![Mexican apophyllite [213 kb]](apo2.jpg)
Above: a large sample of white tabular apophyllite with associated base-metal sulphides. Sample from Irwin Kennedy collection. This is the largest (by far) of three samples from the same locality. The smallest piece, a clear tabular crystal, was subjected to non-destructive testing, leading to the mineral identification and some additional understanding of the mineral occurrence. The three samples, dominated by the same tabular, white to colourless mineral are from Mina Cobresas at La Paz, on the east flank of the Sierra de Catorce, west of the city of Matehuala, which is in the northern tip of the state of San Luis Potosi, 70 km northeast of Charcas.
"Rock of the Month #134, posted for August 2012" ---
Apophyllite is a
potassium-calcium silicate mineral.
A tetragonal phase, it has an unusual structure
with sheets of (Si-O) tetrahedra arranged into 4-fold and 8-fold
rings, sandwiching layers containing the other constituent
atoms and molecules (K, Ca, F and H2O).
The perfect cleavage along the sheet structure imparts a pearly lustre,
in contrast to prism faces, which are vitreous.
The habit is also variable, and includes prisms, blunt pseudocubic crystals,
and flattened, tabular forms. See Deer et al. (2009) for
a modern technical description.
It occurs as a secondary mineral lining and infilling amygdales
in basaltic lava flows,
often in association with zeolites (although it is NOT a zeolite),
and is also found in contact metamorphic environments.
Skarns such as Dal'negorsk (Grant and Wilson, 2001)
and Broadford (on the island of Skye in northwest Scotland: Tilley, 1951)
are well-known.
The occurrences with zeolites in the Deccan Traps of western India
(around Poona in the state of Maharashtra) are especially celebrated,
and well-documented (e.g., Jeffery et al., 1988; Ottens, 2003;
Kothavala, 2003; Makki and Makki, 2008; Wilson, 2009).
Further Information
A more detailed description and
discussion of the samples and their identification
will be found
here, as a 410-kb pdf file,
together
with photos of the two smaller
samples.
Five I.K. apophyllite samples, including the mid-size (63-g) mass
examined here, were viewed and their specific gravity determined
in November 2012.
The calculated specific gravity of apophyllite is circa 2.36-2.37.
The visually-purest sample, from Carazinho, Rio Grande do Sul, Brazil,
collected in 1970, yields a measurement of 2.36.
The sample is a 92-g aggregate
of fine, pale greenish to colourless tetragonal prisms.
The other samples
gave lower values, including 2.27 for the 63-g Mina Cobresas specimen,
which is partially invested with impurities. Measured values
of S.G. for apophyllite are commonly quoted as 2.33-2.37.
References
Deer,WA, Howie,RA and Zussman,J (editors) (2009) Rock
Forming Minerals, Volume 3B: Layered Silicates Excluding Micas
and Clay Minerals (second edition). Geological Society Publishing
House, 314pp.
Grant,R and Wilson,WE (2001) Dal'negorsk: Primorskiy
Kray, Russia. Mineral.Record 32 no.1, 3-30.
Jeffery,KL, Henderson,P, Subbarao,KV and Walsh,JN (1988)
The zeolites of the Deccan basalt - a study of their
distribution. In `Deccan Flood Basalts' (Subbarao,KV editor),
Geol.Soc.India Memoir 10, 393pp., 151-162.
Kothavala,RZ (2003) Recollections of mineral collecting and
dealing in India. Mineral.Record 34, 135-154.
Makki,S and Makki,MF (2008) Green apophyllite from
Bidkin, Aurangabad, India. Mineral.Record 39, 267-273.
Ottens,B (2003) Minerals of the Deccan Traps, India.
Mineral.Record 34, 1-82.
Tilley,CE (1951) The zoned contact-skarns of the Broadford
area, Skye: a study of boron-fluorine metasomatism in
dolomites. Mineral.Mag. 29, 621-666.
Wilson,WE (editor) (2009) Private Mineral Collections in
Texas. Mineral.Record 40 no.1, supplement, 180pp.
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