Definition of Neotype Material for Simonellite, C₁₉H₂₄

Abstract

Simonellite, C₁₉H₂₄, was first described from the lignite deposit of Fognano, Montepulciano, Siena Province, Tuscany, Italy, in 1919. Its crystal structure was solved and refined in the 1960s on the basis of recrystallized individuals. Notwithstanding several studies on this species, no type material has ever been reported. Following a new finding of simonellite, its crystal structure was re-examined using a natural crystal. Simonellite is orthorhombic, with space group Pnaa and unit-cell parameters a = 9.2220(5), b = 9.1269(5), c = 35.8907(17) Å, V = 3020.9(3) ų. Its crystal structure was refined to R₁ = 0.0513 for 2721 unique reflections with F_o > 4σ(F) and 244 refined parameters. Since no type material for this species is currently known, the studied material is defined as a neotype specimen for simonellite. It is deposited in the mineralogical collection of the Museo di Storia Naturale of the University of Pisa under catalogue number 20079.

1. Introduction

Polycyclic Aromatic Hydrocarbon (PAH) minerals represent a very small fraction of the total number of the currently known mineral species, with only eleven species described so far (

Table 1. Valid PAH minerals.

Species	Chemical Formula	Type Locality	Year of Description	Type Material	Ref.
Branchite	C20H34	Monte Vaso, Tuscany, Italy	1839	MSN-UniPi	[1]
Carpathite	C24H12	Olenevo, Zakarpattia Oblast, Ukraine	1955	No data	[2]
Dinite	C20H36	Castelnuovo Garfagnana, Tuscany, Italy	1852	MSN-UniPi	[3]
Fichtelite	C19H34	Hausellohe, Bavaria, Germany	1841	No data	[4]
Freitalite	C14H10	Carola mine, Saxony, Germany	2021	TU-BA-Freiberg	[5]
Hokkaidoite	C22H12	Aibetsu mine and Shikaribetsu lake, Hokkaido, Japan	2022	NMNS	[6]
Idrialite	C22H14	Idria mine, Slovenia	1832	No data	[7]
Kratochvílite	C13H10	Kladno mine, Central Bohemia, Czech Republic	1937	No data	[8]
Ravatite	C14H10	Ravat village, Sughd, Tajikistan	1993	TU-BA-Freiberg	[9]
Simonellite	C19H24	Fognano, Tuscany, Italy	1919	No data	[10]
Wampenite	C18H16	Wampen, Bavaria, Germany	2017	LACMNH	[11]

LACMNH = Los Angeles County Museum of Natural History, Los Angeles, U.S.A.; MSN-UniPi = Museo di Storia Naturale of the University of Pisa, Italy; NMNS = National Museum of Nature and Science, Tsukuba, Japan; TU-BA-Freiberg = TU Bergakademie Freiberg, Germany. [1] Bonaccorsi et al. (2022); [2] Piotrovsky (1955); [3] Franzini et al. (1991); [4] Bromeis (1841); [5] Witzke et al. (2021); [6] Tanaka et al. (2023); [7] Dumas (1832); [8] Rost (1937); [9] Nasdala and Pekov (1993); [10] Boeris (1919); [11] Mills et al. (2017).

). Most of them (seven out of eleven) were described before the establishment of the Commission on New Minerals and Mineral Names of the International Mineralogical Association in 1959 and before the routine deposition of type material. As a consequence, five minerals do not have any data about their type material, i.e., carpathite, fichtelite, idrialite, kratochvílite, and simonellite.

The importance of type material in mineralogy has been well known since the seminal work of Embrey and Hey [12]. In order to fill this gap, recently, we were able to obtain a specimen of simonellite from its type locality, the lignite deposit of Fognano, in the Montepulciano municipality (Siena Province, Tuscany, Italy). The specimen was collected by the mineral amateur Mirko Bonechi, who kindly provided the studied material in the summer of 2024. New crystallographic data confirmed its identification and allowed us to deposit it in the mineralogical collection of the Museo di Storia Naturale of the University of Pisa. Owing to the lack of type material for this PAH mineral, the specimen studied in this work was then proposed as neotype material of simonellite, and the proposal was accepted by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (voting proposal 24-F). The specimen is kept in the mineralogical collection of the Museo di Storia Naturale of the University of Pisa, Via Roma 79, Calci, Pisa Province, Italy, under catalogue number 20079.

