"Rock of the Month #203, posted for May 2018" ---

Qingtian stone

is an aluminous rock composed of three or four key minerals, all sheet silicates, described below. First though, a note from Xuanying Zhao, one of my students at CUGB, from Qingtian, Zhejiang:

"Qingtian stone has a gentle texture, so it’s widely used as a carving and seal cutting material. It may occur in many different colors, like red, yellow, blue, white or even black, which depends on the chemical elements it contains. Qingtian stone is one of the four famous stones in China and almost all of it comes from Qingtian county, Zhejiang province. There are more than ten main mines in Qingtian county: the biggest one is the Shankou mine. The mine follows a northeast-southwest trend, 6 km long".

Mineral Notes

• Pyrophyllite, first discovered in a gold deposit near Ekaterinberg (Sverdlovsk, Urals), is quite often associated with aluminous alteration and gold. Where it is coarse-grained, it typically forms radiating aggregates (e.g., Urals, North Carolina and Georgia, U.S.A.). Generally pale and lustrous, it may also be orange (Guangxi, China: specimen in National Geological Museum, Beijing). A monoclinic mineral, of the same structure as talc, it has similar physical properties to talc but distinct chemistry (it is aluminous rather than magnesian).
• Kaolinite, a triclinic phase, is very important in pottery and ceramics,
• Illite and Dickite, monoclinic clay minerals, and
• Other minerals which may also be present, such as andalusite, sericite (pinite) and quartz, plus traces of dumortierite, corundum, hematite or ilmenite. The rock is very fine-grained, suitable for fine carving. Note that the above minerals are all aluminous, other than quartz, hematite and ilmenite. A related material seems to be agalmatolite or catlinite, an aluminous paleosol (see below).

Kaolinite is a ubiquitous clay mineral, principal constituent of the industrial mineral "china clay" (mined, notably, in the St. Austell district of Cornwall, southwest England).

Dickite may be apple-green, as when associated with mercury mineralization in southwest Alaska (Sainsbury and MacKevett, 1965). Dickite is associated with hydrothermal alteration in a wide range of mineral deposits (e.g., Fe, Hg, Sb, Au, U, kuroko-style VMS, porphyry and epithermal Cu-Au and MVT Pb-Zn).

Illite is actually a family of K-Mg-Fe-bearing clays, of structure similar to muscovite mica, found in marine sediments, including an iron-rich member, the green glauconite of "greensands". These minerals are typically very fine-grained, and are most commonly characterized using techniques of x-ray diffraction (XRD).

Pyrophyllite is of worldwide, yet more localised occurrence, relative to kaolinite and illite. It was first reported in Berezovsk (Berezovskoe), Russia (Wilson, 2016). For a detailed description, see Deer et al. (2009). Pyrophyllite is found with other aluminous minerals, often in hydrothermal alteration zones and breccias. Associated minerals may include andalusite, diaspore, corundum, dumortierite, alunite, as well as pyrite and other ore minerals. Chromian pyrophyllite occurs with the rare Cr oxide merumite, in Guyana (Milton and Narain, 1969; Milton et al., 1976). It has been reported in numerous gold and polymetallic mineral deposits, worldwide. Where sufficiently concentrated, it is an industrial mineral in its own right, with deposits in India, China, Canada (Quebec, Newfoundland) and the U.S.A. (Carolina slate belt: Klein, 1988).

Another popular stone: larderite

Another variety of carving stone popular in China is termed larderite. It may be pale greenish-white to grey and even pink, but especially a pleasing yellow-brown fudge or butterscotch colour. It comes from Fujian. It seems to be used for smaller pieces. The Museum of CUGB has a fine selection of small, 5-8-cm rounded pieces carved to represent - or so it seems to me - a wise man, and maybe a dragon or other creature, in a small cave within the rock. Larderite is a mixture of dickite, kaolinite and nacrite. The latter is a monoclinic polymorph of kaolinite. At the National Geological Museum, a larderite piece from Fujian is identified with fine-grained to massive grossular garnet (indeed, this Al-Ca garnet is often of this brown colour, and is often found in skarns, but it would be a much tougher rock to carve).

Origins

So, what kind of rock is Qingtian stone? The related agalmatolite is an aluminous paleosol, a fossil soil, largely fine-grained muscovite and residual quartz, described from the Lewisian of northwest Scotland, from the Steep Rock Lake Group at Atikokan, Ontario and elsewhere (Allison et al., 1992). Pyrophyllite is typical of hydrothermal alteration zones in volcanic rocks. This mineral assemblage may be found in altered host rocks to a variety of mineral deposits, and this may be its typical setting. The components of Qingtian stone are found in advanced argillic alteration zones atop porphyry copper systems (see, e.g., Kesler et al., 1981; Hedenquist and Taran, 2013). Ultimately, Qingtian stone is probably formed most often from intense alteration of felsic-intermediate volcanic rocks (such as rhyolite, rhyodacite, dacite, ignimbrite and andesite), though, since alteration occurs under a wide range of physical and compositional constraints, it would be unwise to rule out other origins, on the basis of limited data.

As a petrographer, not a crystallographer, I have always found the clay minerals rather elusive and troublesome: too fine-grained and often too similar for ready identification by microscopy. For a wonderful, colourfully- illustrated review of clay minerals, obviously compiled by an enthusiast (who I can identify only as "Anderson"), see: "Clay Minerals".

References

Allison,I, Ferguson,LK, Cardenas,FA and Kronberg,BI (1992) Precambrian muscovite-quartz (agalmatolite) palaeosols from Scotland and Canada. Can.J.Earth Sci. 29, 2523-2529.