Eur. J. Mineral.
2008, 20, 629–633
Published online May 2008

Struvite-(K), KMgPO4 ·6H2O, the potassium equivalentof struvite – a new mineral
Stefan GRAESER1,2, *, Walter POSTL3 , Hans-Peter BOJAR3 , Peter BERLEPSCH4, Thomas ARMBRUSTER4,Thomas RABER5 , Karl ETTINGER6 and Franz WALTER61

Natural History Museum Basel, 4001 Basel, Switzerland*Corresponding author, e-mail: Stefan.graeser@unibas.ch2
Mineralogical Institute, University of Basel, 4056 Basel, Switzerland
3Dept. of Mineralogy, Landesmuseum Joanneum, Raubergasse 10, 8010 Graz, Austria
4Laboratorium f. Chemische und Mineralogische Kristallographie, University of Berne, Freiestr. 3,3012 Berne, Switzerland
5
Edith-Stein-Str. 9, 79110 Freiburg, Germany
6
Institute of Earth Sciences, Dept. of Mineralogy and Petrology, Karl-Franzens-University Graz, Universitätsplatz 2,8010 Graz, Austria
Abstract: The new mineral struvite-(K) is the natural potassium equivalent to struvite NH4 MgPO4 ·6H2 O. It was discovered
independently at two diﬀerent localities: (1) at the famous sulphosalt locality Lengenbach in Binntal, Switzerland, where it occurs
in a dolomitic rock of Triassic age, in close association with various Pb-As-sulphosalts (mainly rathite). It forms extremely fineneedles reaching up to a maximum length of about 0.3 mm. The needles are elongated along the crystallographic a-axis, they
are completely colourless and transparent. The acicular crystals are generally well developed, and several forms such as {0 0 1},{0 1 0}, {1 0 1}, {0 1 2}, {1 1 0}, {1 1 1}, {0 1 2̄} could be identified by optical goniometry. Single-crystal study showed themineral to be orthorhombic, with space group Pmn21 (from analogy to struvite), a = 6.903(3), b = 6.174(2), c = 11.146(3) Å,V = 475.0(2) Å3 (refined from powder data). Qualitative chemical data derived from EDS analyses on an SEM gave major K,Mg, P, and traces of Sb, Fe, and Cu. Attempts to prepare samples for quantitative EMP analyses failed, therefore a structure
determination (using a Bruker AXS three-circle diﬀractometer) and a refinement of K versus NH4 were carried out which provethat the mineral represents the pure K end-member. Optically, the mineral is biaxial positive, with 2VZ = large, α = 1.490(2),β = 1.493(2) (for 589 nm), γ could not be measured, the optic axes plane (OAP) is perpendicular to the needle axis, therefore βparallels the crystallographic a-axis. At the Lengenbach occurrence, struvite-(K) is a product of the latest stage of hydrothermal
activity to supergene alteration.
A second occurrence (2) is Rossblei, Schladminger Tauern, Styria, Austria, an abandoned galena mine. The host rock of thePb-mineralisation is a sericite-schist belonging to the polymetamorphic basis of the Schladminger Tauern. The mineral occurs as
pseudomorphosed aggregates of dirty white colour reaching up to several millimetres . The aggregates represent close intergrowthsof fine-grained struvite-(K) and newberyite Mg(PO3 OH)·7H2 O. Cell parameters refined from the powder data, after deduction ofnewberyite lines, are a = 6.878(1), b = 6.161(1), c = 11.100(1) Å, V = 470.41(9) Å3 . No additional physical, optical, morphological
data could be derived due to the close intergrowth of the two minerals. Struvite-(K) from Schladming obviously represents a recent
alteration product.
Key-words: struvite-(K), new mineral species, potassium magnesium phosphate, crystal structure, Lengenbach, Switzerland,Schladming, Austria.

