Hydrotalcite and quintinite from Dypingdal,Snarum, Buskerud, NorwayGunnar Raadelntroduction
The new mineral species hydrotalcite was described by Hochstetter (1842) from Snarum.
There is no doubt that the type locality must be the Dypingdal serpentine-magnesite deposit.
A sample of the mineral had been presented by Prof. Scheerer to Dr Marchand, who gave itto Hochstetter for investigation. The chemical analysis performed by Hochstetter shows thatthe mineral is essentially a hydrated carbonate of magnesium and aluminium. lt wasassociated with "steatite" which is usually regarded as a synonym of talc, MgaSiaOro(OH)2.
However, the chemical analysis of Hochstetter (1842) shows an appreciable amount of AlzOa(12.52w1%).
Hochstetter's analysis of hydrotalcite gives an Mg:(Al+Fe3') ratio of 2.80:1.00. lts Fe2O3
content of 6.90 wt% is suspiciously high and might indicate contamination by an iron-richmineral. An extensive older literature exists on hydrotalcite, discussing whether it should be
regarded as a distinct mineral or a mixture, and is summarized by Frondel (1941). Thevalidity of hydrotalcite as a species was shown by Manasse ( 1915), who obtained thechemical formula MgsAlzCO3(OH)16'4HrO, with Mg:Al = 3:1. The trigonal crystal structure ofhydrotalcite was first determined by Allmann & Jepsen ( 1969).
Frondel (1941)described manasseite as a hexagonal polytype of hydrotalcite from Snarum
and from Amity, New York. Hydrotalcite and manasseite are frequently intergrown in theDypingdal deposit and are almost indistinguishable in appearance (Neumann 1985)- The
nomenclature of the hydrotalcite supergroup has recently been reviewed by Milts et al.
(2012). Acmrding to IMA rules, polytypes are not to be regarded as distinct species.
Therefore, the name manasseite is discredited. The original hydrotalcite is hydrotalcite-3R,
and manasseite becomes hydrotalcite-2H.
Hydrotalcite belongs to a large group of naturally occurring and synthetic layered doublehydroxide (LDH) compounds (Mills el al. 2012). Their structures are derived from that ofbrucite, tvg(oH)2, in which edge-sharing Mg(oH)o octahedra form continuous sheets parallelto (001), linked by relatively weak hydrogen bonds. ln the LDH mmpounds, the bruciteJikelayers contain both divalent and trivalent cations, creating a surplus positive charge that isbalanced by interlayer anions, mupled with additional water molecules. ln the case ofhydrotalcite, carbonate (CO32) is the interlayer anion. According to Mills et al. (2012), theis recommended for hydrotalcite, with the interlayer anion
formula Mg6Al2(OH)16[CO3].4HrO-square
brackets. The first part of the formula, Mg6Al2(OH)16, indicates lhe
shown incomposition of the brucite-like layer. The water molecules and interlayer anions areintercalated between two brucite-like layers, hence the designation 'layered doublehydroxide'.

On the track of quintinite

ln 1996, I pubtished the results of remnnaissance electron microprobe analyses of fourhydrotalcite samples from Dypingdal (Raade 1986). The mean analytical results ofMg:(Al+Fe) ratios were, with numbers of individual analyses in parentheses: 2.80:1 (1),55

2.30:1 (2), 1.95:1 (4) and 1.92:1 (3). Only the first sample has a ratio close to the ideal 3:1 inhydrotalcite. The two last samples have a ratio close to 2'.1, and the second sample lies in
between. ldid not pay much attention to these results at the time, knowing that synthetic
phases with equivalent structures had been shown to posses highly variable Me2*:lvle3* ratios(Allmann 1970), and structure determination of alleged hydrotalcite from Moravia had given
the formula MgaAl2CO3(OH)rz'3HzO (Allmann & Jepsen 1969).