2. Previous Studies on Simonellite

At the end of the 1910s, Vittorio Simonelli (1860–1929), Italian geologist and paleontologist, collected some white crystalline crusts within fractures of lignite at Fognano, near Montepulciano (Siena Province, Tuscany, Italy), where a lignite deposit hosted in Pliocene clays was just discovered. Two specimens were given to the mineralogist Giovanni Boeris (1867–1946), who held the chair of Mineralogy at the University of Bologna. The study of this material revealed physical and chemical properties that suggested that this phase could be a new hydrocarbon mineral, probably orthorhombic, which was named simonellite in honour of its discoverer [10]. Further data on simonellite were given by Ciusa and Galizzi [13]. On the basis of their chemical analysis, simonellite was defined as C₁₅H₂₀.

Emiliani and Pellizzer [14,15,16] improved our knowledge on simonellite through morphological, optical, and X-ray diffraction studies. In particular, they were able to measure unit-cell parameters, confirming the orthorhombic symmetry suggested by Boeris [10]: a = 9.12, b = 9.27, c = 35.80 Å. According to Weissenberg photographs, Emiliani and Pellizzer [16] proposed the space group Pnaa after a transformation of the unit-cell in order to point out its pseudotetragonal nature. Ghigi and Fabbri [17] examined simonellite using chemical and spectroscopic techniques, whereas Ghigi et al. [18] were able to synthesize this compound. The chemical formula was revised to C₁₉H₂₄.

The crystal structure of simonellite was solved and refined by Foresti and Riva di Sanseverino [19]. They used a sample recrystallized from benzene after the dissolution of natural samples. The unit-cell parameters of simonellite are a = 9.231(3), b = 9.134(3), c = 36.01(1) Å, space group Pnaa. This latter space group was preferred with respect to the conventional Pccn in order to stress the pseudotetragonality of the cell pointed out by Emiliani and Pellizzer [15,16]. Single-crystal X-ray intensity data were measured using photographic techniques, and the crystal structure refinement converged to R = 12%.

3. Experiment

A small bipyramidal crystal of simonellite was sampled from the specimen kindly donated by the mineral collector Mirko Bonechi. Simonellite occurs as colourless tabular crystals with a resinous lustre on the fractures of lignite; rarely, bipyramidal crystals have also been observed (Figure 1). These two morphologies can be compared with those shown in Figures 1 and 2 by Emiliani and Pellizzer [14].

A single-crystal X-ray diffraction study was performed using a Bruker D8 Venture (Bruker AXS Advanced X-ray Solutions SE, Karlsruhe, Germany) diffractometer equipped with an air-cooled Photon III CCD detector (Bruker AXS Inc., Madison, WY, USA) and microfocus MoKα radiation (Centro per l’Integrazione della Strumentazione scientifica dell’Università di Pisa, Pisa, Italy). The detector-to-crystal distance was 52 mm. Data were collected using ω scan mode, in 0.5° slices, with an exposure time of 30 s per frame. A total of 1125 frames was collected. The frames were integrated with the Bruker SAINT software package using a narrow-frame algorithm. Data were corrected for Lorentz polarization, absorption, and background using the Apex4 software package [20]. Unit-cell parameters were refined on the basis of the XYZ centroid of 9993 reflections above 20 σ(I) with 4.604° < 2θ < 53.84°. They are a = 9.2220(5), b = 9.1269(5), c = 35.8907(17) Å, V = 3020.9(3) ų. The statistical tests on the distribution of |E| values (|E² − 1| = 0.942) and systematic absences agree with the space group Pnaa.

The crystal structure of simonellite was refined using Shelxl-2018 [21] starting from the coordinates of C atoms given by Foresti and Riva di Sanseverino [19]. An isotropic model refined to R₁ = 0.1533, improved to R₁ = 0.1173 describing anisotropically the displacement parameters of C atoms. The displacement parameters of H atoms were constrained to be 1.2 times that of the bonded C atoms. The final structural model refined to R₁ = 0.0513 for 2721 unique reflections with F_o > 4σ(F) and 244 refined parameters. Details of the data collection and crystal structure refinement are given in

Table 2. Crystal and experimental details for simonellite.

Crystal Data
Chemical formula	C19H24
Crystal size (mm)	0.175 × 0.150 × 0.110
Cell setting, space group	Orthorhombic, Pnaa
a (Å)	9.2220(5)
b (Å)	9.1269(5)
c (Å)	35.8907(17)
V (Å3)	3020.9(3)
Z	8
Data collection and refinement
Radiation, wavelength (Å)	MoKα, 0.71073
Temperature (K)	293(3)
2θmax (°)	54.94
Measured reflections	32,256
Unique reflections	3450
Reflections with Fo > 4σ(F)	2721
Rint	0.0353
Rσ	0.0196
Range of h, k, l	−11 ≤ h ≤ 11, −11 ≤ k ≤ 10, −46 ≤ l ≤ 43
R1 [Fo > 4σ(F)]	0.0513
R1 (all data)	0.0633
wR (on F2)	0.1505
Goof	1.068
Number of least-squares parameters	244
Maximum and minimum residual peak (e Å−3)	+0.31 [at 1.02 Å from C3] −0.15 [at 1.35 Å from C8]

.

