Mineral Chemistry of Pyroxene Gneiss in Obudu, SE Nigeria, and Its Petrological Significance

Ifeoma Agbi; Emmanuel M. Iroka

doi:10.2478/rmzmag-2023-0001

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Mineral Chemistry of Pyroxene Gneiss in Obudu, SE Nigeria, and Its Petrological Significance

Materials and Geoenvironment

AHEAD OF PRINT

By: Ifeoma Agbi and Emmanuel M. Iroka

Open Access

|Aug 2023

Figures & Tables

Photomicrograph of pyroxene gneiss showing (a) mineral association of orthopyroxene + plagioclase+ biotite + quartz in sample IA5 (PPL); (b) Antiperthite texture in plagioclase of IA5 and apatite inclusion (XPL); (c) Mineral assemblage in sample IA3 showing clinopyroxene occurring alongside orthopyroxene (PPL); (d) BSE image of exsolved laths of hematite in ilmenite in sample IA3.

An-Ab-Or plot for the pyroxene gneiss of Obudu showing andesine as the main plagioclase.

Table 1:

Representative chemical composition (wt%) and atom per formula unit of plagioclase based on 8 oxygen. (sample IA3/5).

Sample	IA3					IA5
Wt%	1	2	3	4	5	1	2	3	4	5
SiO₂	60.55	60.58	60.57	60.48	60.44	60.13	60.07	60.20	59.91	60.95
Al₂O₃	25.33	25.02	25.12	25.43	25.55	25.75	25.18	25.17	25.49	24.90
FeO*	0.09	0.09	0.11	0.10	0.11	0.02	0.05	0.05	0.10	0.07
CaO	6.31	6.48	6.67	6.36	6.55	6.82	6.75	6.72	6.91	6.24
Na₂O	7.40	7.67	7.47	7.49	7.65	7.45	7.32	7.43	7.41	7.66
K₂O	0.68	0.50	0.40	0.55	0.43	0.35	0.53	0.62	0.39	0.46
Total	100.36	100.34	100.35	100.41	100.73	100.52	99.90	100.19	100.20	100.29
apfu
Si	2.692	2.690	2.694	2.686	2.673	2.668	2.684	2.681	2.667	2.709
Al	1.327	1.309	1.317	1.331	1.332	1.347	1.326	1.321	1.337	1.304
Fe³⁺	0.000	0.000	0.000	0.000	0.001	0.000	0.000	0.000	0.000	0.000
Fe²⁺	0.003	0.003	0.004	0.004	0.003	0.001	0.002	0.002	0.004	0.003
Ca	0.301	0.308	0.318	0.303	0.310	0.324	0.323	0.320	0.330	0.297
Na	0.638	0.661	0.644	0.645	0.656	0.641	0.634	0.641	0.640	0.660
K	0.038	0.028	0.023	0.031	0.024	0.020	0.030	0.035	0.022	0.026
Total	5.000	5.000	5.000	5.000	5.000	5.000	5.000	5.000	5.000	5.000
End Member Fractions (mol%)
An	30.77	30.91	32.26	30.93	31.33	32.92	32.72	32.15	33.26	30.21
Ab	65.30	66.27	65.43	65.91	66.21	65.09	64.23	64.33	64.51	67.12
Or	3.93	2.82	2.31	3.16	2.46	1.99	3.06	3.52	2.23	2.67

Pyroxene triangular diagram showing the occurrence of hypersthene and augite in the pyroxene gneiss.

Table 2:

Representative chemical composition (wt%) and atom per formula unit (apfu) of orthopyroxene based on 6 oxygen. (sample IA3/5).

