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Figure 1. Coarse amphibole crystals in a large sample from from the Earle Farm locality near Wilberforce, Haliburton county, in the geological Grenville province of southeastern Ontario. Sample 2356, the amphibole fluororichterite (fluor-richterite). Donated by Brock May, who collected this at the same time as the large mass in the foyer of the Department of Geology, University of Toronto, 2002. This piece is 30x15x15 cm in size, circa 10.5 kg, with innumerable dark amphibole prisms in a grey-white calcitic matrix. In some samples of this rock, the amphiboles define a definite lineation.
"Rock of the Month #166, posted for April 2015" ---
Amphiboles
are a major class of hydrous rock-forming minerals. Silicates with
a chain structure (inosilicates) and often-complex chemistry,
they occur in both igneous and metamorphic rocks, and may also
occur as resistate heavy minerals in clastic sediments.
The idealised formula of the alkali amphibole richterite is
Na2Ca(Mg,Fe,Mn)5Si8O22(OH,F)2.
Richterite occurs in contact-metamorphosed limestones (marbles), skarns, and
in veins in alkaline igneous rocks.
First noted by Breithaupt in 1865,
the type locality for richterite is the Langban mine in Sweden.
Fluororichterite contains more F in replacement of OH,
and K substituting for Na.
Figure 2. Left: Close-up of part of sample 2356.
Right:
A similar piece, sample 2249, was acquired from Allen Cowan
at the Norwood mineral show, summer 2001.
Roughly triangular sample, 20x17x6 cm in size, 3905 grams
(plus 145 grams of sawn fragments and a thin section).
The amphibole prisms vary from <5 mm to 70 mm in length, with a
dark greenish-brown hue, translucent, and a resinous to subvitreous
lustre. The prisms are in overall subparallel alignment,
locally radiating, in a calcitic matrix.
A thin section of this rock contains roughly 80% amphibole, 20% calcite
and traces of other minerals. The rock is presumed to be a skarn. The
amphibole appears optically homogeneous, apart from
traces of inclusions of the accesssory phases.
These include phlogopite mica, up to 0.3 mm long, within the
amphibole prismatic cleavage planes,
and grains of pyrite to 0.05 mm in fractures. The calcite contains traces
of (?) zircon, and goethite is also present (Wilson, 2001).
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The Earle Farm locality yields fluororichterite, as do the Essonville marble roadcut and other localities (Anon, 1982; Sabina, 1986; Fouts, 1998; Lininger, 1999). Potassic fluororichterite is reported in an unusual sheared metasediment-metavolcanic contact zone at Buckingham, Quebec (Hogarth and Dumas, 1998). The nomenclature of this and other amphiboles was reviewed by Mazdab (2003). Fluororichterite is also reported in fenites from Amba Dongar carbonatite complex in Gujarat, western India (Roelofsen et al., 1995) and in the lapis lazuli -bearing mines of the Kokcha river valley, Badakhshan, northeast Afghanistan (Moore and Woodside, 2014).
References
Anon (1982) Bancroft's Guide to Rockhounding. Bancroft and District Chamber of Commerce, Bancroft, Ontario, 17pp.
Fouts,C (1998) Bancroft & District Mineral Collecting Guidebook. Bancroft & District Chamber of Commerce, 50pp.
Hogarth,DD and Dumas,V (1998) Igneous and metasomatic rocks at Buckingham, Quebec. GAC/MAC Abs. 23, 80, Quebec.
Lininger,JL (1999) A mineralogical odyssey: the life and times of Louis Moyd. Matrix, a Journal of the History of Minerals 7 no.4, 147-166.
Mazdab,FK (2003) The diversity and occurrence of potassium-dominant amphiboles. Can.Mineral. 41, 1329-1344.
Moore,TP and Woodside,RWM (2014) The Sar-e-Sang lapis mines, Kuran wa Munjan district, Badakhshan province, Afghanistan. Mineral.Record 45, 280-336.
Roelofsen,JN, Martin,RF, Williams-Jones,A and Veblen,DR (1995) Sequential alteration of mafic minerals in fenites from the Amba Dongar carbonatitic-alkaline complex, Gujarat, India. GAC/MAC Prog.w.Abs. 20, 90, Victoria.
Sabina,AP (1986) Rocks and Minerals for the Collector: Bancroft - Parry Sound area and Southern Ontario. GSC Misc.Rep. 39, 182pp.
Wilson,GC (2001) Meteorites and `meteorwrongs', glassy materials including a `pseudofulgurite', Grenville-province minerals, a suite from Brazil, and ores and ore minerals. TGSL Report 2001-100, 54pp.
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