According to Stoiber and Rose (1973, p. 514), "incrustations at volcanic gas vents offer a convenient approach to studying the gases themselves. In particular, some minor constituents of the gas may be only observable in the associated incrustations. Also, changes in the minerals found at a fumarole reflect changes in the gases.
Though a large number of mineral phases have been described at fumaroles, only nine minerals can be considered as 'very common' and perhaps eight additional ones as 'common.' The orderly arrangement of the fumarolic minerals in zones attests to a common causative process.
The study of many high-temperature fumarolic vents and their associated incrustations has led to the conclusion that in spite of the large number of incrusation minerals found, all can be fairly well explained by a fumarolic gas consisting essentially of H2O, SO2, HCl, HF and possibly CO2 gases, and much smaller concentrations of volatile (and aerosol?) cations (i.e. Na, K, Zn, Cu, V, Mo) passing through an environment of rapidly changing temperature and pO2 and in contact with volcano wallrock."
Dr. John Hughes and others have done considerable work on the fumarolic sublimates found at Izalco Volcano. An interesting fact to notice is the abundance of Vanadium found in these sublimates. It has not, as of yet, been determined why Vanadium is so common. Below is a listing and brief description of the sublimates
ZIESITE: Cu2V2O7
Hughes and Birnie (1980) state that "Ziesite, Cu2V2O7, a new copper vanadate maineral, occurs as anhedral black crystals in the oxide zone of a fumarole at Izalco volcano, El Salvador, Central America. The mineral is associated with stoiberite and other unnamed copper vanadates. It is monoclinic, space group A2/a, Z=2, and D(calc)=3.87 g/cc. It has a metallic luster and a red-brown streak. No cleavages were observed and the mineral shows no fluorescence in short- or long-wave UV radiation. The mineral is opaque and exhibits class C reflectivity. The phase relations between ziesite and other copper vanadates indicate a tempurature of formation between about 760 and 770 degrees C. The mineral is named in honor of Dr. Emanuel G. Zies."
HOWARDEVANSITE: NaCuFe2(VO4)3
Hughes, Drexler, Campana, and Malinconico (1988) write that "Howardevansite, NaCuFe2(VO4)3, has been discovered in the summit crater fumaroles of Izalco volcano, El Salvador. The mineral, which occurs with lyonsite and thenardite, formed as a sublimate from the volcanic gases. Howardevansite occurs as euhedral, black tabular crystals up to 80 microns in greatest dimension. The crystals are triclinic. The atomic arrangement of the mineral has been determined and refined to R=0.026, R(w)=0.036. The structure is formed of ribbons in the (210) plane, consisting of FeO6 octahedra, CuO5 trigonal bipyramids, and VO4 tetrahedra. The ribbons terminate in chains of NaO6 and NaO7 polyhedra that extend infinitely parallel to c at 0, 1/2,z.
Howardevansite is opaque and has a red-brown streak and metallic luster. Z=2 and D(calc)=3.814 g/cm3 for the formula NaCuFe2(VO4)3. In reflected light in air, the mineral is medium gray and exhibits moderate anisotropy. Reflectance values measured on a polished face are (wavelength, min,max) 481nm, 15.5, 20.1%; 547nm, 15.0, 18.4%; 591nm, 14.1, 17.2%, and 644nm, 13.5, 17.1%.
The mineral is named in honor of Dr. Howard Evans, Jr., of the U.S. Geological Survey."
BLOSSITE: Cu2V2O7
Robinson, Hughes, and Malinconico (1987) report "blossite, Cu2V2O7, occurs as a fumarolic sublimate in the "Y" fumarole of Iazlco volcano, El Salvador. The mineral is the natural analogue of the previously described synthetic phase, and is the low-tempurature polymorph of ziesite, Cu2V2O7, which was discovered at the same site. The crystals are orthorhombic, Fdd2. The crystal structure was refined to R=0.047. The atomic arrangement consists of (100) planes of (V2O7)-4 anionic groups; adjacent planes along [100] are linked by Cu +2 in fivefold coordination. On the basis of previously determined atomic arrangements, the blossite-ziesite phase transition is of the non-nearest-neighbor reconstructive type.
Blossite occurs as an equant, black anhedral crystal up to 150 microns in greatest dimension. The mineral has a metallic luster and a red-brown streak. The calculated density (Z=8) is 4.051 g/cm3. Blossite is opaque, and white in blue -filtered white light in air, with weak to moderate bireflectance in shades of creamy-white, and a moderate anisotropy, from gray to creamy brown-gray. Minimum and maximum reflectance values (in air) are 481nm, 14.6, 15.3%; 547nm, 15.4, 16.6%; 591nm, 14.8, 16.7%, and 644nm, 14.5, 15.7%. The mineral is named in honor of Dr. F. Donald Bloss, past-President of the Mineralogical Society of America."
LYONSITE: Cu3Fe4(VO4)6
Hughes, Starkey, Malinconico, and Malinconico (1987) state that "Lyonsite, Cu3Fe4(VO4)6, a new iron-copper vanadate mineral, has been discovered in the summit crater fumaroles of Izalco volcano, El Salvador. The mineral, which occurs with thenardite and a Na-Cu-Fe vanadate, formed as a sublimate product from the volcanic gases. Lyonsite occurs as euhedral, black lathlike crystals up to 230 microns in greatest dimension.
Lyonsite is orthorhombic. The atomic arrangement of the mineral has been determined and refined to R=0.028, R(w)=0.031. The structure is based on a pseudohexagonal close-packed oxygen array. The fundamental building block consists of six isolated VO4 tetrahedra that are linked, through corner sharing, to a central octahedron that is half-occupied by Cu +2 atoms. These "pinwheel" building blocks are connected on four sides by FeO6 octahedra and on the remaining two sides by square-planar CuO4 groups. The fundamental building block is isostructural with the silicate pinwheels in dumortierite.