A specimen of hydrotalcite from Snarum was studied in detail by wet-chemical analysis and
X-ray powder diffractometry by Stanimirova (2001). Three morphological varieties weredistinguished macroscopically, based on differences in colour, transparency, lustre andnature of the aggregates. Samples HT-1 and HT-2 have Mg:Al ratios of 2:1 , whereas forsample HT-3 the ratio is 2-30:1. The X+ay powder data of sample HT-1 were indexed on a2H manasseite cell. For sample HT-2, the X+ay data are interpreted as a mixture ol 2H,3Rand 6R polytypes. Sample HT-3 is predominantly a 3R polytype, with a small amount of the6R polytype.

ln 1997, a hydrotalcite-like mineral with composition MgaAlrCO3(OH)rz.3HzO was describedas the new species quintinite, occurring as 2H and 3f po[types (Chao & Gault 1997).
Quintinite-2H was found in the Jacupiranga mine, Brazil, and quintinite-3f at Mont SaintHilaire, Canada. When that paper appeared, I regrettably failed to couple the new mineralwith my previous, 11 year old results on the composition of Dypingdal hydrotalcite. Being
closer in time, it may seem strange that Stanimirova did not recognize the identity of her
mineral with quintinite. lt was only in 20',2, when we had the hydrotalcite supergroupnomenclature for approval in the Commission on New Minerals, Nomenclature andClassification, that it became clear to me that quintinite occurs at the Dypingdal locality.
Thanks to my mmments, this is mentioned in the paper by Mills et al. (2012), and thereference to Stanimirova (2001) was added.

The cases with Mg:Al close to 2.30:1, observed by both Raade (1986) and Stanimirova(2001), suggest intergrovvths of hydrotalcite and quintinite.

Coda

It is remarkable that two independent investigations on randomly chosen hydrotalcite
samples from Dypingdal, by Raade (1986) and Stanimirova (2001), have resulted in several
cases where the formula corresponds to that of quintinite, MgaAl2(oH)1r[co3].3Hro. This isa slrong indication that quintinite may be widespread at the locality. lt seems at the momentthat the two minerals are not easily distinguished macroscop:cally. From the classicalDypingdal deposit, samples of 'hydrotalcite' are spread all over the world in mineral museums
and private collections- The question is: what is it that you have in your collection, is ithydrotalcite or quintinite?
References
ALLMANN, R.. (1970): Doppelschichtstrukturen mit brucitdhntichen Schichtionen [Me(il)1_,Me(lll),(OH)zl't. Chimia 24, 99-108.
ALLMANN, R. & JEPSEN, H.P. (1969): Die Struktur des Hydrotalkits. Neues Jahtuuch flr
Mineralogie Monatshefte, 544-551 .

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CHAO, G.Y. & GAULT, R.A. (1997): Quintinite-2H, quintinite-3f, charmarite-2H, charmarite3
and caresite-3l, a new group of carbonate minerals related to the hydrotalcite
manasseite group. Ihe Canadian Mineralogist 35, 1541-1549.

f-FRONDEL, C. (1941): Constitution and polymorphism of the pyroaurite and sjdgrenitegroups. American Mineralogist 26, 295-315.
HOCHSTETTER, C. (1842): Untersuchung ilber die Zusammenselzung einiger Mineralien
Joumal fhr praktische Chemie (Erdmanns Journal der Chemie) 27 , 37 5-378.
MANASSE, E. (1915): Rocce eritree e di Aden della mllezione lssel. Afti della Socieh
Toscana di Scienze Naturali, Processi Verbali 24,92. [Publication not seen by the author;
cited from Mills et al. (2012).1MILLS, S.J., CHRISTY, A.G., GENIN, J.-M.R., KAMEDA, T. & COLOMBO, F. (2012):
Nomenclature of the hydrotalcite supergroup: natural layered double hydroxides.
llineralogical Magazine 76, 1289-1 336.
NEUMANN, H. (1985): Norges Mineraler. Norges Geologiske Undersokelse Skrifter 68,278
pp.

RAADE. G. (1986): Petrogenesis of serpentine-magnesite deposits at Modum, south
Norway: A progress report. ln: Metallogeny of Basic and Ultrabasic Rocks (RegionalPresentations). Theophrastus Publications S.A., Athens, 451 -479.

STANIMIROVA, T. (2001): Hydrotalcite polytypes from Snarum, Norway. Annual of theUniversity of Sofia, Facu$ of Geology 94, 73-80.

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