Introduction
The locality Lengenbach in the Valley of Binn, Ct.Valais,
Switzerland is well known for its wealth of rare and uncommon sulphosalt minerals since more than 200 years. At
least 26 members of this fascinating mineral family havebeen discovered for the first time in Lengenbach and weredescribed as new mineral species, starting with the mineral
DOI: 10.1127/0935-1221/2008/0020-1810

dufrénoysite in 1845 (Damour, 1845) up to the latest newsulphosalt member gabrielite (Graeser et al., 2006). Theinterest in sulphosalt minerals, mainly of scientific character, has lead to the surprising fact that, during the summer
months, the locality had been mined for minerals exclusively for scientific research by several syndicates for morethan 150 years. Since 1993 it is a new syndicate which,under the name “Forschungsgemeinschaft Lengenbach
0935-1221/08/0020-1810 $ 2.25
c 2008 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart630

S. Graeser et al.

(FGL)”, is occupied with the exploitation of minerals fromthe dolomitic rocks. It is obvious that the main interest
concerns the numerous sulphides and sulphosalts, nevertheless there may occur also very uncommon minerals of
non-sulphidic character: in 1999, a member of the FGL(Markus Ecker), when looking through material from anearlier (1998) exploitation, discovered a sample with extremely small needle-like crystals of colourless to whitishappearance. A qualitative EDS analysis carried out on an
SEM yielded a chemical composition of mainly K, Mg, andP for the unknown crystals. The subsequent study of thematerial proved that it is the potassium analogue of struvite,
hence representing a new mineral species. Description andname (proposed by the chairman of the CNMMN-IMA)
have been submitted to the Commission on New Minerals
and Mineral names, I.M.A. (proposal number 2003-048)
and were approved by the Commission. Type material isdeposited (under number S176) in the Natural History Museum of Basel.
Struvite-(K) has also been recorded at the abandoned
lead-mine Rossblei near Schladming, Styria, Austria, on
samples collected in the 1960-ies and preserved at theLandesmuseum Joanneum in Graz under the name “phosphorrösslerite”. A recent study showed that this determination was erroneous, and that the material consisted of aclose intergrowth of the two minerals struvite-(K) and newberyite, instead (Postl et al., 2000).

Occurrence(1) Lengenbach: at the moment only one sample containing struvite-(K) is known from this locality. The sample in question is of “thumb-nail” dimension (approx.
2 × 1 × 1 cm) of white dolostone with small cavities
which contain mainly dolomite crystals, some quartz, frequent realgar, and various As-sulphosalts like jordanite,
dufrénoysite, rathite, tennantite. Struvite-(K) has grownon sulphosalt minerals (rathite), and on dolomite crystals
as well. The phosphorous necessary for the formation ofstruvite-(K) may be derived from several phosphate minerals like goyazite SrAl3 [(OH)6 |PO3 OH|PO4 ], gorceixite
BaAl3 [(OH)6 |PO3 OH|PO4 ], and rarely apatite, that haveformed in the dolomitic host-rock under late hydrothermal
conditions. It is not clear whether struvite-(K) has to betreated as a product of hydrothermal activity too, or ratherof supergene alteration.
(2) Rossblei, Schladming: Struvite-(K) occurs as finegrained intergrowth with newberyite in pseudomorphosed
flat tabular crystals (Fig. 3) of an unknown precursor. Thecrystals of struvite-(K) are xenomorphic and up to 20 μm
large, whereas newberyite is hypidiomorphic up to 100 μm.
Both form flat tabular dirty-white aggregates. Struvite-(K)was found in small water puddles in the adit of the abundant galena-mine “Rossblei” south of Schladming. The
host rock is a sericite schist which is a member of the polymetamorphic crystalline of the Schladminger Tauern. Thephosphates are an alteration product of animal bones andcirculating fluids (Laskovic & Weninger, 1967).

Fig. 1. Morphology of struvite-(K), Lengenbach, Switzerland, basedon goniometrical measurement (SHAPE drawing).

