
Hình 1: Một viên tourmalin hai mầu 1,62 ct chứa đồng. Ảnh: Shunsuke Nagai
Một viên đá quý hai mầu (bicolor) sẽ có hai mầu trong một viên đá. Ametrin là một loại đá quý hai mầu nổi tiếng, đó là viên thạch anh mầu tía và vàng – sự kết hợp amethyst và citrin. Ngoài ametrin, những loại đá mầu khác như corindon và tourmalin cũng có thể có hai mầu tự nhiên, mặc dù hầu hết chúng đều không có tên của biến loại cụ thể này. Hầu hết đá hai mầu đều được chế tác sao cho thể hiện được hai mầu khác nhau khi quan sát từ mặt trên xuống.
Phòng thí nghiệm Tokyo gần đây đã kiểm tra một viên đá hai mầu hình chữ nhật, kiểu cắt bậc, nặng 1,62 ct và có kích thước 9,14 × 5,47 × 3,71 mm (hình 1). Mầu sắc thay đổi dần dần từ lục phớt lam tới lục phớt vàng thẫm dọc theo chiều dài. Viên đá dị hướng, với chiết suất 1,620 – 1,640 và tỷ trọng là 3,10. Quan sát dưới phóng đại thấy được các bao thể lỏng dạng mạng lưới và hiệu ứng nhân đôi mạnh. Với những đặc điểm ngọc học này, viên đá được xác định là tourmalin.
Tourmalin Paraiba, một loại tourmalin chứa đồng, là một trong những loại đá quý được tìm kiếm nhiều nhất trên thị trường trong vòng ba thập kỷ qua. Vào năm 2012, Ủy ban Hướng dẫn Hài hòa các Phòng thí nghiệm (the Laboratory Manual Harmonisation Committee – LMHC) đã cập nhật định nghĩa của tourmalin Paraiba như là “một viên tourmalin xanh lam (lam như tia điện, lam neon, lam tím), lục phớt lam tới lam phớt lục, lục (hoặc lục phớt vàng), với độ bão hòa cao và tông mầu trung bình tới sáng (liên quan với biến loại tourmalin này), chủ yếu là do sự hiện diện của đồng và mangan”. Phổ khả kiến-hồng ngoại gần (Vis-NIR) đã được đo bằng máy quang phổ UV-Vis do GIA chế tạo theo đơn đặt hàng để xác định các yếu tố gây mầu. Để được coi là tourmalin Paraiba, sự hấp thụ liên quan đến đồng cần phải chiếm ưu thế và biểu hiện mầu sắc phải nằm trong một phạm vi xác định.

Hình 2. Phổ Vis-NIR của cả viên tourmalin (trên), của phần mầu lục phớt lam (giữa) và phần lục phớt vàng thẫm (dưới). Sự hấp thụ liên quan đến đồng nằm ở vùng mầu đỏ (khoảng 730 nm) và vùng cận hồng ngoại (khoảng 980 nm). Sự hấp thụ liên quan đến sắt chỉ có ở vùng mầu đỏ (khoảng 730 nm). Các giản đồ phổ được chồng lên nhau để thấy được rõ ràng.
Phổ hấp thụ không phân cực của cả viên đá hiển thị hai dải ở vùng đỏ và vùng cận hồng ngoại (hình 2, vạch đen). Để đo phổ của từng mầu riêng biệt, một nửa của viên đá quý được che bằng một miếng bìa cứng mầu đen không thấu quang để cho ánh sáng chỉ truyền qua một mầu tại một thời điểm. Các kết quả chỉ ra rằng yếu tố gây mầu chủ đạo là khác nhau (hình 2; các đường lục phớt lam và lục đậm tương ứng ở giữa và dưới cùng) – phần lục phớt lam được tạo mầu bởi đồng, trong khi phần lục phớt vàng thẫm được tạo mầu bởi sắt (P.B. Merkel and C.M. Breeding, “Spectral differentiation between copper and iron colorants in gem tourmalines,” Summer 2009 G&G, pp. 112–119).

Bảng 1. Hàm lượng Mn, Fe và Cu trong mỗi phần mầu của viên tourmalin hai mầu được đo bằng phương pháp LA-ICP-MS (ppmw, trung bình của ba điểm đo)
Thành phần nguyên tố vết của mỗi vùng mầu khác nhau được phân tích bằng phương pháp khối phổ kế plasma (LA-ICP-MS). Như có thể thấy trong trong bảng 1, phần mầu lục phớt lam cho hàm lượng sắt thấp hơn (566 ppmw) và hàm lượng đồng cao hơn (7850 ppmw), và phần mầu lục phớt vàng thẫm cho hàm lượng sắt cao hơn (11287 ppmw) và hàm lượng đồng thấp hơn (4953 ppmw). So sánh các kiểu phổ hấp thụ Vis-NIR khác nhau của các phần mầu này (hình 2) cho thấy, sự kết hợp của đồng và sắt đối với mỗi phần mầu là phù hợp với công bố của Merkel và Breeding (2009). Do mầu sắc và yếu tố gây mầu của mình mà chỉ phần mầu lục phớt lam là phù hợp với định nghĩa của tourmalin Paraiba.
Đôi khi chúng ta gặp tourmalin chứa đồng có sự phân đới về thành phần với sự thay đổi bên trong về hàm lượng của các nguyên tố vết. Một sự khác biệt lớn, giống như trong trường hợp của viên tourmalin hai mầu này với phần không phải là Paraíba, được tạo mầu bởi sắt, không phải là điển hình. Phần lục phớt vàng thẫm đã có thể được cắt bỏ để cho viên đá trở thành một viên tourmalin Paraiba điển hình.
Theo Yusuke Katsurada
Trong “Gems & Gemology, summer 2022, Vol. 58, No.2”

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