If you can't find a private e-mail in an authors profile, usually you click the bar: "FOLLOW".
Then you can send a message to Marcio by RG-internal Messaging System....
OR, if you won't like to follow Marcio DUARTE, klick on his [ Marcio Duarte ] = https://www.researchgate.net/profile/Marcio_Duarte ,
find the (Menue) bar: "INFO" (top middle) , click there and find (on the right side in the middle to lower part) "SEND MESSAGE". Click there and formulate your message and "SEND".
Also, I wanted to point you to the following thread (on picrosirius staining):