Habit and physical properties(1) Lengenbach: the thumb-nail sample contains a large
number of acicular crystals which rarely exceed 0.5–0.7 mm in length, and 20–30 μm across. They are completely transparent, with a whitish lustre (because of totalreflection eﬀects). The tip of the crystals is always clearlywedge shaped (due to the development of the form {101},
as proven by optical goniometry), in rare cases visible on
both ends. One of the crystals was measured on an opticalgoniometer, after having adjusted it by X-ray diﬀraction ona Weissenberg camera. The crystals are elongated along thecrystallographic a-axis, the habit is determined by the following forms: {0 0 1}, {0 1 0}, {1 0 1}, {0 1 2}, {1 1 0},{1 1 1}, {0 1 2̄} (Fig. 1). Under the polarizing microscope,
struvite-(K) is colourless, has low refraction indices andsmall birefringence. The optic normal lying parallel to theneedle axis, the mineral shows (±) elongation, dependingon the position of the needles. Dispersion of the optic axes
is visible (with r < v). Due to the small crystal size the determination of physical properties must remain incomplete(Table 1).
(2) Rossblei: the mineral occurs here as pseudomorphosed aggregates of dirty white colour reaching up
to several millimetres. The aggregates represent closeintergrowths of fine-grained struvite-(K)and newberyite,Mg(PO3 OH)·7H2 O. Due to the close intergrowth of thetwo minerals no additional physical, optical, morphological data could be derived.

X-ray diﬀraction(1) Lengenbach: Several of the needle-shaped crystals
were studied by X-ray diﬀraction with a Gandolfi camera

Struvite-(K), KMgPO4 ·6H2 O, the potassium equivalent of struvite – a new mineral631

Table 1. Morphology, physical and optical properties of struvite-(K) (Lengenbach).
HabitForms
Twins
a:1:cacicular, elongated along the crystallographic a-axis{0 0 1}, {0 1 0}, {1 0 1}, {0 1 2}, {1 1 0}, {1 1 1}, {0 1 2̄}frequent, presumably contact twins along (1 0 1)
1.15 : 1 : 1.87 (from optical goniometry)
1.118: 1 : 1.805 (from unit-cell parameters)

Colour
LustreStreakCleavage
Fracture
Hardness
Densitycalc

colourless transparent
vitreous, whitish
white
not visible, presumably absent
conchoidal
not determined
1.864 g/cm3nαnβnγ2Vγ (+)
Pleochroism
Dispersion
Orientation

Optical data (589 nm)
1.490 (3)1.493 (2)not determinedlarge (> 70◦ )no pleochroism was detected
weak (r < v)
β // cryst. a-axis, Optic Axes Plane (OAP) perpendicular to needle axis

Fig. 2. SEM picture of struvite-(K) from Lengenbach, Switzerland.

(114.6 mm, FeKα radiation). Powder data of struvite-(K) incomparison with the synthetic compound are presented inTable 2. The patterns were always identical and indicated aclose relationship to the mineral struvite. Yet, as it was obvious from SEM studies that the mineral contains K insteadof NH4 , the true nature as a K-equivalent of struvite could
clearly be predicted. From additional Weissenberg and precession single-crystal diagrams the crystallographic orientation of the needles could be derived, and further proofs
for the isotypy of the two minerals were gained.
(2) Rossblei: Due to the intimate intergrowth of struvite(K) with newberyite always both phases appear in the
powder-pattern (diﬀractometer Siemens D5000, CuKα).
The data for struvite-(K) are in good agreement with theFig. 3. Struvite-(K) (light-grey) beside newberyite (dark-gray) from
Wetterschacht, Rossblei, Schladminger Tauern, BSE image.

pattern of synthetic KMgPO4 ·6H2 O (ICDD 75-1076) (Table 2).

