A Science Crossword - Cryptic

[Matakari]

Good evening, I’m stuck on the following for which I would appreciate some help. Thanks in advance!

11a  River heard just after six? (6) : ???E?N                                                                                         

13, 17 Across US chemist flags summer heartache abroad (8, 7, 5) :
                      M?R?A?E?   M?L?A?E    ?U???

23a Salt for giddy girl say? (8) : S?L?C???

2d Pushing up mountains around Congo border region (8) : O?O???I?

[newmodarmy]

Severn seems too obvious?

[newmodarmy]

Apologies if there's some extra instructions 

[newmodarmy]

Silicone? But its not a salt so maybe not

[Toorak]

23 silicate

[newmodarmy]

Margaret someone?

[newmodarmy]

Of course, well done Toorak

[cashier]

2   Orogenic?

[newmodarmy]

orogenic?

[newmodarmy]

Anagram C(ong)O +region

[mallyh]

11a severn

[jazzyjen]

Margaret Melhase Fuchs

anagram of flag summer heartache

[Matakari]

Perfect! Many thanks, all!