Biotite oxidation

C2 Biotite Oxidation and hydrolysis of iron Fe(II-IlI) + H2O + 02- Fe00H + H+ (Primary Fe-Al-silicate) -1- H 4- H2O—> (secondary Al-silicate) + (Fe-oxide/ hydroxide) + 4- H4Si04 Very slow incongruent dissolution... 

Kleiman LE, Saragovi C, Puglisi C, de Kanter FL (1992) Biotite oxidation processes in ash-flow tuffs (Mendoza, Argentina) A Mossbauer spectroscopy and chemical study. Chem Geol 97 251-264 Kliem W, Lehmaim G (1979) A reassigmnent of the optical absorption bands in biotites. Phys Chem Minerals 4 65-75... 

Treasure Lake Group, overlain by felsic ignimbrite sheets of the Faber Group (Goad et al. 2000). Ore minerals mainly consists of Fe-, As-, Co- and Cu-sulphides, native Au and Bi. Intense, pervasive, polyphase iron-oxide (magnetite-dominant)-hornblende-biotite-tourmaline-K-feldspar-carbonate replacive alteration occurs in the upper metasedimentary sequence below the volcanic-sedimentary unconformity... 

The flotation feed at the Valkoomesky plant contains tourmaline (18%), biotite (13%), muscovite (17%), limonite (2%) and sulphides (5%). Tin assays in the flotation feed averaged about 0.5% Sn, of which the bulk was contained in the -48 to +12 pm fractions. Flotation of tin was carried out with sea water using oxidized petroleum solution in kerosene (1 2 ratio). 

Figure 17. Comparison of 5 Fe values for bulk igneous rocks (O), suspended river loads ( ) and individual minerals from volcanic rocks (right panel). Left panel shows the fraction of Fe that exists as oxides (data for igneous rocks arbitrarily plotted as 0.02). Right panel shows 6 Fe values for coexisting magnetite ( ), olivine (+), hornblende (O), and biotite (O) in four volcanic rocks. Data from Canfield (1997), Beard et al. (2003a), and Beard and Johnson (2004).

Biotite and magnetite are also usually present and visible in hand specimen, muscovite may be present, and more rarely other oxides may be seen. Field estimates of modes ranged from 20-35 vol.% quartz, 15-35 vol.% plagioclase, 30-50 vol.% potassium feldspar, and 1-10 vol.% biotite. Accessory minerals include magnetite, muscovite, monazite, xenotime, zircon, apatite, epidote, ilmenite, titanite, allanite, molybdenite, and galena. The major U and Th minerals are uraninite and uranothorite. 

A few years ago it was pointed out to me by Sterling Hendricks that ordinary black mica, biotite, which has an intensely black color, owes this color to the presence of iron in both the ferrous and ferric oxidation state. Black tourmalines also usually contain both ferrous and ferric iron. Another intensely black mineral, with black streak, is ilvaite, with composition Ca(Fe )aFe+++(Si04)20H. 

The autoradiographs of the rock and mineral thin sections (Figures 4 to 6) also confirm the importance of iron oxides although biotite.(K(Mg,Fe Si AlCLQ(0H) ) and hornblende ((Na,Ca2)(Mg,Fe )(Al,Fe )(Si AlO OH ) contain ferrous iron, sorption appears to take place solely on the small opaque (iron-oxide) inclusions. In the case of biotite, these oxides are located between the basal planes, and are randomly distributed in the hornblende. Similar distributions are observed for olivine, pyroxene, and epidote. The results for pyroxene further confirm the low sorption results obtained with gabbro, where it is one of the major minerals. 

The autoradiographs also show some sorption on the granite obtained from the Eye-Dashwa Lakes pluton (Figure 6). Even though the iron concentration is low, hydrothermal alteration of the granite has resulted in the infilling of the minute fractures in the microcline feldspar with iron oxides, which show technetium sorption, while there is no sorption on the biotite crystals. 

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