Chemistry and structure refinement(1) Lengenbach: the original material found by M. Ecker
among Lengenbach samples was primarily tested by qualitative EDS analyses on an SEM, yielding K, Mg, andP as major components, with traces of Sb, Fe, and Cu.
With the result of the single-crystal study as a presumably new K-Mg-P-mineral with struvite structure, quantitative EMP analyses were indispensable for a further study.
Unfortunately, any attempts to produce proper preparations

632

S. Graeser et al.

Table 2. X-ray powder data of struvite-(K).

hkl*1 0 1002
011
110
111
012
112
200
103
021
211
004
022
212
122
114
213
221
214
115
223
215
230

dobs (Å)
5.830
5.573
5.400
4.604
4.255
4.136
3.547

1
†dcalc (Å)
5.869
5.573
5.401
4.602
4.254
4.137
3.548

I/I o
40
30
10
20
100
80
10

3.271

3.272

90

2.905
2.787
2.699
2.650
2.513
2.387
2.3412.256
2.045
2.007
1.95411.79281.7691

2.908
2.787
2.700
2.650
2.515
2.384
2.340
2.253
2.046
2.006
1.95611.79201.7676

50
40
50
70
30
30
40
10
1040
50
40
40

dobs (Å)
5.849
5.552
5.387

2
dcalc (Å)
5.847
5.550
5.387

I/I o
13
45
26

4.241
4.125
3.537
3.439
3.258
2.969
2.899
2.775
2.694
2.641
2.507

4.241
4.124
3.537
3.439
3.259
2.968
2.899
2.775
2.693
2.641
2.508

100
75
625
37
19
52
47
64
33
17

1.94991.9500143d (Å)
5.842
5.543
5.385
4.587
4.239
4.121
3.534
3.436
3.255
2.968
2.897
2.772
2.692
2.639
2.507
2.372
2.330
2.246
2.036
1.9964
1.94881.7834
1.7626

I/I o
1128
16
15
100
48
416
38
1257
29
39
41
11
7
13
5
5
11
17
99

Notes:
1 = Lengenbach, Binntal, Switzerland (Gandolfi camera, 114.6 mm, FeKα, this work)* not used for refinement†dcalc for a = 6.903(3), b = 6.174(2), c = 11.146(3) Å
I/I0 = intensities visually estimated
2 = Rossblei, Schladminger Tauern, Styria, Austria(Diﬀractometer, Siemens D5000, CuKα, this work)
3 = synthetic KMgPO4 ·6H2 O (ICDD 75-1076)

Table 3. Unit-cell parameters of struvite-(K) (calculated from X-ray
powder data), synthetic KMgPO4 ·6H2 O, and struvite.
Sp-Gr.
a (Å)
b (Å)
c (Å)
V (Å3 )
Z1
Pmn21
6.892 (2)
6.166 (2)11.139 (4)473.4 (3)2

1aPmn216.903(3)6.174(2)11.146(3)475.0(2)22
Pmn21
6.878 (1)6.161(1)11.100 (1)470.41 (9)23Pmn21
6.873
6.160
11.087469.4024Pmn21
6.95
6.14
11.22
478.8
2Notes:
1 = Lengenbach, Binntal, Switzerland (from structure determination, this work).
1a = Lengenbach, Binntal, Switzerland (refined from X-ray powderdata, this work).
2 = Rossblei, Schladminger Tauern, Styria, Austria (from X-raypowder data, this work).
3 = synthetic KMgPO4 ·6H2 O (ICDD 75-1076).
4 = Struvite NH4 MgPO4 ·6H2 O (Strunz & Nickel, 2001).