4. Crystal Structure Description

Atom coordinates and isotropic or equivalent isotropic displacement parameters are reported in

Table 3. Site, site occupancy (s.o.), fractional atomic coordinates and equivalent isotropic or isotropic (*) displacement parameters (in Ų) for simonellite.

Site	s.o.	x/a	y/b	z/c	Ueq/iso*
C1	C1.00	0.09304(16)	0.58044(17)	0.18091(4)	0.0536(4)
C2	C1.00	0.11399(19)	0.4744(2)	0.21351(5)	0.0629(4)
H21	H1.00	0.046(2)	0.509(2)	0.2337(5)	0.075 *
H22	H1.00	0.082(2)	0.364(2)	0.2061(5)	0.075 *
C3	C1.00	0.26913(19)	0.4638(2)	0.22583(4)	0.0635(4)
H31	H1.00	0.274(2)	0.402(2)	0.2492(6)	0.076 *
H32	H1.00	0.308(2)	0.571(2)	0.2323(5)	0.076 *
C4	C1.00	0.36154(18)	0.4011(2)	0.19462(4)	0.0563(4)
H41	H1.00	0.468(2)	0.4100(19)	0.2023(5)	0.068 *
H42	H1.00	0.3390(18)	0.289(2)	0.1921(5)	0.068 *
C5	C1.00	0.56465(17)	0.37044(18)	0.13188(4)	0.0554(4)
H51	H1.00	0.5799(18)	0.320(2)	0.1544(5)	0.067 *
C6	C1.00	0.66364(18)	0.35868(19)	0.10373(5)	0.0599(4)
H61	H1.00	0.748(2)	0.295(2)	0.1083(5)	0.072 *
C7	C1.00	0.64820(17)	0.43693(17)	0.07008(4)	0.0545(4)
C8	C1.00	0.52816(18)	0.52221(17)	0.06603(4)	0.0537(4)
H81	H1.00	0.5147(19)	0.5774(18)	0.0410(5)	0.064 *
C9	C1.00	0.29681(18)	0.62351(19)	0.08977(4)	0.0590(4)
H91	H1.00	0.2835(19)	0.673(2)	0.0648(5)	0.071 *
C10	C1.00	0.19720(18)	0.63498(19)	0.11729(4)	0.0588(4)
H101	H1.00	0.112(2)	0.700(2)	0.1127(5)	0.071 *
C11	C1.00	0.21297(15)	0.56090(15)	0.15184(4)	0.0471(3)
C12	C1.00	0.33480(15)	0.47632(15)	0.15767(4)	0.0447(3)
C13	C1.00	0.43997(15)	0.45995(15)	0.12860(4)	0.0449(3)
C14	C1.00	0.42185(16)	0.53536(16)	0.09431(4)	0.0488(3)
C15	C1.00	0.0921(2)	0.7400(2)	0.19460(6)	0.0733(5)
H151	H1.00	0.009(2)	0.757(2)	0.2122(5)	0.088 *
H152	H1.00	0.186(2)	0.764(2)	0.2098(5)	0.088 *
H153	H1.00	0.077(2)	0.806(2)	0.1740(6)	0.088 *
C16	C1.00	0.94449(19)	0.5443(3)	0.16343(6)	0.0756(5)
H161	H1.00	0.943(2)	0.438(3)	0.1512(6)	0.091 *
H162	H1.00	0.866(2)	0.539(2)	0.1840(6)	0.091 *
H163	H1.00	0.921(2)	0.620(3)	0.1450(6)	0.091 *
C17	C1.00	0.7618(2)	0.4253(2)	0.03965(5)	0.0636(4)
H171	H1.00	0.7314(19)	0.493(2)	0.0169(5)	0.076 *
C18	C1.00	0.9105(2)	0.4704(3)	0.05352(7)	0.0868(6)
H181	H1.00	0.986(3)	0.466(3)	0.0335(6)	0.104 *
H182	H1.00	0.911(3)	0.568(3)	0.0663(6)	0.104 *
H183	H1.00	0.943(3)	0.392(3)	0.0716(7)	0.104 *
C19	C1.00	0.7630(3)	0.2742(3)	0.02207(6)	0.0877(6)
H191	H1.00	0.660(3)	0.254(3)	0.0101(6)	0.105 *
H192	H1.00	0.837(3)	0.271(3)	0.0012(6)	0.105 *
H193	H1.00	0.788(3)	0.206(3)	0.0431(7)	0.105 *

, whereas

Table 4. Carbon–C distances (in Å) in simonellite.