Lyonsite is opaque and has a dark gray streak and metallic luster. Z=2 and D(calc)=4.215 g/cm3. Lyonsite displays a good {001} cleavage. The mineral is creamy white in reflected light in air. Reflectance values measured on (010) are ( wavelenght, min, max) 481nm, 17.5, 23.1%; 547nm, 16.6, 22.3%; 591nm, 14.7,20.7%, and 644nm, 14.4, 18.8%.
The mineral is named after Dr. John B. Lyons of Dartmouth College. Milligram amounts of the phase exist and are on deposit at the NMNH, Smithsonian Institution."
MCBIRNEYITE: Cu3(VO4)2
Hughes, Christian, Finger, and Malinconico (1987) write "mcbirneyite, Cu3(VO4)2, a new copper vanadate mineral, has been discovered in the summit crater fumaroles of Izalco volcano, El Salvador. The mineral formed as a sublimate from the volcanic gases and is associated with stoiberite, fingerite, ziesite, euchlorine and thenardite. Mcbirneyite is the naturally occurring analog of synthetic Cu3V2O8.
Mcbirneyite is triclinic. The five most intense lines in the diffraction pattern are 4.01, 3.12, 2.82, 2.641, and 2.428 angstoms. The crystal structure of the mineral has been determined; the independent structure solution demonstrates that mcbirneyite is isostructural with stranskiite, CuZn2(AsO4)2, and a synthetic Cu phosphate, Cu3(PO4)2. The structure is constructed of (110) layers of Cu atoms in square-planar and square-based pyramidal coordination with oxygen; adjacent layers are linked by isolated VO4 tetrahedra.
Mcbirneyite is black with a metallic luster. The calculated density (Z=1) is 4.50 g cm-3. The mineral is opaque to transmitted light; in reflected light in aur the mineral is medium-dark gray and nonpleochroic, with reflectance values as follows: 470nm, 18.5%; 546nm, 17.5%; 589nm, 18.7%; 650nm, 20.6%.
The mineral is named in honor of Alexander R. McBirney, Editor-in-Chief, Journal of Volcanology and Geothermal Research."
FINGERITE: Cu11O2(VO4)6
Hughes and Hadidiacos (1985) report that "fingerite, Cu11O2(VO4)6, a new copper vanadate mineral, has been discovered in the summit crater fumaroles of Izalco volcano, El Salvador, Central America. The mineral, which occurs with thenardite and euchlorine, forms as a sublimate product from the volcanic gases. Fingerite occurs as euhedral, subhedral and anhedral crystals up to 150 microns in greatest dimension.
Fingerite is triclinic. The five most intense lines on the Gandolfi pattern are 6.48, 3.894, 3.042, 2.822, and 2.025.
Fingerite is black with a metallic luster and opaque. The calculated density offingerite is 4.776 g/cc, Z=1. In reflected light in air, fingerite is medium gray, non-pleochroic and exhibits 2 extinctions per complete rotation of the stage. The reflectance of randomly oriented grains in blue-filtered white light in air is 21 +/- 2%.
Milligram amounts of the mineral exist. Type specimens are on deposit at the National Museum of Natural History, Smithsonian Institution and the Geophysical Laboratory, Carnegie Institution of Washington. The mineral is named in honor of Dr. Larry W. Finger, Geophysical Laboratory."
SHCHERBINAITE: V2O5
Hughes and Finger (1983) write that "shcherbinaite, naturally occurring V2O5, has been discovered in the high-tempurature fumaroles of Izalco volcano, El Salvador, and Bezymyanny volcano, Kamchatka, USSR. The Izalco mineral occurs as euhedral golden-yellow needles and laths up to 1 cm in length. The crystals, which formed as sublimates from volcanic gases, are found coating basaltic breccia fragments in the summit crater fumaroles. The mineral is associated with bannermanite."
BANNERMANITE: NaxV6O15
Hughes and Finger (1983) reported in 1983 that "bannermanite, NaxV6O15, is a natural vanadium oxide bronze compound, occurs as black subhedral to euhedral crystals in the oxide zone of a fumarole at Izalco volcano, El Salvador, Central America. The mineral is associated with shcherbinaite, stoibrite, and ziesite. Bannermanite is monoclinic, space group C2/m, D(obs)=3.5 +/-0.2 g/cm3, and D(calc) =3.55 g/cm3. The crystal structure of bannermanite has been refined to wR=0.074 from the vanadium oxide bronze structure model. The structure consists of a clinolattice of VO5 square-based pyramids and VO6 octahedra. This array of V-O polyhedra froms rectangular tunnels in which Na and K ions occur. The vanadium ions are in two charge states, V +4, and V +5, with the number of vanadium ions in the reduced state equal to the number of alkali ions in the tunnel sites; thus, charge balance is maintained.
Bannermanite transmits light on thin edges, with a mean index of refraction of 2.2, on the basis of the rule of Gladstone and Dale. The mineral has two cleavages and a dark gray-black streak. It is named in honor of the late Dr. Harold M. Bannerman."
STOIBERITE: Cu5V2O10
Birnie and Hughes (1979), in 1979 wrote that "stoiberite, Cu5V2O10, a new coppervanadate mineral, occurs as black platy crystals in the oxide zone of a fumarolein Izalco volcano, El Salvador, Central America. It is monoclinic, space group P2/n. The mineral is opaque with a medium reflectivity less than that of galena. It's calculated density is 4.96 g/cc. It is named after Dr. Richard E. Stoiber."