Gugiaite
General
Category	Sorosilicate
Formula (repeating unit)	Ca2[BeSi2O7]
IMA symbol	Gug[1]
Strunz classification	9.BB.10
Dana classification	55.04.02.06
Crystal system	Tetragonal
Crystal class	Scalenohedral (42m) H-M symbol: (4 2m)
Space group	P421m
Unit cell	a = 7.43, c = 5.024 [Å]; Z = 2
Identification
Color	Colorless
Cleavage	Perfect on {010}, distinct {001}, indistinct on {110}
Fracture	Uneven - Flat surfaces (not cleavage) fractured in an uneven pattern
Mohs scale hardness	5
Luster	Vitreous, glassy
Streak	White
Diaphaneity	Transparent
Density	3.03
Optical properties	uniaxial (+)
Refractive index	nω = 1.664 nε = 1.672
Birefringence	δ = 0.008
Other characteristics	strongly piezoelectric
References	[2][3][4]

Gugiaite is a melilite mineral, named for the Chinese village of Gugia where it was first discovered. Its chemical formula is Ca₂BeSi₂O₇. It occurs mostly in skarns with melanite adjacent to an alkali syenite and has no economic value. Its crystals are small tetragonal tablets with vitreous luster and perfect cleavage. It is colorless and transparent with a density of three. The mineral belongs to space group P-421m and is strongly piezoelectric.

Shortly after the discovery of gugiaite, it was noted that a new name was unnecessary as it could have been considered an end member of meliphanite, (Ca,Na)₂Be(Si,Al)₂(O,F)₂ differing mainly in containing much less Na and F (Fleischer 1963). Recent data have confirmed that gugiaite differs from meliphanite optically and structurally (Grice and Hawthorne 2002). Gugiaite is a melilite and is distinctly different from other beryllium minerals such as meliphanite and leucophanite (Grice and Hawthorne 2002). Gugiaite is named for its locality near the village of Gugia, China (Peng et al. 1962). Incongruent information exists regarding Gugia; consequently the actual location of this village within China is unclear (de Fourestier 2005). Gujia is most often referenced as being in either Jiangsu Province or Liaoning Province (Yang et al. 2001; Mandarino 2005).

Composition

Gugiaite has an ideal chemical formula of Ca₂BeSi₂O₇ and is a member of the melilite and sorosilicate (Si₂O₇) groups (Peng et al. 1962). It is chemically similar to jeffreyite (Ca,Na)₂[(Be,Al)Si₂(O,OH)₇], meliphanite (Ca,Na)₂[Be(Si,Al)₂O₆(O,OH,F)], and leucophanite (Ca,Na)₂[Be(Si,Al)₂O₆(O,F)] in that they all contain essential calcium, beryllium, and silicon (Hawthorne and Huminicki 2002). Two chemical analyses gave similar results and one is as follows: SiO₂ 44.90, Al₂O₃ 2.17, Fe₂O₃ 0.11, MnO 0.07, MgO 0.38, CaO 40.09, BeO 9.49, Na₂O 0.72, K₂O 0.20, H₂O⁻ 0.36, H₂O⁺ 0.90, F 0.25, Cl 0.18, P₂O₅ 0.08, TiO₂ trace, -O=(F,Cl)2 0.15, sum 99.94, 99.79% (Fleischer 1963). Common impurities are Ti, Zr, Hf, Al, Fe, Mn, Mg, Na, K, F, Cl, and P (Fleischer 1963).

Geologic occurrence

Gugiaite is usually found in skarn in contact with alkaline syenite with melanite, orthoclase, aegirine, titanite, apatite, vesuvianite, and prehnite (Peng et al. 1962). It occurs as thin square tablets, to 3 mm, in small cavities in skarn and enclosed in melanite (Peng et al. 1962). Skarns are often formed at the contact zone between granite intrusions and carbonate sedimentary rocks through metasomatism. Gugiaite has also been found in a miarolitic cavity in granite (Grew 2002). This type of cavity is crystal lined, irregular, and known for being a source of rare minerals, such as beryllium, that are not normally found in abundance in igneous rocks (https://web.archive.org/web/20091028021704/http://geocities.com/oklahomamgs/London/Pegmatite2.html). While initially found in Gugia, China, its localities have expanded to include Piedmont, Italy, Ehime Prefecture, Japan, Eastern Siberian Region, Russia, and most recently Telemark, Norway (http://www.mindat.org/min-1769.html).

Crystal structure

Gugiaite is composed of infinite sheets of tetrahedra with Be-Si-Si linkages and interstitial Ca (Hawthorne and Huminicki 2002). As shown in Figure 1, the oxygen atom bonds to a [4]-coordinated high-valence cation, Si, to produce a discontinuous polymerization of tetrahedra linked by interstitial Ca (Hawthorne and Huminicki 2002). It is isostructural with akermanite (Ca₂MgSi₂O₇) with Be occupying the Mg site of akermanite (Hawthorne and Huminicki 2002). X-ray studies by the Weissenberg method show gugiaite to be tetragonal, space group P-421m, space group number 113, and H-M Symbol -42m (Peng et al. 1962). Cell dimensions are: a=b=7.48(2) Ȧ, c=5.044(3) Ȧ, V=277.35 Ȧ, α=β=γ=90◦, and Z=2 (Peng et al. 1962). The axial ratio is a:c=1:0.67617 (Peng et al. 1962). Structurally A is Ca2, T1 is Be(54), T2 is Si2(53), and X is O7 (Yang et al. 2001). The three strongest lines of the X-ray powder data for gugiaite are 2.765(10), 1.485(7), and 1.709(7) (Peng et al. 1962).

Physical properties

The crystal form of gugiaite occurs as thin tetragonal tablets mostly 2-3 mm across and 0.3-0.5 mm thick, shown in Figure 2 below (Fleischer 1963). The cleavages are {010} perfect, {001} distinct, and {110} poor (Peng et al. 1962). It is transparent, optically uniaxial (+), and strongly piezoelectric (Peng et al. 1962). See Table for additional physical properties.

Significance

Gugiaite does not appear to have any political significance or economic value. From a historical perspective, gugiaite was the first beryllium mineral found in skarn systems at contacts between alkaline rocks and limestones (Peng et al. 1962). Also, thermodynamic equilibrium studies involving gugiaite have been conducted to determine the distribution of beryllium between gaseous and solid phases as a function of temperature in attempts to deduce the processes that formed the solar system (Lodders and Lauretta 1997).

References