Distribution and Characteristics of Nanotubular Halloysites in the Thach Khoan Area, Phu Tho, Vietnam oleh Bui Hoang Bac, Nguyen Tien Dung, Luong Quang Khang, Khuong The Hung, Nguyen Van Lam, Do Manh An, Phan Viet Son, Tran Thi Van Anh, Doan Van Chuong, Bui Thanh Tinh