Peter B. Leavens

Reidite: An impact-produced high-pressure polymorph of zircon found in marine sediments

Abstract Reidite is a high-pressure polymorph of zircon with the scheelite structure. It has been found in an upper Eocene impact ejecta layer in marine sediments on the upper continental slope off New Jersey and on Barbados. Reidite occurs (epitaxially oriented) in shock-metamorphosed zircons. It is associated with impact glass (tektites), shocked quartz and feldspar with multiple sets of planar deformation features, coesite, and trace amounts of stishovite. This phase was first produced in high-pressure laboratory experiments in 1969 and has also been produced in shock recovery experiments. Reidite is brittle with an irregular fracture, a hardness of 7.5, a calculated density of 5.2 g/cm3, a white streak, adamantine luster, and it does not fluoresce. In index oil in transmitted light, shocked zircon grains consisting almost entirely of reidite are transparent. Pleochroism was not observed. Reidite appears to have parallel extinction and is length slow. The maximum birefringence is roughly 0.015. Reidite appears to be uniaxial positive. It is tetragonal, space group I41/a, a = 4.738 (1) Å, c = 10.506 (2) Å, V = 235.84(2) Å3. Previous shock-loading experiments on zircons indicate that the transition to reidite starts at about 30 GPa and is completed around 53 GPa. Reidite should be a useful indicator of peak pressure in shock metamorphosed rocks. Reidite is named after Alan Reid who first produced this phase in the laboratory.

Chemical Variation in Vesuvianite

SummaryMicroprobeanalyses of 44 vesuvianite specimens of different color, morphology, and occurrence indicate the chemical variation of the species. Vesuvianites can be divided into four types, based on chemistry and color; most vesuvianites can be assigned to one of these types with confidence. Type 1 vesuvianites contain ≈ 2 atoms of Mg, <0.25 atoms of Ti, and variable Fe, apparently trivalent, in a formula based on 50 non-H cations. They are dark to pale green, white, or pink. Type 2 vesuvianites contain > 2.5 atoms of Mg, variable (divalent?) Fe, and <0.5 atoms of Ti. In color they are yellow, yellow-brown, or yellow-green. Type 3 vesuvianites contain divalent Fe, 0.5 to 1.5 atoms of Ti and close to 18 atoms of Si (full occupancy of tetrahedral sites). They are yellow, brown, red-brown, or black. Type 4 vesuvianites are manganoan or cuprian; the studied samples are from Pajsberg, Sweden, Franklin, New Jersey, and Telemark, Norway. In color they are red-brown, purple, blue (“cyprine”), or green. Iron and much of the Mn is apparently trivalent.Unlike garnets, which they resemble structurally, vesuvianites contain almost exclusively Ca in the eight-fold sites in the structure. Silicon occupies 95% or more of the tetrahedral sites. Aluminum fills the smaller octahedral site, A. Chemical variation occurs predominantly in the more open, octahedral, general or G site and in the five-coordinated B site. Simple substitutions in G include Fe3+ or Mn3+ for Al3+, and Fe2+, Mn2+, or Zn2+ for Mg2+. Coupled substitutions include TiO = AIOH, MgTi = AlAl and AlAl = MgSi. The B site may contain Cue2+, Fe2+, Fe3+, or A13+. Changes in the amounts of O and OH in two different positions give a range of anion charge from 146 (0670H12) to about 148 (0690H10).ZusammenfassungMikrosondenanalysen von 44 Vesuvian-Proben, die in Farbe, Morphologie and Vorkommen unterschiedlich sind, zeigen die chemische Variation der Spezies. Aufgrund von Chemismus und Farbe können Vesuviane in vier Typen eingeteilt werden; die meisten Vesuviane konnen einem dieser Typen klar zugeordnet werden. Vesuviane vom Typ 1 enthalten in einer Formel, die sich auf 50 nicht-H Kationen bezieht, ≈ 2 Mg-Atome, < 0,25 Ti-Atome and variables, anscheinend dreiwertiges Fe. Sie sind dunkel- bis blaßgrün, weiß oder rosa. Vesuviane vom Typ 2 enthalten > 2,5 Mg-Atome, variables (zweiwertiges?) Fe and < 0,5 Ti-Atome. Sie sind gelb, gelbbraun oder gelbgrün gefärbt. Vesuviane vom Typ 3 enthalten zweiwertiges Fe, 0,5 bis 1,5 Ti-Atome and beinahe 18 Si-Atome (völlige Besetzung der Tetraederpositionen). Sie sind gelb, braun, rotbraun oder schwarz. Vesuviane vom Typ 4 sind Mangan- oder Kupfer-haltig; die untersuchten Proben sind von Pajsberg, Schweden, Franklin, New Jersey, and Telemark, Norwegen. Sie sind rotbraun, purpur, blau (“Cyprin”) oder grün gefarbt. Eisen und ein großer Teil des Mangns sind anscheinend dreiwertig.Im Unterschied zu den strukturell ähnlichen Granaten enthalten die Vesuviane auf den acht-koordinierten Positionen der Struktur fast nur Ca. Silizium besetzt 95% oder mehr der Tetraederpositionen. Aluminium füllt die kleinere Oktaederposition A. Chemische Variabilität tritt hauptsächlich in der offeneren, oktaedrischen, allgemeinen oder G Position and in der fiinf-koordinierten B Position auf. Einfache Substitutionen auf G umfassen Fe3+ und Mn3+ fur Al3+, sowie Fe2+, Mn2+ and Zn2+ für Mgt2+. Gekoppelte Substitutionen beinhalten TiO = AlOH, MgTi = AlAl and AlAl = MgSi. Die B Position kann Cu2+, Fe2+, Fe3+ and A13+ enthalten. Wechsel in den Beträgen an O und OH auf zwei unterschiedlichen Positionen gibt für die Anionenladung einen Bereich von 146 (O67OH12) bis etwa 148 (O69OH10).[/p]