Quantitative characterization appreciation of golden citrine golden by the irradiation of [FeO₄]⁴⁻

3.1 IR test

Through gemological tests of all samples, they are testified genuine citrines from refractive index is 1.54–1.55, special density is 2.66, and positive uniaxial crystal (1+). The further identification of natural citrine, heat-treated citrine and synthetic citrine were proceeded by Fourier transform infrared spectrum (FT-IR), RUKER, German.

With voltage 85–265 V, frequency 47–65 Hz, temperature scope 18–35 °C, humidity range <70%, sample scanning time 3 scans, background scanning time 8 scans, resolution 4 cm⁻¹, diaphragm setting 6 mm and 10 KHz, spectrum scope 2000–6000 cm⁻¹, samples scanning repeated 50–100, and transmission mode (Halim et al., 2017; Rahman et al., 2017).

There's almost horizontal curve at 5200 cm⁻¹ of sample CF01 and CF02 (Fig. 1), while is exactly differ from the combined spectrum of stretching vibration and bending vibration at the same position (Fig. 2); and the high transmission at the 3556 cm⁻¹ (Fig. 1) is much differ from natural citrine too (Fig. 2), which contributed by less OH⁻ or H₂O. Finally, it results that these two samples are Heat-Treated Citrine.

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Fig. 1 IR spectrum of facet heat-treated citrine.

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Fig. 2 IR spectrum of facet natural citrine.

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There’s many different absorptions at 5199 cm⁻¹ and 3200–3600 cm⁻¹ in IR spectrum, they are the same as the natural citrine’s standard IR spectrum, so these are all-natural citrine samples (Fig. 2).

Sample CR01 shows similar as sample CF01 and CF02, which reveals the high transmissivity (higher than natural crystal), that is the low absorptivity at 5237 cm⁻¹; and transmitted high at 3557 cm⁻¹. All these illustrates the possibility of low OH⁻ or H₂O of the sample, so, the sample CR01 could be judged as heat-teated Citrine (Fig. 3).

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Fig. 3 IR spectrum of cabochon heat-teated citrine.

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The curve almost tends to horizontal while less than 3500 cm⁻¹ in the IR spectrum of sample CR03 and CR04, it shows the incompatibility of natural citrine standard IR spectrum, so these two sample were testified as synthetic citrine (Fig. 4).

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Fig. 4 IR spectrum of cabochon synthetic citrine.

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There’s many different absorptions at 5185 cm⁻¹ and 3200–3600 cm⁻¹ in IR spectrum except sample CR01, CR03 and CR04, all these are exactly the same as the natural citrine’s standard IR spectrum, so these cabochon stones are all-natural citrine samples (Fig. 5).

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Fig. 5 IR spectrum of cabochon natural citrine.

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Among all 32-citrine sample, CF01, CF02 and CR01 are heat-treated for their low absorptivity at 3500 cm⁻¹, CR03 and CR04 are synthetics for their horizontal curve when less than 3500 cm⁻¹, and the rest 27 pieces citrine are identified natural citrine without treatment for both their combined spectrum at 5200 cm⁻¹ and stretching vibration during 3200–3600 cm⁻¹, as a result, the further investigation will focus on these 27 citrines (see Fig. 6).

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Fig. 6 Citrine plotson chromatic diagram (L^* = 50).

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3.2 Quantitative characterization of citrine golden

To get rid of the color-test disturbance from faint dichroism and uneven color distribution of citrine, polarizing test were used to confirm that table facet of faceted citrine or back surface of cabochon citrine is perpendicular to vertical axis C, which is the single direction shows no dichroism (Shamsudin et al., 2017a,b; Ali et al., 2017). So, the test surface was selected as where color concentrated and much more intense which parallel with the very top facet of faceted citrine or very bottom surface of cabochon citrine.

By the consideration of hue, citrine can be classified into two groups: orange yellow (OY) h₀ ∈ (49°, 75°) and yellow (Y) h₀ ∈ (75°, 91°); by the consideration of chroma, citrine can be classified into five groups: faint C^∗ ∈ (8, 20), light C^∗ ∈ (20, 28), medium C^∗ ∈ (28, 40), fancy C^∗ ∈ (40, 50) and fancy intense C^∗ ∈ (50, 58); by the consideration of lightness, citrine can be classified into five groups too: dark L^∗ ∈ (37, 46), dull L^∗ ∈ (46, 55), medium L^∗ ∈ (55, 64), bright L^∗ ∈ (64, 73), brilliance L^∗ ∈ (73, 83) (see Table 1).

Statistical analysis of natural citrine color parameters shows the relationships of C^∗ and h₀, and of C^∗ and L^∗ (Figs. 7 and 8). It reveals that citrine chroma decreases with the increase of h₀ and L^∗, that is the negative correlation of C^∗ with h₀ and L^∗.

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Fig. 7 The relation of C^* and ho of citrine.

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Fig. 8 The relation of C^* and L^* of citrine.

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Although the C^∗ and h₀ of citrine’s golden show the moderate negative correlation, with the help of SPSS partial correlation analysis, C^∗ and h₀ are independent from each other while after the control of lightness to chroma. In the same way, it is illustrated that there’s no distinct correlation between citrine’s golden and chroma while after the control of hue to chroma (Ismail and Hanafiah, 2017). All these reveals the moderate effect to citrine’s chroma are come from its hue and lightness together.

L^∗ of citrines are in the scope of 38–84, it is different with different hue: L^∗ of orange yellow citrines are concentrated in the whole scope; while L^∗ of yellow citrines concentrated in the scope of 55–64, and it shows moderate intensity; and much more dark or bright citrine are not found in nature yet (Aziz and Hanafiah, 2017).

Natural citrine’s lightness will increase with the increase of hue (Fig. 9), when take no consideration of the effect from chroma to lightness, the correlation coefficient of lightness and hue r = 0.924, P = 0.00 testified the high linear correlation of L^∗ and h₀; and when take consideration of the effect from chroma to lightness, the correlation coefficient of lightness and hue r = 0.897, P = 0.00 with the help of SPSS partial correlation analysis, it testified the high linear correlation of L^∗ and h₀ similarity (Fig. 11).

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Fig. 9 The relation of L^* and ho of citrine.

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Fig. 10 Citrine’s color sequenced by L^* increase.

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Fig. 11 Citrine’s color sequenced by ho increase.

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3.3 Weights analysis of color parameters

In order to analyze the contribution to citrine golden from lightness, chroma and hue, hierarchical cluster analysis and R cluster analysis were used to classify each color parameter (Table 2), to discuss the parameters classification of each color. In the color correlation matrix of citrine, L^∗ and h₀ correlated much more with coefficient r = 0.927, much higher than the correlation of L^∗ and C^∗ or of C^∗ and h₀, and then the high correlation of lightness and hue-angle were testified.

The colors of 32 pieces citrine were sequenced by the increase of lightness or the increase of hue-angle by computer simulation (Figs. 10 and 11). Combined with the observation by naked eye, it is reveals the sequence by lightness increase is accord with visual effect, which is the darker the lightness is, the darker the citrine is.

Combined with sequenced color and cluster analysis result, it illustrates that based on the citrine golden, the contribution weighs to visual color is L^∗, h₀ and C^∗ in sequence.

So, for citrine color appreciation, L^∗ should be considered as the primary factor because it can correspond visual effect typically and directly, and then take hue and chroma into account together comprehensively.