C1–C2	1.530(2)	C5–C6	1.366(2)	C9–C14	1.415(2)
C1–C11	1.5310(19)	C5–C13	1.415(2)	C10–C11	1.420(2)
C1–C15	1.536(2)	C6–C7	1.410(2)	C11–C12	1.3792(19)
C1–C16	1.543(2)	C7–C8	1.361(2)	C12–C13	1.4323(18)
C2–C3	1.501(2)	C7–C17	1.517(2)	C13–C14	1.4201(19)
C3–C4	1.519(2)	C8–C14	1.416(2)	C17–C18	1.516(3)
C4–C12	1.5134(19)	C9–C10	1.353(2)	C17–C19	1.517(3)

and

Table 5. Carbon–H distances (in Å) in simonellite.

C2–H21	1.008(19)	C8–H81	1.039(17)	C16–H163	0.98(2)
C2–H22	1.09(2)	C9–H91	1.013(17)	C17–H171	1.062(19)
C3–H31	1.01(2)	C10–H101	0.999(19)	C18–H181	1.00(3)
C3–H32	1.06(2)	C15–H151	1.00(2)	C18–H182	1.00(3)
C4–H41	1.024(18)	C15–H152	1.05(2)	C18–H183	1.01(2)
C4–H42	1.051(19)	C15–H153	0.97(2)	C19–H191	1.06(2)
C5–H51	0.941(18)	C16–H161	1.07(2)	C19–H192	1.01(2)
C6–H61	0.987(19)	C16–H162	1.04(2)	C19–H193	1.01(2)

give selected bond distances. The Crystallographic Information File (CIF) is made available in the Supplementary Materials linked to this article.

Simonellite is a molecular solid. Its molecule, C₁₉H₂₄, is shown in Figure 2. It corresponds to 1,1-dimethyl-1,2,3,4-tetrahydro-7-isopropylphenantrene, in accord with Ghigi et al. [18].

Figure 2. The C₁₉H₂₄ molecule occurring in simonellite. Black and pink circles are C and H atoms, respectively. C–C and C–H bonds are shown as thick and thin black lines, respectively. Drawn using CrystalMaker v.9.2.8 [22].

Figure 2. The C₁₉H₂₄ molecule occurring in simonellite. Black and pink circles are C and H atoms, respectively. C–C and C–H bonds are shown as thick and thin black lines, respectively. Drawn using CrystalMaker v.9.2.8 [22].

Two different kinds of C atoms can be identified: three-fold (sp²) and four-fold coordinated (sp³). Three-fold coordinated C atoms show two different environments: some C atoms are bonded to three other C atoms (i.e., C7, C11, C12, C13, and C14), whereas others are bonded to two C atoms and one H atom (i.e., C5, C6, C8, C9, and C10). The C–C distance ranges between 1.353(2) and 1.432(2) Å (average value 1.399 Å, to be compared with 1.401 Å given by Foresti and Riva di Sanseverino [19]). Four-fold coordinated C atoms have different neighbours: C at C1 site is bonded to four other C atoms, whereas C at C17 is bonded to three C atoms and one H atom. Two C and two H atoms are bonded to C atoms at C2, C3, and C4, whereas C at the C15, C16, C18, and C19 sites are bonded to one C atom only and three H ones. Carbon atoms in sp³ configuration have C–C distances in the range 1.500(2)–1.543(2) Å (average value 1.523 Å, to be compared with 1.539 Å reported by Foresti and Riva di Sanseverino [19]).

Figure 3 shows the projections of the crystal structure of simonellite. As previously noted by Foresti and Riva di Sanseverino [19], the C₁₉H₂₄ molecules lie almost linearly along a.

5. Conclusions

Modern X-ray diffraction analysis carried on natural crystals of simonellite confirmed the previous crystallographic investigations performed on recrystallized samples (e.g., [19]), also allowing for the deposit of a neotype material in a public mineral collection, i.e., that of the Museo di Storia Naturale of the University of Pisa.

Simonellite is the third well-characterized hydrocarbon mineral from the Tertiary lignite deposits of Tuscany, after dinite [3,23] and branchite [1,24]. Type materials of all these three PAH compounds are currently kept in the mineralogical collections of the Museo di Storia Naturale of the University of Pisa.