High-silica, topaz-bearing rhyolites of Cenozoic age are widely distributed across the western United States and Mexico. Topaz rhyolites are characteristically enriched in fluorine (>0.2 wt%) and contain topaz crystallize during post-magmatic vapor-phase alteration. In the United States, their ages span much of the Cenozoic Era (50 to 0.06 Ma). Their emplacement followed or was contemporaneous with calc-alkaline and basaltic magmatism in the Basin and Range province, along the Rio Grande rift, and in Montana, and coincided with episodes of extensional tectonism in these regions.
Aenigmatite-group compositions conform to the general formulaA 2B6T6O2p, where t8lA cations are Na and Ca, t6J3 - tz*, Fe3*, Mg, Al, Cr, Ti, and Sb5r, and talz are Si, Al, B, and Be in named end-members. These include the sodic minerals aenigmatite, krinovite, and wilkinsonite, and the calcic minerals rhdnite, serendibite, dorrite, welshite, and h6gtuvaite (a new Be-bearing end-member: Grauch et al. 1994). Graphical rcpresentafion of the compositional relations among these phases is made possible by the vector method, which has recently been applied to a number of other mineral groups. The fundamental principle is that a chemical displacement such as CaAl(NaSi)-1, having both direction and magtitude, can be thought of as a vector of unit length and arbitrary orientation. Compositional relations are simplified by condensing on vectors of isovalent substitution, such as Fe3*Al-, and Fe2+Mg-r. In vector diagrams, bound vectors are attached to a point and show the relations among basis and derived vectors. The conespondingfee vectors generate the accessible compositional range (e.g., triangle, quadrilateral, or pentagon), starting from an initial formula or composition (additive component of J.B. Thompson, Jr.). When this approach is used on the aenigmatite group, the compositions of the main minerals are derived ftom that of aenigmatite by application of the basis vecton CaAl(NaSi)-1, Al2(MgTi)-,, and Al2(MgSi)-r. Hogtuvaite is derived by BeSiAl-2 acting on rhtlnite, and welshite by AlSbBe(MgTiSi)_1 acting on hpgtuvaite. Other theoretical end-members can be generated by the vector method; in the aenigmatite group, many are probably unstable with regmd to a mixture of clinopytoxene plus olivine or spinel, especially if they lack Ti or excess A1.
Analyse par microscopie electronique a transmission et microscopie electronique analytique. Existence de chlorite, sous forme de fine lamelle (epaisseur 0,1 a 0,3 μm) dans les cristaux d'andalusite, de composition Al 4,32 Fe 0,07 (Si 2,83 Al 1,17 )O 10 (OH) 8 . Des rapports molaires Al/Si similaires pour l'andalusite et la donbassite suggerent que la degradation de l'andalusite en donbassite s'effectue par l'intermediaire d'une reaction de deshydratation
Wolframite-bearing quartz veins flanked by greisen alteration occur at and near the Black Pearl mine, Yavapai County, Arizona. The veins are genetically related to a small albitite stock, and cut a series of Proterozoic metasedimentary and intrusive rocks. The largest vein, the only one mined, is located at the apex of the stock. Field relations imply that this stock is a late-stage differentiate of the 1.4-Ga anorogenic Lawler Peak batholith, which crops out about 3 km to the south. Other, similar, albitites occur locally. Sharp contacts, relatively unaltered xenoliths, and uniform mineralogy indicate that the albitites are of igneous origin...
'%3e%3cpath fill='%23001489' d='M0 0v6h9V0z'/%3e%3cpath fill='%23e03c31' d='M0 0v3h9V0z'/%3e%3cg stroke='%23fff' stroke-width='2'%3e%3cpath id='d' d='m0 0 4.5 3L0 6m4.5-3H9'/%3e%3cuse xlink:href='%23b' stroke='%23ffb81c' clip-path='url(%23c)'/%3e%3c/g%3e%3cuse xlink:href='%23d' fill='none' stroke='%23007749' stroke-width='1.2'/%3e%3c/g%3e%3c/svg%3e)