Sample	IA3					IA5
Wt%	1	2	3	4	5	1	2	3	4	5
SiO₂	51.68	51.86	52.20	52.05	52.20	52.94	52.82	52.87	52.98	52.98
TiO₂	0.10	0.09	0.05	0.11	0.09	0.08	0.10	0.09	0.08	0.10
Al₂O₃	0.87	0.85	0.92	0.77	0.80	0.49	0.58	0.53	0.57	0.64
Cr₂O₃	0.02	0.01	0.00	0.03	0.00	0.05	0.10	0.05	0.05	0.08
FeO*	25.46	25.42	24.74	25.07	26.72	24.49	22.65	23.40	23.30	23.15
MnO	0.66	0.64	0.61	0.64	0.76	0.69	0.61	0.63	0.61	0.59
MgO	19.69	19.83	20.38	20.03	18.94	20.89	21.07	21.49	21.23	21.35
CaO	0.82	0.64	0.51	0.80	0.46	0.57	1.69	0.73	0.85	1.04
Na₂O	0.00	0.02	0.01	0.03	0.01	0.00	0.05	0.00	0.04	0.04
K₂O	0.00	0.01	0.00	0.00	0.01	0.03	0.01	0.00	0.01	0.00
Total	99.32	99.37	99.43	99.54	99.99	100.23	99.67	99.80	99.72	99.99
apfu
Si	1.971	1.975	1.979	1.976	1.990	1.989	1.985	1.985	1.992	1.985
Ti	0.003	0.003	0.002	0.003	0.003	0.002	0.003	0.002	0.002	0.003
Al	0.039	0.038	0.041	0.035	0.036	0.022	0.026	0.024	0.025	0.028
Cr	0.001	0.000	0.000	0.001	0.000	0.001	0.003	0.001	0.001	0.003
Fe³⁺	0.013	0.007	0.000	0.008	0.000	0.000	0.000	0.000	0.000	0.000
Fe²⁺	0.799	0.802	0.785	0.788	0.852	0.770	0.712	0.735	0.733	0.725
Mn	0.021	0.021	0.020	0.020	0.025	0.022	0.019	0.020	0.020	0.019
Mg	1.119	1.126	1.152	1.134	1.076	1.170	1.181	1.203	1.190	1.192
Ca	0.033	0.026	0.021	0.033	0.019	0.023	0.068	0.029	0.034	0.042
Na	0.000	0.001	0.001	0.002	0.001	0.000	0.004	0.000	0.003	0.003
Total	4.000	4.000	4.000	4.000	4.000	4.000	4.000	4.000	4.000	4.000
X_Mg	0.58	0.58	0.59	0.59	0.56	0.60	0.62	0.62	0.62	0.62
End Member Fractions (%)
Wo	1.70	1.32	1.07	1.67	0.97	1.18	3.47	1.50	1.76	2.13
En	56.98	57.40	58.85	57.77	55.28	59.61	60.22	61.15	60.80	60.85
Fs	41.32	41.28	40.08	40.57	43.75	39.22	36.31	37.36	37.44	37.02

Table 3:

Representative chemical composition (wt%) and atom per formula unit (apfu) of calcic amphibole and clinopyroxene based on 23 oxygen and 6 oxygen, respectively. (sample IA3/5)

Amphibole	Clinopyroxene
Sample	IA3					IA3		IA5
Wt%	1	2	3	4	5	1	2	1	2
SiO₂	42.71	42.70	42.86	41.79	43.34	52.67	52.74	53.24	52.89
TiO₂	2.59	2.39	2.06	2.49	2.48	0.16	0.13	0.07	0.16
Al₂O₃	11.11	10.57	10.77	11.40	10.51	1.49	1.39	1.44	1.50
Cr₂O₃	0.13	0.05	0.06	0.10	0.08	0.00	0.02	0.08	0.08
FeO*	14.09	15.61	15.77	15.84	15.30	9.41	10.37	9.66	9.17
MnO	0.11	0.15	0.15	0.16	0.15	0.31	0.30	0.31	0.32
MgO	11.58	11.30	11.01	10.55	11.46	13.66	13.42	14.27	14.30
CaO	11.42	11.18	11.51	11.48	11.29	21.48	20.74	20.17	19.95
Na₂O	1.35	1.53	1.01	1.22	1.35	0.41	0.48	0.36	0.41
K₂O	1.61	1.37	1.43	1.55	1.33	0.00	0.00	0.00	0.00
Total	96.70	96.86	96.63	96.58	97.29	99.59	99.58	99.60	98.76
apfu
Si	6.382	6.379	6.414	6.296	6.427	1.972	1.980	1.991	1.991
Ti	0.291	0.269	0.231	0.282	0.277	0.004	0.004	0.002	0.004
Al	1.956	1.861	1.900	2.024	1.836	0.066	0.062	0.063	0.066
Cr	0.016	0.005	0.007	0.011	0.010	0.000	0.000	0.002	0.002
fe³	0.326	0.555	0.548	0.448	0.518	0.012	0.006	0.000	0.000
Fe²⁺	1.435	1.394	1.426	1.548	1.379	0.283	0.319	0.302	0.289
Mn	0.013	0.019	0.019	0.020	0.019	0.010	0.009	0.010	0.010
Mg	2.581	2.517	2.456	2.371	2.534	0.762	0.751	0.796	0.802
Ca	1.829	1.790	1.845	1.854	1.794	0.861	0.834	0.808	0.804
Na	0.391	0.442	0.293	0.356	0.390	0.030	0.035	0.026	0.030
K	0.306	0.262	0.273	0.297	0.251
Total	15.527	15.493	15.411	15.506	15.434	4.000	4.000	4.000	4.000
X_Mg	0.59	0.56	0.55	0.54	0.57	0.72	0.70	0.72	0.74
End Member Fractions (mol%)
Wo						44.90	43.66	42.40	42.44
En						39.74	39.31	41.75	42.33
Fs						15.36	17.03	15.85	15.22

