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Figure 1. A large sphene crystal, sample 2881, believed to have been collected from the Mesoproterozoic Grenville province in southeastern Ontario. Sample donated by Bruce Geller, provenance Bob Jordan. Locality unknown, but probably a skarn in the region, as discussed below. This remarkably large, flattened axe-head crystal of waxy brown sphene, 12x9x5 cm, weighs 814 g, magnetic susceptibility 0.229x10-3 SI units. Ultraviolet light reveals tiny bright-blue specks, probably tiny calcite inclusions, the only other mineral evident in this remarkable piece. Subtle lineations and ridge-like features are seen upon slow rotation of the crystal faces.
"Rock of the Month #167, posted for May 2015" ---
Titanite
also known as sphene, is a very common accessory mineral in granites
and other rocks. It is commonly found as well-formed lozenge-shaped crystals, typically
no more than 1-2 mm in length.
Sphene is a monoclinic nesosilicate, ideal formula
CaTi[SiO4(O,OH,F).
Chemical substitutions are common, including:
Na, REE, Mn and Sr for Ca;
Al, Fe, Nb, Ta, V and Cr for Ti; and
OH, F or Cl for O.
It is readily identified in transmitted light, and often
shows euhedral rhomboid cross-sections.
It is a widespread accessory mineral in felsic-intermediate igneous rocks such as
granites and diorites, where it may be the main host mineral for titanium, absent ilmenite.
It occurs also in silica-undersaturated rocks such as nepheline syenites, in skarns,
metamorphic rocks and Alpine-type veins.
It is well-known that the lanthanides (rare earth elements, REE) and actinides (notably, thorium and uranium) are two families
of chemical elements with similar, gradually varying properties.
Both can occur as minor components of sphene.
Thus sphene has been used as an analogue
for possible media for disposal of radioactive waste.
The replacement of translucent, dark-coloured sphene by very fine-grained aggregates of REE -rich minerals leads to
LREE and Th enrichment in the secondary minerals, and HREE and U losses, the HREE loss increasing with atomic number
(Pan et al., 1993).
Irradiation of a host mineral during the gradual radioactive
decay of actinide atoms can damage the crystal structure
of the host, forming what is called a metamict grain,
and ultimately to an amorphous material. Such sphene has been
studied, including material from the Cardiff mine (Zhang et al., 2000).
Sphene is found widely alkaline rock suite of the Bancroft region of
Ontario (Adams, 1904; Peterson, 1978; Anon, 1982; Robinson and Chamberlain, 1982).
One mode of occurrence is in
the metre-scale calcitic veins and
dykes in the region (Anderson et al., 1995).
It has been suggested (Moecher et al., 1997) that these peculiar rocks may be igneous
carbonatites, as opposed to hybrid melts formed by reaction
of silicate magma with carbonate sediments.
Sphene also occurs in shear zones with zircon, allanite and uranium mineralization,
as in Cardiff township (Fyson et al., 1980).
Further east,
one district noted for `calcite dykes' is the
Lake Clear area in Sebastopol township, home to several
noted localities, including old mines.
The Meany mine was opened in 1880 on a N.E.-S.W. vein 6-16 feet [1.8-4.9 m] wide: the deposit contains apatite, pyroxene and late veins of `pink' to orange calcite, the calcite enclosing coarse crystals of apatite, orthoclase K-feldspar, sphene, hornblende, scapolite, chlorite and mica (Grice and Gault, 1982).
The famed road trips of Sabina (1969, 1971, 1983, 1987)
list numerous Grenville localities for sphene, with 37 sphene
occurrences in Sabina (1971) alone.
The Grenville mineral localities across southeast Ontario may yield
large crystals, as in the Kingston area (Baker, 1916, p.30).
Baker's observation was that granites penetrated
Grenville sediments in great volumes, the intrusives
generally assuming sheet-like form within the regional structures. Thus the
so-called “pyroxenite dykes”, with coarse pyroxene, apatite, sphene and zircon,
are simply contact metamorphic aureoles around thin felsic
intrusives, formed by reaction with local limestones and dolostones.
The Otter Lake area, in the Pontiac region of southwest Quebec, northwest of Ottawa, is an eastward extension of rocks similar
to those of the Bancroft region, including biotite syenite and
nepheline syenite. The dark minerals in the suite include
hornblende, biotite and sphene (Kretz, 2005).
