Spanish term (edited): diaclasa

It's diaclasa with an A, and yes, it's definitely joint.

TERM diaclasa

Reference Amstutz, Gloss. of Mining Geology
(2)
TERM juntura

Reference Amstutz, Gloss. of Mining Geology

Definition a) line of cleavage in a coal seam; b) divisional plane or surface that divides a rock and along which there has been no visible movement parallel to the plane or surface

Reference usbm
(1)
TERM joint

Reference usbm;amstutz 28,43 defn

http://europa.eu.int/eurodicautom/Controller

Diaclasas
EN Joints
FR Diaclases
http://pci204.cindoc.csic.es/tesauros/Geologia/HTML/GEO_D1.H...

***Diaclasas (juntas; inglés: joints)***: Fracturas sin desplazamiento transversal detectable, solo con poco movimiento extensional. Son las fracturas más frequentes en todos los tipos de rocas. En la superficie son más frequentes como en altas profundidades.
http://www.mineranet.com.ar/geoestructural.asp

joint, in geology
in geology, fracture in rocks along which no appreciable movement has occurred (see fault). Nearly vertical, or sheet, joints that result from shrinkage during cooling are commonly found in igneous rocks. Similar joints occur in thick beds of sandstone and gneiss, with the sheets resembling the structure of a sliced onion....
http://www.bartleby.com/65/jo/joint-geo.html