for EMP analyses from the completely transparent minutecrystals failed. The only solution to this problem was thecomplete structure determination, including a refinementof K versus NH4 .
A prismatic, colourless, and transparent crystal ofstruvite-(K) from Lengenbach (Switzerland), about 0.20 ×
0.04 × 0.04 mm in size, was measured on a Bruker
AXS three-circle diﬀractometer (equipped with a CCD
1000 K area detector and a flat graphite monochromator)
using Mo Kα X-radiation from a fine focus sealed tube. A
SMART system of programs was used for crystal-latticedetermination and X-ray data collection, and SAINT+ forthe data reduction including intensity integration, background and Lorentz-polarization corrections. The program
XPREP was used for empirical absorption correction basedon pseudo ψ-scans. The structure was solved in the spacegroup Pmn21 by direct methods (program SHELXS) andrefined with the program SHELXL. The refinement converged at R1 = 6.35 % for 758 reflections with I > 2sigma(I); R1 = 7.01 % for all 818 reflections; wR2 =12.92 %; Rint = 4.90 %; Rsigma = 4.19 %. The refinement ofK versus NH4 yielded the pure K end-member (Fig. 4).
(2) Rossblei: Electron-microprobe analyses were done on
a SEM JEOL JSM-6310 with energy- and wavelength dispersive spectrometers. No quantitative data were achieved,
due to instability of these phosphates under the micro-beam

Struvite-(K), KMgPO4 ·6H2 O, the potassium equivalent of struvite – a new mineral633

possession at our disposal for the study. Samples from
Rossblei were provided by Prof. J.G. Haditsch (Graz). The
help and advice of Ernst A.J. Burke (chairman of the Commission on New Minerals and Mineral Names, I.M.A.) isgreatly acknowledged. The comments of the two reviewers,
Th. Witzke and I.V. Pekov, helped to improve the quality ofthe publication.

References

Fig. 4. Crystal structure of struvite-(K), projected along [010], anddisplaying H... O hydrogen bonds.

and at vacuum condition (fractures as seen in Fig. 3). Only
the main elements Mg, K and P were detected.

Discussion
The Lengenbach sample, with a more complete data collection, has to be designated as the holotype sample forthe new species struvite-(K), KMgPO4 ·H2 O. Moreover, its
formation is undoubtedly the result of a natural process.
According to the definitions given by Dunn & Mandarino(1987) for type mineral specimens, the Rossblei occurrence
represents a cotype for the species.
Struvite-(K) is a member of the schertelite-struvite groupand belongs to the Strunz class 8.CH.25 according toStrunz & Nickel (2001).
Acknowledgements: Markus Ecker (member of theLengenbach syndicate FGL) kindly put all material in hisDamour, A.A. (1845): Dufrénoysit, eine neue Mineralgattung. Ann.
Chim. Phys., 14, 379.
Dunn, P.J. & Mandarino, J.A. (1987): Formal definitions of typemineral specimens. Can. Mineral., 25, 571-572
Graeser, S., Topa, D., Balić-Žunić, T., Makovicky, E. (2006):
Gabrielite (Tl2 AgCu2 As3 S7 ) – a new species of Thalliumsulfosalt from Lengenbach, Binntal, Switzerland. Can.
Mineral., 44, 135-140
ICDD (International Centre for Diﬀraction Data), 1601 Park Lane,
Swarthmoore, Pennsylvania 19081-2389 U.S.A.
Laskovic, F. & Weninger, H. (1967): Phosphorrösslerit
MgH(PO4 )·7H2 O und Newberyit MgH(PO4 )·3H2 O aus dem
Revier Roßblei der Eschachalm im Obertal bei Schladming. –Arch. f. Lagerst.forsch. i.d. Ostalpen, 5, 132-140.
Postl, W., Walter, F., Ettinger, K., Bojar, H.P. (2000): Erster
Nachweis des Kalium-Analogons MgK(PO4 )·6H2 O vonStruvit, und der kristallinen Phase Mg2 KH(PO4 )·15H2 O
aus dem ehemaligen Bleibergbau Rossblei, Eschachalm,
Schladminger Tauern, Steiermark, Österreich. Joannea –Mineralogie, 1, 45-52.
Strunz, H. & Nickel, E.H. (2001): Strunz Mineralogical Tables.
Chemical structural classification system. 9th edition. E.
Schweizerbart’sche Verlagsbuchhandlung, Stuttgart, 870 p.

Received 4 June 2007
Modified version received 7 January 2008
Accepted 10 March 2008