compositions are combined with modal and microprobe data for the principal phenocrysts to calculate model residual liquid compositions for mafic phonolites, minettes and a syenitic rock. On the basis of phase-equilibria, it is suggested that the main controls of differentiation are polybaric involving crystallization during transport of primary magmas from the mantle for the minettes, and low-pressure differentiation for the mafic phonolites. Whereas magma mixing might have contributed to petrogenesis, many of the disequilibrium features exhibited by clinopyroxene and biotite phenocrysts can also be attributed to pre-existing phenocrysts undergoing decompression melting during magma uprise from its mantle source, followed by rapid crystal growth and episodic volatile loss in sub-volcanic magma chambers.