Suitable crystals of sphene can be used for Pb-Pb age dating in the region.
Heaman (1985) dated a range of mafic to felsic intrusive rocks in Chandos township.
Another example of Pb-Pb dating of sphene is the
Olden wollastonite skarn (Grammatikopoulos et al., 2005).
The technique is especially useful because sphene may be the only unambiguously metamorphic mineral in marbles and calc-silicate gneisses (Mezger et al., 1991).
References
Adams,FD (1904) On a new nepheline rock from the province of Ontario, Canada. Amer.J.Sci. 17, 269-276.
Anderson,ED, Cook,CA and Moecher,DP (1995) Geochemical constraints on the origin of carbonatite-like rocks in the Central Metasedimentary Belt, Grenville Province, southern Ontario. GSA Abs.w.Progs. 27 no.6, 119, New Orleans.
Anon (1982) Bancroft's Guide to Rockhounding. Bancroft and District Chamber of Commerce, Bancroft, Ontario, 17pp.
Baker,MB (1916) The geology of Kingston and vicinity. OBM Ann.Rep. 25 part 3, 1-36.
Fyson,WK, Baer,AJ and Habib,MK (1980) Structural fabric and uranium distribution in shear zones near Cardiff, Ontario. OGS Misc.Pap. 93, 83-85.
Grammatikopoulos,TA, Clark,AH, Pearce,TH and Archibald,DA (2005) Genesis of the Olden wollastonite skarn, Sharbot Lake domain, Central Metasedimentary Belt, Grenville province, southeastern Ontario, Canada. Can.J.Earth Sci. 42, 1401-1417.
Grice,JD and Gault,RA (1982) The Clear Lake-Kuehl Lake area, Renfrew County, Ontario. Mineral.Record 13 no.4, 209-213.
Heaman,LM (1985) A Geochemical and Isotopic Study of Plutonic and High Grade Metamorphic Rocks from the Chandos Township area, Grenville Province, Ontario. PhD Thesis, McMaster University, 307pp.
Kretz,R (2005) Subsolidus evolution of gneissic biotite syenite and nepheline syenite, Otter Lake area, Quebec. Can.Mineral. 43, 1107-1122.
Mezger,K, Van der Pluijm,BA, Essene,EJ and Halliday,AN (1991) Synorogenic collapse: a perspective from the Middle Crust, the Proterozoic Grenville orogen. Science 254, 695-698, 01 November.
Moecher,DP, Anderson,ED, Cook,CA and Mezger,K (1997) Petrogenesis of Grenville "carbonatites". GAC/MAC Abs. 22, 104, Ottawa.
Palache,C (1932) The largest crystal. Amer.Mineral. 17, 362-363.
Pan,Y, Fleet,ME and MacRae,ND (1993) Late alteration in titanite (CaTiSiO5): redistribution and remobilization of rare earth elements and implications for U/Pb and Th/Pb geochronology and nuclear waste disposal. Geochim.Cosmochim.Acta 57, 355-367.
Peterson,H (1978) Peterson Guide to Mineral Collecting, Bancroft Area. H. Peterson, Bancroft, 2nd edition, 60pp.
Robinson,G and Chamberlain,SC (1982) An introduction to the mineralogy of Ontario's Grenville province. Mineral.Record 13 no.2, 71-86.
Sabina,AP (1969) Rocks and Minerals for the Collector: Buckingham - Mont-Laurier - Grenville, Quebec: Hawkesbury - Ottawa, Ontario. GSC Pap. 68-51, 107pp.
Sabina,AP (1971) Rocks and Minerals for the Collector: Ottawa to North Bay, Ontario: Hull to Waltham, Quebec. GSC Pap. 70-50, 130pp.
Sabina,AP (1983) Rocks and Minerals for the Collector: Kingston, Ontario to Lac St-Jean, Quebec. GSC Misc.Rep. 32 (revised version of GSC Pap. 67-51), 130pp.
Sabina,AP (1987) Rocks and Minerals for the Collector: Hull-Maniwaki, Quebec, Ottawa- Peterborough, Ontario. GSC Misc.Rep. 41 (revised version of GSC Pap. 69-50), 141pp.
Zhang,M, Salje,EKH, Malcherek,T, Bismayer,U and Groat,LA (2000) Dehydration of metamict titanite: an infrared spectroscopic study. Can.Mineral. 38, 119-130.
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