Si Vs Mg/(Mg +Fe²⁺) diagram for amphiboles from sample IA3 shown as tschermakite.

Table 4:

Representative chemical composition (wt%) and atom per formula unit (apfu) of biotite based on 22 oxygen (sample IA3/5).

Sample	IA3				IA5
Remarks	Core adjacent Opx	Rim adjacent Opx	Matrix 1	Matrix 2	Inclusion	Rim adjacent Opx	Core adjacent Opx	Matrix 1	Matrix 2
SiO₂	36.65	36.42	35.82	35.79	37.68	38.06	37.54	37.34	37.29
TiO₂	5.05	4.80	5.18	5.07	5.54	5.27	5.15	5.80	5.88
Al₂O₃	14.87	13.94	13.99	14.03	14.18	14.39	14.36	14.23	13.87
Cr₂O₃	0.04	0.04	0.05	0.07	0.45	0.44	0.40	0.44	0.41
FeO*	14.65	16.62	16.40	16.81	12.74	12.19	13.12	13.69	13.67
MnO	0.01	0.09	0.11	0.06	0.03	0.05	0.04	0.07	0.04
MgO	13.81	13.23	12.27	12.66	14.80	15.45	15.12	14.26	14.04
CaO	0.00	0.00	0.02	0.00	0.07	0.00	0.01	0.07	0.00
Na₂O	0.03	0.03	0.03	0.04	0.11	0.05	0.05	0.11	0.04
K₂O	9.37	9.34	9.42	9.34	9.50	9.64	9.67	9.41	9.63
Total	94.49	94.51	93.28	93.86	95.09	95.53	95.47	95.41	94.87
apfu
Si	5.519	5.547	5.535	5.504	5.591	5.601	5.563	5.550	5.580
Ti	0.572	0.550	0.602	0.586	0.618	0.584	0.574	0.648	0.661
Al	2.640	2.504	2.548	2.543	2.480	2.497	2.509	2.493	2.447
Cr	0.005	0.005	0.006	0.009	0.053	0.052	0.047	0.052	0.049
Fe²⁺	1.845	2.117	2.119	2.162	1.581	1.500	1.626	1.702	1.711
Mn	0.002	0.011	0.014	0.008	0.003	0.006	0.005	0.008	0.005
Mg	3.099	3.003	2.825	2.901	3.273	3.388	3.338	3.159	3.132
Ca	0.000	0.000	0.003	0.000	0.011	0.000	0.001	0.011	0.000
Na	0.009	0.007	0.009	0.013	0.031	0.013	0.015	0.031	0.012
K	1.800	1.816	1.857	1.832	1.799	1.810	1.828	1.783	1.839
Total	15.491	15.560	15.519	15.557	15.439	15.452	15.507	15.437	15.436
X_Mg	0.63	0.59	0.57	0.57	0.67	0.69	0.67	0.65	0.65