REE-apatite from Chernigovka carbonatitic complex
Dubyna A.V., Kryvdik S.G.
N.P. Semenenko Institute of geochemistry, mineralogy and ore formation NAS of Ukraine, Kyiv, Ukraine
Apatite is present both accessory and rock-forming mineral in all types of alkaline silicate rocks and carbonatites of Chernigovka massif (Azov area). As in most of alkaline ultrabasic and carbonatitic massifs, apatites of Chernigovka have increased content of REE (0,85-3,41% REE2Î3) and Sr (0,18-4,6% SrÎ). In different types of rocks the concentration of these elements rises together with increasing SiO2 content in apatite (tveitasite, ringite) or Na2O (beforsite). The considered REE-apatites occurs as orbed grains with irregular edges. Apatites with significant REE concentration (up to 7.34-11.0%) and high SiO2 contents (up to 3,6%) are kwon in tveitasites, tveitasite-pyroxenites and ringites in this massif. The color of such apatites changes from dark-red to brown because there are exsolution inclusions of cellular, columnar or without any defined forms that were formed during apatite destruction. Collection of these mineral phases can change in different rocks of massif. Earlier  britholite (rarely orthite) as one main phase was found in Chernigovka REE-apatites from tveitasites and ringites by microprobe investigation. By subsequent investigation  there were determined bastnasite, quartz, calcite, strontianite and monazite in exolution texture of REE-apatite from tveitasite-pyroxenites.
REE-apatites from Chernigovka ringites and beforsite have been investigated by microanalyzer JEOL JXA 8200 and scanning microproanalyzer JEOL JED-6700. The unhomogeneous texture of REE-apatite from tveitasites and rengites is distinctly displayed by own and prior accomplished microprobe investigation. The matrix of apatite is well-defined. This matrix is presented by apatite it contains isometric and irregular-shaped of neogenic apatite in addition to above mentioned ecssolution minerals. Apatite of matrix is lighter as regards to more dark apatite and has higher concentration SiO2 and REE2O3. Essential distinction in the other elements (Ca, P, Na, F, Sr) isn’t observed. At the same time the REE-apatites of Chernigovka beforsites have a homogeneous structure.
Mostly among the minerals formed by destruction of REE-apatite predominate one of them. Based on data  britholite is the main phase in apatites of tveitasites and ringites. In some investigated samples of REE-apatite from ringites the prevalent phase is bastnaesite and subordinate quantity of calcite and quartz. Although there are grains with calcite and monazite only.
The grains of bastnaesite from 10 to 150 μm have more often irregular and rarely isometric shapes. Frequently in the bigger bastnaesite grains are dark parts of inconstant composition. Apparently they consists of fine-grained aggregate mixture (<1 μm) of hematite, quartz, bastnaesite and apatite. The interesting peculiarities of these aggregates is higher thorium content (1,02% ThO2) whereas in apatite matrix and exsolution bastnaesite its concentration is distinctly less. The thin margin (~1 μm) of hematite is formed on the rim of these fine-grained aggregates.
Besides bastnaesite there are isometric grains of calcite with increased strontium content (2,41% SrÎ). Maybe there is strontianite inclusions (<1 μm) in calcite from apatite but it failed to analyse. Though the fine inclusions of strontianite are in coexist calcite of ringites.
The monazite grains are changeable both shapes and sizes. More fine grains (less 20 μm) are more isometric or elongated shapes whereas the bigger grains (20-50 μm) are complicated and irregular-shaped.
The quartz grains are irregular-shaped (as bastnaesite too) from investigated samples of apatite. Sometimes there are orthite, rarely britholite and undiagnosed rare metal silicate (cerite?) in association with quartz grains. Orthite as margin is confined to contacts of calcite and alkali feldspar. Moreover it should note that some variety of Chernigovka’s tvaitasites are characterized by high content (up to 20%) of orthite.
There are fixed 5,74-7,39% REE2O3 in REE-apatite from beforsites of Chernigovka massif. Furthermore, apatite from beforsites have high strontium content (4,31-4,55% SrO). Unlike to investigated apatites from ringites, in apatites from beforsites the consentration of REE is increasing with Na2O content with negligible SiO2 content. The investigated REE-apatites from beforsites have a homogeneous texture of grains without a visible exsolution of neogenic mineral phases. By the present investigations it is established availability the fine inclusions (or segregations?) of undiagnosed carbonate mineral (intermediate composition between carbocernaite and ancylite) with high content of REE and Sr. Besides there are fine inclusions of magnetite in this apatite.
The increasing Sr content in apatites agree with high Sr content in befosites. The supersaturation of alkali in magmatic melt (especially sodium) promote sodium entering in apatite coupled with REE by belovitic scheme Na++TR3+=2Ca+2. In melt more enriched in SiO2 REE enter in apatite by britholitic scheme Ca+2+P+5=TR+3+Si+4.
Table. The composition of apatites and their exsolution products from ringites and beforsites of Chernigovka massif
138,20% CO2; 2 1,79% SO3
1-6 – apatites from ringites (5-6 more dark-colored apatite); 7-9 – apatite from beforsite; 10-13 – inclusions of bastnaesite in apatite from ringites; 14-18 – fine-grained aggregates in bastnaesite; 19 – calcite; 20 – cerite (?); 21 – orthite from ringites; 22 – inclusion of stroncianite in calcite from ringites; 23 – monazite; 24-25 - carbonate inclusions in apatite from beforsites.
Analysis: microprobe – 1-14, 21, 22, 24, 25; scanning microprobe – 15-20, 23.
There is reason to believe that in a primary Si-REE apatite took place a breakdown of solid solution by more rich and poor in REE and Si apatites (8-10% REE2O3 and 2,3-3,6% SiO2; 4-5% REE2O3 and 1,5-2% SiO2 accordingly) without any certain regularities and shapes of exsolution. Some later the britholite exsolution (isostructural with apatite) are formed from enriched REE-apatite. As is known britholite is an unstable mineral and it is usually substituted by bastnaesite, quarz, monazite, carbonate and REE silicate (cerite-like?). Sometimes the preserved exolution britholite is fixed.
It looks as if the Na-REE-apatite is more stable phase because no any typical exsolution textures aren’t found in it. Although in those apatites are fine inclusions of mentioned REE-Sr-carbonate.
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