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Scapolite and analcime are uncommon alteration minerals in syenite. Here we describe a syenite body in a fault zone that hosts these minerals and we elucidate the unusual chemical conditions that led to their formation. The Clarke Head syenite is part of a regional late Devonian–early Carboniferous back-arc A-type granitoid suite in the Cobequid Highlands of Nova Scotia, Canadian Appalachians. The syenite contains magmatic rutile and analcime. Magmatic K-feldspar has been largely replaced by scapolite of marialite composition, which shows a temporal transition from more Cl rich to more carbonate rich varieties. Scapolite veins cut both the syenite and associated gabbro and diorite. Analcime I occurs as a late stage interstitial magmatic mineral. Analcime II forms by alteration of albite lamellae in perthite replaced by scapolite, analcime III vugs that cross-cut scapolite and analcime IV occurs in late veins cross-cutting earlier analcime phases and scapolite. The regional A-type granite plutonism was Na- and halogen rich, with widespread late-magmatic albitization and F-related mobility of REE minerals. However, only at Clarke Head is there extreme Na- and Cl-rich metasomatism, indicated by scapolite and Cl-rich hastingsite (with >1 wt% Cl content). Cl-rich fluids were derived from regional halite evaporites at faulted basin margins. The main circulating fluids were likely of magmatic origin, based on the δ18O of both replacement and vein scapolite. As the NaCl supply was reduced, scapolite became more Ca-rich. Halogen-rich fluids also mobilized Ti and Zr, which were reprecipitated in hydrothermal rutile and zircon as the halogen activity diminished. Scapolite replacement of K-feldspar released large amounts of K+ into the circulating fluids, which converted ferromagnesian silicates in fault zones to secondary biotite.

Compositional trends of natural samples of scapolite extend toward the ideal (anhydrous) end-members marialite (Ma) Na4Al3Si9O24Cl and meionite (Me) Ca4Al6Si6O24CO3. Intermediate members do not follow the plagioclase substitution, although Si contents closely correlate with Me% [calculated as 100{Σ(divalent cations)}/4]. The solid solution may be explained by substitutional interactions among the M, T and A sites, as constrained by net charge-balance and maximum site-occupancy requirements of the structural formula M4T12O24A. Two changes in compositional trends located at [(Na3.4Ca0.6)(Al3.6Si8.4)O24]+1 and [(Na1.4Ca2.6)(Al4.7Si7.3)O24]+1.9 divide the series into three portions. Nomenclature consistent with these divisions is marialite (Me 0–15), calcian marialite (15 < Me ≤ 50), sodian meionite (50 < Me < 65) and meionite (Me 65–100). A large number of scapolite occurrences have Si values near 7.3 apfu, corresponding to the 14/m-P42/n symmetry transition and to the stabilization of scapolite in a Cl-poor environment. Trends of Cl with Si or (Na + K) apfu indicate complex substitutional mechanisms, and should not be used to suggest that “mizzonite” or “dipyre” are end-members. Calculation of excess positive charge from measurement of M and T cations indicates that scapolite typically has the maximum content of Cl expected by charge-balance. Anion substitution is complicated by substition of H as hydroxyl or bicarbonate-bisulfate anions. The calculation CO2 = 1−ClFS may apply only to anhydrous scapolites which are rare, otherwise it over-estimates CO2 because it ignores the role of H. CO2 content calculated by charge-balance equations gives better agreement with constraints of the known substitutional mechanism. More H is present than is required by stoichiometry, and molecular H2O may be present in channels along the c-axis.

The heat capacities of five mineral samples from the scapolite solid-solution series, Na 1 Al 3 Si 9 O 24 Cl (marialite) to Ca 4 Al 6 Si 6 O 24 CO 3 (meionite), were measured by the adiabatic method from T = 8 K to T = 350 K and by the differential scanning calorimetry (d.s.c.) method from T = 300 K to T = 1000 K. The meionite (Me) content in per cent {Me = 100 Ca*/(Ca* + Na*)} (where the asterisk indicates that possible substituents are included) and molar heat capacity ( C p.m / R ) at T = 298.15 K for each sample is: Me 28 , 82.07; Me 44 , 82.09; Me 55 , 83.95; Me 69 , 85.80; Me 88 , 84.54. The standard molar entropies, { S m o (298.15 K) - S m o (0K)} R ?1 ( R = 8.31451 J·K -1 ·mol -1 ), at T = 298.15 K for the respective compositions are : 85.05 ± 0.26, 83.78 ± 0.50, 85.22 ± 0.24, 85.76 ± 0.21, and 84.17 ± 0.59. The calculated standard molar entropies (as above) at T = 298.15 K for the end-members marialite and meionite, and for an intermediate composition (mizzonite=Me 75 ) are 84.85, 83.94 and 86.15, respectively. Values of the coefficients in the equation C p.m / R = a + bT + cT 2 + dT ?1/2 + eT -2 (valid from T = 300 K to T = 1000 K) are: (Me x , a , b /K, c /K 2 , d /K ?1/2 , e /K -2 ) Me 88 , 315.580, −0.0795676, 1.52825·10 -5 , −3954·83, 1808460; Me 69 , 261.285, −0.0415017, 8.73053·10 -7 , −3028.28, 1083666; Me 55 , 232.236, −0.0352222, 6.49875·10 -6 , 2505.99, 601750; Me 44 , 276.696, −0.0756614, 2.39722·10 -5 , -3210.40, 1044363; Me 28 , 149.917, 0.0229399, -1.23180·10 -5 , 1208.87; -318470. Smoothed thermodynamic functions for the five samples are also presented. The enthalpies of solution for five natural scapolites were measured in 2PbO·B 2 O 3 melts at T = 973 K by Calvet-type calorimetry.

Scapolite solid solution has been synthesized at 750°C and 4 kbar and is stable relative to plagioclase + calcite + halite over the range of plagioclase compositions from Ab 85 An 15 to Ab 70 An 30 , although albite + halite is stable relative to marialite, Na 4 Al 3 Si 9 O 24 Cl, and anorthite + calcite is stable relative to meionite, Ca 4 Al 6 Si 6 O 24 CO 3 . A chloride-free scapolite, mizzonite, has been synthesized at the approximate composition NaCa 3 Al 5 Si 7 O 24 CO 3 (Ab. 2An. CaCO 3 ). In the absence of chloride, a three-phase invariant assemblage, sodic plagioclase (~Ab 60 An 40 ) + scapolite + calcite is stable relative to plagioclase + calcite over the approximate range of plagioclase composition Ab 60 An 40 -Ab 35 An 65 and another three-phase invariant assemblage, calcic plagioclase (~Ab 15 An 85 ) + scapolite + calcite is stable over the approximate range Ab 30 An 70 -An 15 An 85 . Unit-cell dimensions and refractive indices have been determined for the scapotite synthesized in these experiments and are compared with values for chemically analyzed natural scapolites. Scapolite must be regarded as a ternary solid solution in which, at a given equivalent An-content, the Cl/CO 3 ratio in the large anion site can vary as a function of NaCl and CaCO 3 activities.