Kornerupine, (□,Fe,Mg)(Mg,Fe,Al)
9(Si,Al,B)
5 (O,OH,F)
22, has been reported with talc in rocks from six localities worldwide, but only at Chilapila Hill in the Lufilian Arc, Zambia
do textural relationships imply that kornerupine (Krn) equilibrated with talc (Tlc) during a prograde metamorphic event at
T≈ 640 °C,
P≈ 13 kbar; a prograde Krn + Tlc assemblage has also been reported from Mautia Hill, Tanzania (
P ≤ 13 kbar). In order to estimate possible constraints on the stability range for the kornerupine + talc paragenesis in nature,
we constructed a
P-
T diagram in the model system MgO-Al
2O
3-SiO
2-H
2O (MASH) for seven phases quartz (Qtz), B-free kornerupine sensu stricto, anthophyllite (Ath), chlorite (Chl), cordierite
(Crd), kyanite (Ky), and talc. The minimum pressure for Krn + Tlc + Ky stability in MASH is close to that for Ky + Tlc stability,
i.e., 6–8 kbar, at
T≤ 780 °C. However, in the natural system, B
2O
3 and Na
2O are major constituents in Krn and orthoamphibole (Oam), respectively, and dravitic tourmaline (Tur) is widespread. The critical
assemblage alternative to Krn + Tlc in nature is Tur + Oam. The upper pressure limit of Tur + Ath is determined by the upper
pressure for anthophyllite: 7.7–10.5 kbar at 682–794 °C in the MgO-SiO
2-H
2O system (Chernosky et al. 1985, Am Mineral 70:223–236), and is undoubtedly higher in the presence of Na
2O, CaO, and Al
2O
3. At three of the six localities, talc is a retrograde phase; nonetheless, it possibly equilibrated with kornerupine on the
retrograde path or during a later metamorphic event at
P-
T conditions appropriate for Ky + Tlc. At the sixth locality (Mulvoj, southwestern Pamir Mountains, Tajikistan), Krn is found
in the same thin section as talc and kyanite and all three minerals formed during a prograde metamorphic event at
T≥ 650 °C,
P near 7 kbar. However, Krn is restricted to a lens 4 to 6 mm thick of phlogopite + anthophyllite + Tur and it does not touch
either talc or kyanite. A reaction relating the Mulvoj and Chilapila Hill (Krn + Tlc + Ky + Qtz + Tur) parageneses is calculated
from compositions in the Mulvoj rock to be 0.40Tur + 2.55Ath + 1.33H
2O + 0.27F = Krn + 2.16Tlc + 0.36B
2O
3 + 0.02Rutile + 0.19Na
2O + 0.17CaO. Given the difference in metamorphic pressures estimated for Mulvoj and Chilapila Hill, Krn + Tlc is inferred
to be favored by increasing pressure as well as by low Na
2O and CaO contents. Some FeO, F, Fe
2O
3, and BeO are present in measurable amounts in at least one of the phases in the Mulvoj and Chilapila Hill whiteschists (e.g.,
Krn contains 0.24–0.67 wt% BeO), but the effect of these constituents is subordinate to that of Na
2O, CaO and B
2O
3. The Krn + Tlc could be a more important assemblage in B-bearing whiteschists than has been reported to date, particularly
at pressures where orthoamphibole is no longer stable.
Received: 21 April 1997 / Accepted: 13 October 1997
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